U.S. patent application number 17/484831 was filed with the patent office on 2022-01-13 for methods and apparatuses for video coding.
This patent application is currently assigned to Tencent America LLC. The applicant listed for this patent is Tencent America LLC. Invention is credited to Guichun LI, Xiang LI, Shan LIU, Meng XU, Xiaozhong XU, Xin ZHAO.
Application Number | 20220014781 17/484831 |
Document ID | / |
Family ID | |
Filed Date | 2022-01-13 |
United States Patent
Application |
20220014781 |
Kind Code |
A1 |
XU; Meng ; et al. |
January 13, 2022 |
METHODS AND APPARATUSES FOR VIDEO CODING
Abstract
Aspects of the disclosure provide methods and apparatuses for
video encoding/decoding. An apparatus for video decoding includes
processing circuitry that decodes prediction information for a
current block in a current coded picture. The prediction
information indicates a motion vector predictor index (MVP_idx) for
selecting a motion vector predictor in a motion vector predictor
list. The processing circuitry determines whether the MVP_idx is
smaller than a threshold. When the MVP_idx is determined to be
smaller than the threshold, the processing circuitry decodes a
motion vector difference (MVD) corresponding to the motion vector
predictor and reconstructs the current block based on the motion
vector predictor and the MVD. When the MVP_idx is determined to be
equal to or larger than the threshold, the processing circuitry
reconstructs the current block based on the motion vector predictor
without the MVD which is not signaled in the coded video
sequence.
Inventors: |
XU; Meng; (San Jose, CA)
; LI; Xiang; (Saratoga, CA) ; XU; Xiaozhong;
(State College, PA) ; LI; Guichun; (Milpitas,
CA) ; LIU; Shan; (San Jose, CA) ; ZHAO;
Xin; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tencent America LLC |
Palo Alto |
CA |
US |
|
|
Assignee: |
Tencent America LLC
Palo Alto
CA
|
Appl. No.: |
17/484831 |
Filed: |
September 24, 2021 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
16698322 |
Nov 27, 2019 |
|
|
|
17484831 |
|
|
|
|
62776350 |
Dec 6, 2018 |
|
|
|
62787044 |
Dec 31, 2018 |
|
|
|
International
Class: |
H04N 19/52 20060101
H04N019/52; H04N 19/577 20060101 H04N019/577; H04N 19/70 20060101
H04N019/70; H04N 19/56 20060101 H04N019/56 |
Claims
1. A method for motion vector difference (MVD) coding in image
coding, comprising: determining a motion vector predictor and a
corresponding MVD representing motion information for a current
block; and encoding prediction information by processing circuitry,
the prediction information indicating the motion vector predictor
and the motion vector difference (MVD) for the current block,
wherein the MVD includes plural syntax elements and no more than
one of the plural syntax elements of the MVD is coded using context
coding.
2. The method according to claim 1, wherein each one of the plural
syntax elements comprises two flags, each of the two flags
indicating a value of the respective syntax element for one of two
directional components.
3. The method according to claim 2, wherein all of the plural
syntax elements of the MVD are coded with equal probability and
none of the plural syntax elements are coded using context
coding.
4. The method according to claim 2, wherein the two flags
corresponding to a first syntax element of the MVD are context
coded, the two flags of the first syntax element indicating whether
an absolute value of the MVD is greater than 0 in each of the two
directional components, and all syntax elements of the MVD that are
not the first syntax element are coded with equal probability.
5. The method according to claim 2, wherein the two flags
corresponding to a first syntax element of the MVD are context
coded, the two flags of the first syntax element indicating whether
an absolute value of the MVD is greater than 1 in each of the two
directional components, and all syntax elements of the MVD that are
not the first syntax element are coded with equal probability.
6. The method according to claim 2, wherein the plural syntax
elements consist of a first syntax element indicating whether an
absolute value of the MVD is greater than 0, a second syntax
element indicating the absolute value of the MVD minus 1, and a
third syntax element indicating a sign of the MVD.
7. The method according to claim 6, wherein all of the plural
syntax elements of the MVD are context coded, and the second syntax
element is binarized with a K-th order Exponential-Golomb
binarization, where K is one of 0, 2, or 3.
8. The method according to claim 2, wherein a later signaled one of
the two flags of one of the plural syntax elements is context coded
based on an earlier signaled one of the two flags of the one of the
plural syntax elements.
9. The method according to claim 2, wherein, a first syntax element
of the plural syntax elements indicates whether the MVD is greater
than 0, and in response to an earlier signaled one of the two flags
of the first syntax element indicating that a corresponding
component is not greater than 0, a later signaled one of the two
flags of the first syntax element is bypass coded.
10. A apparatus for coding a motion vector difference (MVD) in
image coding, comprising: processing circuitry configured to
determine a motion vector predictor and a corresponding MVD
representing motion information for a current block; and encode
prediction information, the prediction information indicating the
motion vector predictor and the motion vector difference (MVD) for
the current block, wherein the MVD includes plural syntax elements
and no more than one of the plural syntax elements of the MVD is
coded using context coding.
11. The apparatus according to claim 10, wherein each one of the
plural syntax elements comprises two flags, each of the two flags
indicating a value of the respective syntax element for one of two
directional components.
12. The apparatus according to claim 11, wherein all of the plural
syntax elements of the MVD are coded with equal probability and
none of the plural syntax elements are coded using context
coding.
13. The apparatus according to claim 11, wherein the two flags
corresponding to a first syntax element of the MVD are context
coded, the two flags of the first syntax element indicating whether
an absolute value of the MVD is greater than 0 in each of the two
directional components, and all syntax elements of the MVD that are
not the first syntax element are coded with equal probability.
14. The apparatus according to claim 11, wherein the two flags
corresponding to a first syntax element of the MVD are context
coded, the two flags of the first syntax element indicating whether
an absolute value of the MVD is greater than 1 in each of the two
directional components, and all syntax elements of the MVD that are
not the first syntax element are coded with equal probability.
15. The apparatus according to claim 11, wherein the plural syntax
elements consist of a first syntax element indicating whether an
absolute value of the MVD is greater than 0, a second syntax
element indicating the absolute value of the MVD minus 1, and a
third syntax element indicating a sign of the MVD.
16. The apparatus according to claim 15, wherein all of the plural
syntax elements of the MVD are context coded, and the second syntax
element is binarized with a K-th order Exponential-Golomb
binarization, where K is one of 0, 2, or 3.
17. The apparatus according to claim 11, wherein a later signaled
one of the two flags of one of the plural syntax elements is
context coded based on an earlier signaled one of the two flags of
the one of the plural syntax elements.
18. The apparatus according to claim 11, wherein, a first syntax
element of the plural syntax elements indicates whether the MVD is
greater than 0, and in response to an earlier signaled one of the
two flags of the first syntax element indicating that a
corresponding component is not greater than 0, a later signaled one
of the two flags of the first syntax element is bypass coded.
19. A non-transitory computer-readable storage medium storing
computer-readable instructions thereon, which, when executed by a
computer, cause the computer to perform a method for motion vector
difference (MVD) coding in image coding, the method comprising:
determining a motion vector predictor and a corresponding MVD
representing motion information for a current block; and encoding
prediction information, the prediction information indicating the
motion vector predictor and the motion vector difference (MVD) for
the current block, wherein the MVD includes plural syntax elements
and no more than one of the plural syntax elements of the MVD is
coded using context coding.
Description
INCORPORATION BY REFERENCE
[0001] This application is a continuation of application Ser. No.
16/698,322, filed Nov. 27, 2019, which claims the benefit of
priority to U.S. Provisional Application No. 62/776,350, "FLEXIBLE
MOTION VECTOR PREDICTION" filed on Dec. 6, 2018, and U.S.
Provisional Application No. 62/787,044, "IMPROVED MOTION VECTOR
DIFFERENCE CODING" filed on Dec. 31, 2018, which are incorporated
by reference herein in their entirety.
TECHNICAL FIELD
[0002] The present disclosure describes embodiments generally
related to video coding.
BACKGROUND
[0003] The background description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent the work is
described in this background section, as well as aspects of the
description that may not otherwise qualify as prior art at the time
of filing, are neither expressly nor impliedly admitted as prior
art against the present disclosure.
[0004] Video coding and decoding can be performed using
inter-picture prediction with motion compensation. Uncompressed
digital video can include a series of pictures, each picture having
a spatial dimension of, for example, 1920.times.1080 luminance
samples and associated chrominance samples. The series of pictures
can have a fixed or variable picture rate (informally also known as
frame rate) of, for example, 60 pictures per second or 60 Hz.
Uncompressed video has significant bitrate requirements. For
example, 1080p60 4:2:0 video at 8 bit per sample (1920.times.1080
luminance sample resolution at 60 Hz frame rate) requires close to
1.5 Gbit/s bandwidth. An hour of such video requires more than 600
GBytes of storage space.
[0005] One purpose of video coding and decoding can be the
reduction of redundancy in the input video signal, through
compression. Compression can help reduce the aforementioned
bandwidth or storage space requirements, in some cases by two
orders of magnitude or more. Both lossless and lossy compression,
as well as a combination thereof can be employed. Lossless
compression refers to techniques where an exact copy of the
original signal can be reconstructed from the compressed original
signal. When using lossy compression, the reconstructed signal may
not be identical to the original signal, but the distortion between
original and reconstructed signals is small enough to make the
reconstructed signal useful for the intended application. In the
case of video, lossy compression is widely employed. The amount of
distortion tolerated depends on the application; for example, users
of certain consumer streaming applications may tolerate higher
distortion than users of television distribution applications. The
compression ratio achievable can reflect that: higher
allowable/tolerable distortion can yield higher compression
ratios.
[0006] A video encoder and decoder can utilize techniques from
several broad categories, including, for example, motion
compensation, transform, quantization, and entropy coding.
[0007] Video codec technologies can include techniques known as
intra coding. In intra coding, sample values are represented
without reference to samples or other data from previously
reconstructed reference pictures. In some video codecs, the picture
is spatially subdivided into blocks of samples. When all blocks of
samples are coded in intra mode, that picture can be an intra
picture. Intra pictures and their derivations such as independent
decoder refresh pictures, can be used to reset the decoder state
and can, therefore, be used as the first picture in a coded video
bitstream and a video session, or as a still image. The samples of
an intra block can be exposed to a transform, and the transform
coefficients can be quantized before entropy coding. Intra
prediction can be a technique that minimizes sample values in the
pre-transform domain. In some cases, the smaller the DC value after
a transform is, and the smaller the AC coefficients are, the fewer
the bits that are required at a given quantization step size to
represent the block after entropy coding.
[0008] Traditional intra coding such as known from, for example
MPEG-2 generation coding technologies, does not use intra
prediction. However, some newer video compression technologies
include techniques that attempt, from, for example, surrounding
sample data and/or metadata obtained during the encoding/decoding
of spatially neighboring, and preceding in decoding order, blocks
of data. Such techniques are henceforth called "intra prediction"
techniques. Note that in at least some cases, intra prediction is
only using reference data from the current picture under
reconstruction and not from reference pictures.
[0009] There can be many different forms of intra prediction. When
more than one of such techniques can be used in a given video
coding technology, the technique in use can be coded in an intra
prediction mode. In certain cases, modes can have submodes and/or
parameters, and those can be coded individually or included in the
mode codeword. Which codeword to use for a given
mode/submode/parameter combination can have an impact in the coding
efficiency gain through intra prediction, and so can the entropy
coding technology used to translate the codewords into a
bitstream.
[0010] A certain mode of intra prediction was introduced with
H.264, refined in H.265, and further refined in newer coding
technologies such as joint exploration model (JEM), versatile video
coding (VVC), and benchmark set (BMS). A predictor block can be
formed using neighboring sample values belonging to already
available samples. Sample values of neighboring samples are copied
into the predictor block according to a direction. A reference to
the direction in use can be coded in the bitstream or may be
predicted itself.
[0011] Referring to FIG. 1A, depicted in the lower right is a
subset of nine predictor directions known from H.265's 33 possible
predictor directions (corresponding to the 33 angular modes of the
35 intra modes). The point where the arrows converge (101)
represents the sample being predicted. The arrows represent the
direction from which the sample is being predicted. For example,
arrow (102) indicates that sample (101) is predicted from a sample
or samples to the upper right, at a 45 degree angle from the
horizontal. Similarly, arrow (103) indicates that sample (101) is
predicted from a sample or samples to the lower left of sample
(101), in a 22.5 degree angle from the horizontal.
[0012] Still referring to FIG. 1A, on the top left there is
depicted a square block (104) of 4.times.4 samples (indicated by a
dashed, boldface line). The square block (104) includes 16 samples.
Each sample is labelled with an "S", its position in the Y
dimension (e.g., row index), and its position in the X dimension
(e.g., column index). For example, sample S21 is the second sample
in the Y dimension (from the top) and the first (from the left)
sample in the X dimension. Similarly, sample S44 is the fourth
sample in block (104) in both the Y and X dimensions. As the block
is 4.times.4 samples in size, S44 is at the bottom right. Further
shown are reference samples that follow a similar numbering scheme.
A reference sample is labelled with an R, its Y position (e.g., row
index) and X position (column index) relative to block (104). In
both H.264 and H.265, prediction samples neighbor the block under
reconstruction; therefore no negative values need to be used.
[0013] Intra picture prediction can work by copying reference
sample values from the neighboring samples as appropriated by the
signaled prediction direction. For example, assume the coded video
bitstream includes signaling that, for this block, indicates a
prediction direction consistent with arrow (102)--that is, samples
are predicted from a prediction sample or samples to the upper
right, at a 45 degree angle from the horizontal. In that case,
samples S41, S32, S23, and S14 are predicted from the same
reference sample R05. Sample S44 is then predicted from reference
sample R08.
[0014] In certain cases, the values of multiple reference samples
may be combined, for example through interpolation, in order to
calculate a reference sample; especially when the directions are
not evenly divisible by 45 degrees.
[0015] The number of possible directions has increased as video
coding technology has developed. In H.264 (year 2003), nine
different direction could be represented. That increased to 33 in
H.265 (year 2013), and JEM/VVC/BMS, at the time of disclosure, can
support up to 65 directions. Experiments have been conducted to
identify the most likely directions, and certain techniques in the
entropy coding are used to represent those likely directions in a
small number of bits, accepting a certain penalty for less likely
directions. Further, the directions themselves can sometimes be
predicted from neighboring directions used in neighboring, already
decoded, blocks.
[0016] FIG. 1B shows a schematic (105) that depicts 65 intra
prediction directions according to JEM to illustrate the increasing
number of prediction directions over time.
[0017] The mapping of intra prediction directions bits in the coded
video bitstream that represent the direction can be different from
video coding technology to video coding technology; and can range,
for example, from simple direct mappings of prediction direction to
intra prediction mode, to codewords, to complex adaptive schemes
involving most probable modes, and similar techniques. In all
cases, however, there can be certain directions that are
statistically less likely to occur in video content than certain
other directions. As the goal of video compression is the reduction
of redundancy, those less likely directions will, in a well working
video coding technology, be represented by a larger number of bits
than more likely directions.
[0018] Motion compensation can be a lossy compression technique and
can relate to techniques where a block of sample data from a
previously reconstructed picture or part thereof (reference
picture), after being spatially shifted in a direction indicated by
a motion vector (MV henceforth), is used for the prediction of a
newly reconstructed picture or picture part. In some cases, the
reference picture can be the same as the picture currently under
reconstruction. MVs can have two dimensions X and Y, or three
dimensions, the third being an indication of the reference picture
in use (the latter, indirectly, can be a time dimension).
[0019] In some video compression techniques, an MV applicable to a
certain area of sample data can be predicted from other MVs, for
example from those related to another area of sample data spatially
adjacent to the area under reconstruction, and preceding that MV in
decoding order. Doing so can substantially reduce the amount of
data required for coding the MV, thereby removing redundancy and
increasing compression. MV prediction can work effectively, for
example, because when coding an input video signal derived from a
camera (known as natural video) there is a statistical likelihood
that areas larger than the area to which a single MV is applicable
move in a similar direction and, therefore, can in some cases be
predicted using a similar motion vector derived from MVs of
neighboring areas. That results in the MV found for a given area to
be similar or the same as the MV predicted from the surrounding
MVs, and that in turn can be represented, after entropy coding, in
a smaller number of bits than what would be used if coding the MV
directly. In some cases, MV prediction can be an example of
lossless compression of a signal (namely: the MVs) derived from the
original signal (namely: the sample stream). In other cases, MV
prediction itself can be lossy, for example because of rounding
errors when calculating a predictor from several surrounding
MVs.
[0020] Various MV prediction mechanisms are described in H.265/HEVC
(ITU-T Rec. H.265, "High Efficiency Video Coding", December 2016).
Out of the many MV prediction mechanisms that H.265 offers,
advanced motion vector prediction (AMVP) mode and merge mode are
described here.
[0021] In AMVP mode, motion information of spatial and temporal
neighboring blocks of a current block can be used to predict motion
information of the current block, while the prediction residue is
further coded. Examples of spatial and temporal neighboring
candidates are shown in FIG. 1C and FIG. 1D, respectively. A
two-candidate motion vector predictor list is formed. The first
candidate predictor is from the first available motion vector of
the two blocks A0 (112), A1 (113) at the bottom-left corner of the
current block (111), as shown in FIG. 1C. The second candidate
predictor is from the first available motion vector of the three
blocks B0 (114), B1 (115), and B2 (116) above the current block
(111). If no valid motion vector can be found from the checked
locations, no candidate will be filled in the list. If two
available candidates have the same motion information, only one
candidate will be kept in the list. If the list is not full, i.e.,
the list does not have two different candidates, a temporal
co-located motion vector (after scaling) from C0 (122) at the
bottom-right corner of a co-located block (121) in a reference
picture will be used as another candidate, as shown in FIG. 1D. If
motion information at C0 (122) location is not available, the
center location C1 (123) of the co-located block in the reference
picture will be used instead. In the above derivation, if there are
still not enough motion vector predictor candidates, a zero motion
vector will be used to fill up the list. Two flags mvp_10_flag and
mvp_11_flag are signaled in the bitstream to indicate the AMVP
index (0 or 1) for MV candidate list L0 and L1, respectively.
[0022] In a merge mode for inter-picture prediction, if a merge
flag (including a skip flag) is signaled as TRUE, a merge index is
then signaled to indicate which candidate in a merge candidate list
will be used to indicate the motion vectors of the current block.
At the decoder, the merge candidate list is constructed based on
spatial and temporal neighbors of the current block. As shown in
FIG. 1C, up to four MVs derived from five spatial neighboring
blocks (A0-B2) are added into the merge candidate list. In
addition, as shown in FIG. 1D, up to one MV from two temporal
co-located blocks (C0 and C1) in the reference picture is added to
the list. Additional merge candidates include combined
bi-predictive candidates and zero motion vector candidates. Before
taking the motion information of a block as a merge candidate,
redundancy checks are performed to check whether it is identical to
an element in the current merge candidate list. If it is different
from each element in the current merge candidate list, it will be
added to the merge candidate list as a merge candidate.
MaxMergeCandsNum is defined as the size of the merge candidate list
in terms of candidate number. In HEVC, MaxMergeCandsNum is signaled
in the bitstream. A skip mode can be considered as a special merge
mode with zero residual.
[0023] In VVC, a sub-block based temporal motion vector prediction
(SbTMVP) method, similar to the temporal motion vector prediction
(TMVP) in HEVC, can use the motion field in the collocated picture
to improve motion vector prediction and merge mode for CUs in the
current picture. The same collocated picture used by TMVP is used
for SbTMVP. SbTMVP differs from TMVP in the following two main
aspects: (1) TMVP predicts motion at CU level but SbTMVP predicts
motion at sub-CU level; and (2) whereas TMVP fetches the temporal
motion vectors from the collocated block in the collocated picture
(the collocated block is the bottom-right or center block relative
to the current CU), SbTMVP applies a motion shift before fetching
the temporal motion information from the collocated picture, where
the motion shift is obtained from the motion vector from one of the
spatial neighboring blocks of the current CU.
[0024] The SbTMVP process is illustrated in FIG. 1E and FIG. 1F.
SbTMVP predicts the motion vectors of the sub-CUs within the
current CU in two steps. In the first step, as shown in FIG. 1E,
the spatial neighbors of a current block (131) are examined in the
order of A1 (132), B1 (133), B0 (134), and A0 (135). Once the first
available spatial neighboring block that has a motion vector that
uses the collocated picture as its reference picture is identified,
this motion vector is selected to be the motion shift to be
applied. If no such motion vector is identified from the spatial
neighbors, then the motion shift is set to (0, 0).
[0025] In the second step, the motion shift identified in the first
step is applied (i.e., added to the coordinates of the current
block) to obtain sub-CU-level motion information (e.g., motion
vectors and reference indices) from the collocated picture as shown
in FIG. 1F. The example in FIG. 1F assumes the motion shift (149)
is set to the motion vector of the spatial neighboring block A1
(143). Then, for a current sub-CU (e.g., sub-CU (144)) in the
current block (142) of the current picture (141), the motion
information of a corresponding collocated sub-CU (e.g., collocated
sub-CU (154)) in the collocated block (152) of the collocated
picture (151) is used to derive the motion information for the
current sub-CU. The motion information of the corresponding
collocated sub-CU (e.g., collocated sub-CU (154)) is converted to
the motion vectors and reference indices of the current sub-CU
(e.g., sub-CU (144)) in a similar way as the TMVP process in HEVC,
where temporal motion scaling is applied to align the reference
pictures of the temporal motion vectors to the reference picture of
the current CU.
[0026] In VVC, a combined sub-block based merge list which contains
both a SbTMVP candidate and affine merge candidates can be used in
sub-block based merge mode. The SbTMVP mode is enabled/disabled by
a sequence parameter set (SPS) flag. If the SbTMVP mode is enabled,
the SbTMVP predictor is added as the first entry of the sub-block
based merge list, and followed by the affine merge candidates. The
maximum allowed size of the sub-block based merge list is 5 in some
applications. The sub-CU size used in SbTMVP is fixed to be
8.times.8, for example. As done for affine merge mode, SbTMVP mode
is only applicable to a CU when both width and height are larger
than or equal to 8.
[0027] The encoding logic of an additional SbTMVP merge candidate
is the same as the encoding logic for other merge candidates. That
is, for each CU in a P or B slice, an additional rate distortion
(RD) check is performed to decide whether to use the SbTMVP
candidate.
[0028] In VVC, a history-based MVP (HMVP) method includes a HMVP
candidate that is defined as the motion information of a previously
coded block. A table with multiple HMVP candidates is maintained
during the encoding/decoding process. The table is emptied when a
new slice is encountered. Whenever there is an inter-coded
non-affine block, the associated motion information is added to the
last entry of the table as a new HMVP candidate. The coding flow of
the HMVP method is depicted in FIG. 1G.
[0029] The table size S is set to be 6, which indicates up to 6
HMVP candidates may be added to the table. When inserting a new
motion candidate into the table, a constrained FIFO rule is
utilized such that a redundancy check is first applied to determine
whether an identical HMVP is in the table. If found, the identical
HMVP is removed from the table and all the HMVP candidates
afterwards are moved forward, i.e., with indices reduced by 1. FIG.
1H shows an example of inserting a new motion candidate into the
HMVP table.
[0030] HMVP candidates could be used in the merge candidate list
construction process. The latest several HMVP candidates in the
table are checked in order and inserted into the candidate list
after the TMVP candidate. Pruning is applied on the HMVP candidates
to the spatial or temporal merge candidate excluding sub-block
motion candidate (i.e., ATMVP).
[0031] To reduce the number of pruning operations, the number of
HMVP candidates to be checked (denoted by L) is set as L=(N<=4)?
M: (8-N), where N indicates a number of available non-sub-block
merge candidates and M indicates a number of available HMVP
candidates in the table. In addition, once the total number of
available merge candidates reaches the signaled maximally allowed
merge candidates minus 1, the merge candidate list construction
process from the HMVP candidate list is terminated. Moreover, the
number of pairs for combined bi-predictive merge candidate
derivation is reduced from 12 to 6.
[0032] HMVP candidates could also be used in the AMVP candidate
list construction process. The motion vectors of the last K HMVP
candidates in the table are inserted after the TMVP candidate. Only
HMVP candidates with the same reference picture as the AMVP target
reference picture are used to construct the AMVP candidate list.
Pruning is applied on the HMVP candidates. In some applications, K
is set to 4 while the AMVP candidate list size is kept unchanged,
i.e., equal to 2.
[0033] Pairwise average candidates are generated by averaging
predefined pairs of candidates in the current merge candidate list.
In VVC, the number of pairwise average candidates is 6, and the
predefined pairs are defined as {(0, 1), (0, 2), (1, 2), (0, 3),
(1, 3), (2, 3)}, where the numbers denote the merge indices to the
merge candidate list. The averaged motion vectors are calculated
separately for each reference list. If both motion vectors are
available in one list, these two motion vectors are averaged even
when they point to different reference pictures. If only one motion
vector is available, the one motion vector is directly used. If no
motion vector is available, this list is considered as invalid. The
pairwise average candidates can replace the combined candidates in
the HEVC standard.
[0034] Multi-hypothesis prediction is applied to improve
uni-prediction of AMVP mode. One flag is signaled to enable or
disable multi-hypothesis prediction. Moreover, one additional merge
index is signaled when the flag is true. In this way,
multi-hypothesis prediction turns uni-prediction into
bi-prediction, where one prediction is acquired using the original
syntax elements in AMVP mode while the other prediction is acquired
using the merge mode. The final prediction uses 1:1 weights to
combine these two predictions as in bi-prediction. The merge
candidate list is first derived from merge mode with sub-CU
candidates (e.g., affine, alternative temporal motion vector
prediction (ATMVP)) excluded. Next, the merge candidate list is
separated into two individual lists, one for list 0 (L0) containing
all L0 motions from the candidates, and the other for list 1 (L1)
containing all L1 motions. After removing redundancy and filling
vacancy, two merge lists are generated for L0 and L1, respectively.
There are two constraints when applying multi-hypothesis prediction
for improving AMVP mode. First, it is enabled for those CUs with
the luma coding block (CB) area larger than or equal to 64. Second,
it is only applied to L1 for low delay B pictures.
SUMMARY
[0035] Aspects of the disclosure provide methods and apparatuses
for video encoding/decoding. In some examples, an apparatus for
video decoding includes processing circuitry that decodes
prediction information for a current block in a current coded
picture that is part of a coded video sequence. The prediction
information indicates a motion vector predictor index for selecting
a motion vector predictor in a motion vector predictor list. The
processing circuitry determines whether the motion vector predictor
index is smaller than a threshold. When the motion vector predictor
index is determined to be smaller than the threshold, the
processing circuitry decodes a motion vector difference (MVD)
corresponding to the motion vector predictor and reconstructs the
current block based on the motion vector predictor and the MVD.
[0036] In some embodiments, when the motion vector predictor index
is determined to be equal to or larger than the threshold, the
processing circuitry reconstructs the current block based on the
motion vector predictor without the MVD which is not signaled in
the coded video sequence.
[0037] In some embodiments, the processing circuitry determines a
reference picture in a reference picture list for the current block
according to one of a reference index to the reference picture in
the reference picture list and a reference picture associated with
the motion vector predictor.
[0038] In an embodiment, the threshold is a preset number. In
another embodiment, the threshold is signaled in the coded video
sequence.
[0039] In some embodiments, the processing circuitry determines
whether an inter-prediction hypothesis of a new motion vector
predictor is different from an inter-prediction hypothesis of the
motion vector predictor list.
[0040] In some embodiments, the motion vector predictor index is
smaller than the threshold when the current block has one reference
picture list.
[0041] In some embodiments, the motion vector predictor list
includes a motion vector predictor associated with a non-zero MVD
and a motion vector predictor associated with a zero MVD.
[0042] According to aspects of the disclosure, an apparatus for
video decoding includes processing circuitry that decodes
prediction information for a current block in a current picture
that is a part of a coded video sequence. The prediction
information indicates a first motion vector difference (MVD)
corresponding to a first motion vector predictor in a first motion
vector predictor list and a plurality of syntax elements associated
with the first MVD. No more than one of the plurality of syntax
elements is context coded. The processing circuitry decodes the
first MVD according to the plurality of syntax elements and
reconstructs the current block based on the first MVD and the first
motion vector predictor.
[0043] In some embodiments, a total number of the plurality of
syntax elements associated with the first MVD is smaller than
four.
[0044] In some embodiments, one of the plurality of syntax elements
associated with the first MVD has a first component and a second
component, and the second component is decoded based on the first
component.
[0045] In some embodiments, when the prediction information
indicates a bi-directional prediction mode for the current block,
the processing circuitry further decodes a second MVD corresponding
to a second motion vector predictor in a second motion vector
predictor list based on the first MVD. The processing circuitry
reconstructs the current block based on the first motion vector
predictor, the first MVD, the second motion vector predictor, and
the second MVD.
[0046] In some embodiments, when the prediction information
indicates a first affine mode for the current block and the first
motion vector predictor corresponds to a first control point of the
current block, the processing circuitry decodes a third MVD
corresponding to a third motion vector predictor based on the first
MVD. The third motion vector predictor corresponds to a second
control point of the current block. The processing circuitry
further reconstructs the current block based on the first motion
vector predictor, the first MVD, the third motion vector predictor,
and the third MVD.
[0047] In some embodiments, when the prediction information
indicates a second affine mode with bi-directional prediction for
the current block, the processing circuitry decodes a fourth MVD
corresponding to a fourth motion vector predictor in a fourth
motion vector predictor list based on the first MVD. The fourth
motion vector predictor and the first motion vector predictor
correspond to a same control point of the current block. The
processing circuitry reconstructs the current block based on the
first motion vector predictor, the first MVD, the fourth motion
vector predictor, and the fourth MVD.
[0048] Aspects of the disclosure also provide one or more
non-transitory computer-readable mediums storing instructions which
when executed by a computer for video decoding cause the computer
to perform any one of a combination of the methods for video
decoding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Further features, the nature, and various advantages of the
disclosed subject matter will be more apparent from the following
detailed description and the accompanying drawings in which:
[0050] FIG. 1A is a schematic illustration of an exemplary subset
of intra prediction modes;
[0051] FIG. 18 is an illustration of exemplary intra prediction
directions;
[0052] FIG. 1C is a schematic illustration of a current block and
its surrounding spatial merge candidates in one example;
[0053] FIG. 1D is a schematic illustration of a co-located block
and temporal merge candidates in one example;
[0054] FIG. 1E is a schematic illustration of a current block and
its surrounding spatial merge candidates for sub-block based
temporal motion vector prediction (SbTMVP) according to one
example;
[0055] FIG. 1F is an exemplary process of deriving SbTMVP according
to one example;
[0056] FIG. 1G is a decoding flow of a history based motion vector
prediction (HMVP) method in one example;
[0057] FIG. 1H is an exemplary process of updating a table in HMVP
according to one example;
[0058] FIG. 2 is a schematic illustration of a simplified block
diagram of a communication system in accordance with an
embodiment:
[0059] FIG. 3 is a schematic illustration of a simplified block
diagram of a communication system in accordance with an
embodiment;
[0060] FIG. 4 is a schematic illustration of a simplified block
diagram of a decoder in accordance with an embodiment;
[0061] FIG. 5 is a schematic illustration of a simplified block
diagram of an encoder in accordance with an embodiment;
[0062] FIG. 6 shows a block diagram of an encoder in accordance
with another embodiment;
[0063] FIG. 7 shows a block diagram of a decoder in accordance with
another embodiment;
[0064] FIG. 8 shows a flowchart outlining an exemplary process
according to some embodiments of the disclosure;
[0065] FIG. 9A shows a syntax table for coding an MVD for list 0
(L0) or bi-prediction according to some embodiments of the
disclosure;
[0066] FIG. 9B shows a syntax table for an MVD coding according to
some embodiments of the disclosure;
[0067] FIG. 10 shows a syntax table for an MVD coding according to
an embodiment of the disclosure;
[0068] FIG. 11 shows a flowchart outlining an exemplary process
according to some embodiments of the disclosure; and
[0069] FIG. 12 is a schematic illustration of a computer system in
accordance with an embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0070] FIG. 2 illustrates a simplified block diagram of a
communication system (200) according to an embodiment of the
present disclosure. The communication system (200) includes a
plurality of terminal devices that can communicate with each other,
via, for example, a network (250). For example, the communication
system (200) includes a first pair of terminal devices (210) and
(220) interconnected via the network (250). In the FIG. 2 example,
the first pair of terminal devices (210) and (220) performs
unidirectional transmission of data. For example, the terminal
device (210) may code video data (e.g., a stream of video pictures
that are captured by the terminal device (210)) for transmission to
the other terminal device (220) via the network (250). The encoded
video data can be transmitted in the form of one or more coded
video bitstreams. The terminal device (220) may receive the coded
video data from the network (250), decode the coded video data to
recover the video pictures and display video pictures according to
the recovered video data. Unidirectional data transmission may be
common in media serving applications and the like.
[0071] In another example, the communication system (200) includes
a second pair of terminal devices (230) and (240) that performs
bidirectional transmission of coded video data that may occur, for
example, during videoconferencing. For bidirectional transmission
of data, in an example, each terminal device of the terminal
devices (230) and (240) may code video data (e.g., a stream of
video pictures that are captured by the terminal device) for
transmission to the other terminal device of the terminal devices
(230) and (240) via the network (250). Each terminal device of the
terminal devices (230) and (240) also may receive the coded video
data transmitted by the other terminal device of the terminal
devices (230) and (240), and may decode the coded video data to
recover the video pictures and may display video pictures at an
accessible display device according to the recovered video
data.
[0072] In the FIG. 2 example, the terminal devices (210), (220),
(230) and (240) may be illustrated as servers, personal computers
and smart phones but the principles of the present disclosure may
be not so limited. Embodiments of the present disclosure find
application with laptop computers, tablet computers, media players
and/or dedicated video conferencing equipment. The network (250)
represents any number of networks that convey coded video data
among the terminal devices (210), (220), (230) and (240), including
for example wireline (wired) and/or wireless communication
networks. The communication network (250) may exchange data in
circuit-switched and/or packet-switched channels. Representative
networks include telecommunications networks, local area networks,
wide area networks and/or the Internet. For the purposes of the
present discussion, the architecture and topology of the network
(250) may be immaterial to the operation of the present disclosure
unless explained herein below.
[0073] FIG. 3 illustrates, as an example for an application for the
disclosed subject matter, the placement of a video encoder and a
video decoder in a streaming environment. The disclosed subject
matter can be equally applicable to other video enabled
applications, including, for example, video conferencing, digital
TV, storing of compressed video on digital media including CD, DVD,
memory stick, and the like.
[0074] A streaming system may include a capture subsystem (313)
that can include a video source (301), for example a digital
camera, creating for example a stream of video pictures (302) that
are uncompressed. In an example, the stream of video pictures (302)
includes samples that are taken by the digital camera. The stream
of video pictures (302), depicted as a bold line to emphasize a
high data volume when compared to encoded video data (304) (or
coded video bitstreams), can be processed by an electronic device
(320) that includes a video encoder (303) coupled to the video
source (301). The video encoder (303) can include hardware,
software, or a combination thereof to enable or implement aspects
of the disclosed subject matter as described in more detail below.
The encoded video data (304) (or encoded video bitstream (304)),
depicted as a thin line to emphasize the lower data volume when
compared to the stream of video pictures (302), can be stored on a
streaming server (305) for future use. One or more streaming client
subsystems, such as client subsystems (306) and (308) in FIG. 3 can
access the streaming server (305) to retrieve copies (307) and
(309) of the encoded video data (304). A client subsystem (306) can
include a video decoder (310), for example, in an electronic device
(330). The video decoder (310) decodes the incoming copy (307) of
the encoded video data and creates an outgoing stream of video
pictures (311) that can be rendered on a display (312) (e.g.,
display screen) or other rendering device (not depicted). In some
streaming systems, the encoded video data (304), (307), and (309)
(e.g., video bitstreams) can be encoded according to certain video
coding/compression standards. Examples of those standards include
ITU-T Recommendation H.265. In an example, a video coding standard
under development is informally known as Versatile Video Coding
(VVC). The disclosed subject matter may be used in the context of
VVC.
[0075] It is noted that the electronic devices (320) and (330) can
include other components (not shown). For example, the electronic
device (320) can include a video decoder (not shown) and the
electronic device (330) can include a video encoder (not shown) as
well.
[0076] FIG. 4 shows a block diagram of a video decoder (410)
according to an embodiment of the present disclosure. The video
decoder (410) can be included in an electronic device (430). The
electronic device (430) can include a receiver (431) (e.g.,
receiving circuitry). The video decoder (410) can be used in the
place of the video decoder (310) in the FIG. 3 example.
[0077] The receiver (431) may receive one or more coded video
sequences to be decoded by the video decoder (410); in the same or
another embodiment, one coded video sequence at a time, where the
decoding of each coded video sequence is independent from other
coded video sequences. The coded video sequence may be received
from a channel (401), which may be a hardware/software link to a
storage device which stores the encoded video data. The receiver
(431) may receive the encoded video data with other data, for
example, coded audio data and/or ancillary data streams, that may
be forwarded to their respective using entities (not depicted). The
receiver (431) may separate the coded video sequence from the other
data. To combat network jitter, a buffer memory (415) may be
coupled in between the receiver (431) and an entropy decoder/parser
(420) ("parser (420)" henceforth). In certain applications, the
buffer memory (415) is part of the video decoder (410). In others,
it can be outside of the video decoder (410) (not depicted).
Instill others, there can be a buffer memory (not depicted) outside
of the video decoder (410), for example to combat network jitter,
and in addition another buffer memory (415) inside the video
decoder (410), for example to handle playout timing. When the
receiver (431) is receiving data from a store/forward device of
sufficient bandwidth and controllability, or from an isosynchronous
network, the buffer memory (415) may not be needed, or can be
small. For use on best effort packet networks such as the Internet,
the buffer memory (415) may be required, can be comparatively large
and can be advantageously of adaptive size, and may at least
partially be implemented in an operating system or similar elements
(not depicted) outside of the video decoder (410).
[0078] The video decoder (410) may include the parser (420) to
reconstruct symbols (421) from the coded video sequence. Categories
of those symbols include information used to manage operation of
the video decoder (410), and potentially information to control a
rendering device such as a render device (412)(e.g., a display
screen) that is not an integral part of the electronic device (430)
but can be coupled to the electronic device (430), as was shown in
FIG. 4. The control information for the rendering device(s) may be
in the form of Supplemental Enhancement Information (SEI messages)
or Video Usability Information (VUI) parameter set fragments (not
depicted). The parser (420) may parse/entropy-decode the coded
video sequence that is received. The coding of the coded video
sequence can be in accordance with a video coding technology or
standard, and can follow various principles, including variable
length coding, Huffman coding, arithmetic coding with or without
context sensitivity, and so forth. The parser (420) may extract
from the coded video sequence, a set of subgroup parameters for at
least one of the subgroups of pixels in the video decoder, based
upon at least one parameter corresponding to the group. Subgroups
can include Groups of Pictures (GOPs), pictures, tiles, slices,
macroblocks, Coding Units (CUs), blocks, Transform Units (TUs),
Prediction Units (PUs) and so forth. The parser (420) may also
extract from the coded video sequence information such as transform
coefficients, quantizer parameter values, motion vectors, and so
forth.
[0079] The parser (420) may perform an entropy decoding/parsing
operation on the video sequence received from the buffer memory
(415), so as to create symbols (421).
[0080] Reconstruction of the symbols (421) can involve multiple
different units depending on the type of the coded video picture or
parts thereof (such as: inter and intra picture, inter and intra
block), and other factors. Which units are involved, and how, can
be controlled by the subgroup control information that was parsed
from the coded video sequence by the parser (420). The flow of such
subgroup control information between the parser (420) and the
multiple units below is not depicted for clarity.
[0081] Beyond the functional blocks already mentioned, the video
decoder (410) can be conceptually subdivided into a number of
functional units as described below. In a practical implementation
operating under commercial constraints, many of these units
interact closely with each other and can, at least partly, be
integrated into each other. However, for the purpose of describing
the disclosed subject matter, the conceptual subdivision into the
functional units below is appropriate.
[0082] A first unit is the scaler/inverse transform unit (451). The
scaler/inverse transform unit (451) receives a quantized transform
coefficient as well as control information, including which
transform to use, block size, quantization factor, quantization
scaling matrices, etc. as symbol(s) (421) from the parser (420).
The scaler/inverse transform unit (451) can output blocks
comprising sample values that can be input into aggregator
(455).
[0083] In some cases, the output samples of the scaler/inverse
transform (451) can pertain to an intra coded block; that is: a
block that is not using predictive information from previously
reconstructed pictures, but can use predictive information from
previously reconstructed parts of the current picture. Such
predictive information can be provided by an intra picture
prediction unit (452). In some cases, the intra picture prediction
unit (452) generates a block of the same size and shape of the
block under reconstruction, using surrounding already reconstructed
information fetched from the current picture buffer (458). The
current picture buffer (458) buffers, for example, partly
reconstructed current picture and/or fully reconstructed current
picture. The aggregator (455), in some cases, adds, on a per sample
basis, the prediction information that the intra prediction unit
(452) has generated to the output sample information as provided by
the scaler/inverse transform unit (451).
[0084] In other cases, the output samples of the scaler/inverse
transform unit (451) can pertain to an inter coded, and potentially
motion compensated block. In such a case, a motion compensation
prediction unit (453) can access reference picture memory (457) to
fetch samples used for prediction. After motion compensating the
fetched samples in accordance with the symbols (421) pertaining to
the block, these samples can be added by the aggregator (455) to
the output of the scaler/inverse transform unit (451) (in this case
called the residual samples or residual signal) so as to generate
output sample information. The addresses within the reference
picture memory (457) from where the motion compensation prediction
unit (453) fetches prediction samples can be controlled by motion
vectors, available to the motion compensation prediction unit (453)
in the form of symbols (421) that can have, for example X, Y, and
reference picture components. Motion compensation also can include
interpolation of sample values as fetched from the reference
picture memory (457) when sub-sample exact motion vectors are in
use, motion vector prediction mechanisms, and so forth.
[0085] The output samples of the aggregator (455) can be subject to
various loop filtering techniques in the loop filter unit (456).
Video compression technologies can include in-loop filter
technologies that are controlled by parameters included in the
coded video sequence (also referred to as coded video bitstream)
and made available to the loop filter unit (456) as symbols (421)
from the parser (420), but can also be responsive to
meta-information obtained during the decoding of previous (in
decoding order) parts of the coded picture or coded video sequence,
as well as responsive to previously reconstructed and loop-filtered
sample values.
[0086] The output of the loop filter unit (456) can be a sample
stream that can be output to the render device (412) as well as
stored in the reference picture memory (457) for use in future
inter-picture prediction.
[0087] Certain coded pictures, once fully reconstructed, can be
used as reference pictures for future prediction. For example, once
a coded picture corresponding to a current picture is fully
reconstructed and the coded picture has been identified as a
reference picture (by, for example, the parser (420)), the current
picture buffer (458) can become a part of the reference picture
memory (457), and a fresh current picture buffer can be reallocated
before commencing the reconstruction of the following coded
picture.
[0088] The video decoder (410) may perform decoding operations
according to a predetermined video compression technology in a
standard, such as ITU-T Rec. H.265. The coded video sequence may
conform to a syntax specified by the video compression technology
or standard being used, in the sense that the coded video sequence
adheres to both the syntax of the video compression technology or
standard and the profiles as documented in the video compression
technology or standard. Specifically, a profile can select certain
tools as the only tools available for use under that profile from
all the tools available in the video compression technology or
standard. Also necessary for compliance can be that the complexity
of the coded video sequence is within bounds as defined by the
level of the video compression technology or standard. In some
cases, levels restrict the maximum picture size, maximum frame
rate, maximum reconstruction sample rate (measured in, for example
megasamples per second), maximum reference picture size, and so on.
Limits set by levels can, in some cases, be further restricted
through Hypothetical Reference Decoder (HRD) specifications and
metadata for HRD buffer management signaled in the coded video
sequence.
[0089] In an embodiment, the receiver (431) may receive additional
(redundant) data with the encoded video. The additional data may be
included as part of the coded video sequence(s). The additional
data may be used by the video decoder (410) to properly decode the
data and/or to more accurately reconstruct the original video data.
Additional data can be in the form of, for example, temporal,
spatial, or signal noise ratio (SNR) enhancement layers, redundant
slices, redundant pictures, forward error correction codes, and so
on.
[0090] FIG. 5 shows a block diagram of a video encoder (503)
according to an embodiment of the present disclosure. The video
encoder (503) is included in an electronic device (520). The
electronic device (520) includes a transmitter (540)(e.g.,
transmitting circuitry). The video encoder (503) can be used in the
place of the video encoder (303) in the FIG. 3 example.
[0091] The video encoder (503) may receive video samples from a
video source (501) (that is not part of the electronic device (520)
in the FIG. 5 example) that may capture video image(s) to be coded
by the video encoder (503). In another example, the video source
(501) is a part of the electronic device (520).
[0092] The video source (501) may provide the source video sequence
to be coded by the video encoder (503) in the form of a digital
video sample stream that can be of any suitable bit depth (for
example: 8 bit, 10 bit, 12 bit, . . . ), any colorspace (for
example, BT.601 YCrCB, RGB, . . . ), and any suitable sampling
structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). In a media
serving system, the video source (501) may be a storage device
storing previously prepared video. Ina videoconferencing system,
the video source (501) may be a camera that captures local image
information as a video sequence. Video data may be provided as a
plurality of individual pictures that impart motion when viewed in
sequence. The pictures themselves may be organized as a spatial
array of pixels, wherein each pixel can comprise one or more
samples depending on the sampling structure, color space, etc. in
use. A person skilled in the art can readily understand the
relationship between pixels and samples. The description below
focuses on samples.
[0093] According to an embodiment, the video encoder (503) may code
and compress the pictures of the source video sequence into a coded
video sequence (543) in real time or under any other time
constraints as required by the application. Enforcing appropriate
coding speed is one function of a controller (550). In some
embodiments, the controller (550) controls other functional units
as described below and is functionally coupled to the other
functional units. The coupling is not depicted for clarity.
Parameters set by the controller (550) can include rate control
related parameters (picture skip, quantizer, lambda value of
rate-distortion optimization techniques, . . . ), picture size,
group of pictures (GOP) layout, maximum motion vector search range,
and so forth. The controller (550) can be configured to have other
suitable functions that pertain to the video encoder (503)
optimized for a certain system design.
[0094] In some embodiments, the video encoder (503) is configured
to operate in a coding loop. As an oversimplified description, in
an example, the coding loop can include a source coder (530) (e.g.,
responsible for creating symbols, such as a symbol stream, based on
an input picture to be coded, and a reference picture(s)), and a
(local) decoder (533) embedded in the video encoder (503). The
decoder (533) reconstructs the symbols to create the sample data in
a similar manner as a (remote) decoder also would create (as any
compression between symbols and coded video bitstream is lossless
in the video compression technologies considered in the disclosed
subject matter). The reconstructed sample stream (sample data) is
input to the reference picture memory (534). As the decoding of a
symbol stream leads to bit-exact results independent of decoder
location (local or remote), the content in the reference picture
memory (534) is also bit exact between the local encoder and remote
encoder. In other words, the prediction part of an encoder "sees"
as reference picture samples exactly the same sample values as a
decoder would "see" when using prediction during decoding. This
fundamental principle of reference picture synchronicity (and
resulting drift if synchronicity cannot be maintained, for example
because of channel errors) is used in some related arts as
well.
[0095] The operation of the "local" decoder (533) can be the same
as of a "remote" decoder, such as the video decoder (410), which
has already been described in detail above in conjunction with FIG.
4. Briefly referring also to FIG. 4, however, as symbols are
available and encoding/decoding of symbols to a coded video
sequence by an entropy coder (545) and the parser (420) can be
lossless, the entropy decoding parts of the video decoder (410),
including the buffer memory (415) and the parser (420) may not be
fully implemented in the local decoder (533).
[0096] An observation that can be made at this point is that any
decoder technology except the parsing/entropy decoding that is
present in a decoder also necessarily needs to be present, in
substantially identical functional form, in a corresponding
encoder. For this reason, the disclosed subject matter focuses on
decoder operation. The description of encoder technologies can be
abbreviated as they are the inverse of the comprehensively
described decoder technologies. Only in certain areas a more detail
description is required and provided below.
[0097] During operation, in some examples, the source coder (530)
may perform motion compensated predictive coding, which codes an
input picture predictively with reference to one or more
previously-coded picture from the video sequence that were
designated as "reference pictures". In this manner, the coding
engine (532) codes differences between pixel blocks of an input
picture and pixel blocks of reference picture(s) that may be
selected as prediction reference(s) to the input picture.
[0098] The local video decoder (533) may decode coded video data of
pictures that may be designated as reference pictures, based on
symbols created by the source coder (530). Operations of the coding
engine (532) may advantageously be lossy processes. When the coded
video data may be decoded at a video decoder (not shown in FIG. 5),
the reconstructed video sequence typically may be a replica of the
source video sequence with some errors. The local video decoder
(533) replicates decoding processes that may be performed by the
video decoder on reference pictures and may cause reconstructed
reference pictures to be stored in the reference picture cache
(534). In this manner, the video encoder (503) may store copies of
reconstructed reference pictures locally that have common content
as the reconstructed reference pictures that will be obtained by a
far-end video decoder (absent transmission errors).
[0099] The predictor (535) may perform prediction searches for the
coding engine (532). That is, for a new picture to be coded, the
predictor (535) may search the reference picture memory (534) for
sample data (as candidate reference pixel blocks) or certain
metadata such as reference picture motion vectors, block shapes,
and so on, that may serve as an appropriate prediction reference
for the new pictures. The predictor (535) may operate on a sample
block-by-pixel block basis to find appropriate prediction
references. In some cases, as determined by search results obtained
by the predictor (535), an input picture may have prediction
references drawn from multiple reference pictures stored in the
reference picture memory (534).
[0100] The controller (550) may manage coding operations of the
source coder (530), including, for example, setting of parameters
and subgroup parameters used for encoding the video data.
[0101] Output of all aforementioned functional units may be
subjected to entropy coding in the entropy coder (545). The entropy
coder (545) translates the symbols as generated by the various
functional units into a coded video sequence, by lossless
compressing the symbols according to technologies such as Huffman
coding, variable length coding, arithmetic coding, and so
forth.
[0102] The transmitter (540) may buffer the coded video sequence(s)
as created by the entropy coder (545) to prepare for transmission
via a communication channel (560), which may be a hardware/software
link to a storage device which would store the encoded video data.
The transmitter (540) may merge coded video data from the video
coder (503) with other data to be transmitted, for example, coded
audio data and/or ancillary data streams (sources not shown).
[0103] The controller (550) may manage operation of the video
encoder (503). During coding, the controller (550) may assign to
each coded picture a certain coded picture type, which may affect
the coding techniques that may be applied to the respective
picture. For example, pictures often may be assigned as one of the
following picture types:
[0104] An Intra Picture (1 picture) may be one that may be coded
and decoded without using any other picture in the sequence as a
source of prediction. Some video codecs allow for different types
of intra pictures, including, for example Independent Decoder
Refresh ("IDR") Pictures. A person skilled in the art is aware of
those variants of I pictures and their respective applications and
features.
[0105] A predictive picture (P picture) may be one that may be
coded and decoded using intra prediction or inter prediction using
at most one motion vector and reference index to predict the sample
values of each block.
[0106] A bi-directionally predictive picture (B Picture) may be one
that may be coded and decoded using intra prediction or inter
prediction using at most two motion vectors and reference indices
to predict the sample values of each block. Similarly,
multiple-predictive pictures can use more than two reference
pictures and associated metadata for the reconstruction of a single
block.
[0107] Source pictures commonly may be subdivided spatially into a
plurality of sample blocks (for example, blocks of 4.times.4,
8.times.8, 4.times.8, or 16.times.16 samples each) and coded on a
block-by-block basis. Blocks may be coded predictively with
reference to other (already coded) blocks as determined by the
coding assignment applied to the blocks' respective pictures. For
example, blocks of 1 pictures may be coded non-predictively or they
may be coded predictively with reference to already coded blocks of
the same picture (spatial prediction or intra prediction). Pixel
blocks of P pictures may be coded predictively, via spatial
prediction or via temporal prediction with reference to one
previously coded reference picture. Blocks of B pictures may be
coded predictively, via spatial prediction or via temporal
prediction with reference to one or two previously coded reference
pictures.
[0108] The video encoder (503) may perform coding operations
according to a predetermined video coding technology or standard,
such as ITU-T Rec. H.265. In its operation, the video encoder (503)
may perform various compression operations, including predictive
coding operations that exploit temporal and spatial redundancies in
the input video sequence. The coded video data, therefore, may
conform to a syntax specified by the video coding technology or
standard being used.
[0109] In an embodiment, the transmitter (540) may transmit
additional data with the encoded video. The source coder (530) may
include such data as part of the coded video sequence. Additional
data may comprise temporal/spatial/SNR enhancement layers, other
forms of redundant data such as redundant pictures and slices, SEI
messages, VUI parameter set fragments, and so on.
[0110] A video may be captured as a plurality of source pictures
(video pictures) in a temporal sequence. Intra-picture prediction
(often abbreviated to intra prediction) makes use of spatial
correlation in a given picture, and inter-picture prediction makes
uses of the (temporal or other) correlation between the pictures.
In an example, a specific picture under encoding/decoding, which is
referred to as a current picture, is partitioned into blocks. When
a block in the current picture is similar to a reference block in a
previously coded and still buffered reference picture in the video,
the block in the current picture can be coded by a vector that is
referred to as a motion vector. The motion vector points to the
reference block in the reference picture, and can have a third
dimension identifying the reference picture, in case multiple
reference pictures are in use.
[0111] In some embodiments, a bi-prediction technique can be used
in the inter-picture prediction. According to the bi-prediction
technique, two reference pictures, such as a first reference
picture and a second reference picture that are both prior in
decoding order to the current picture in the video (but may be in
the past and future, respectively, in display order) are used. A
block in the current picture can be coded by a first motion vector
that points to a first reference block in the first reference
picture, and a second motion vector that points to a second
reference block in the second reference picture. The block can be
predicted by a combination of the first reference block and the
second reference block.
[0112] Further, a merge mode technique can be used in the
inter-picture prediction to improve coding efficiency.
[0113] According to some embodiments of the disclosure,
predictions, such as inter-picture predictions and intra-picture
predictions are performed in the unit of blocks. For example,
according to the HEVC standard, a picture in a sequence of video
pictures is partitioned into coding tree units (CTU) for
compression, the CTUs in a picture have the same size, such as
64.times.64 pixels, 32.times.32 pixels, or 16.times.16 pixels. In
general, a CTU includes three coding tree blocks (CTBs), which are
one luma CTB and two chroma CTBs. Each CTU can be recursively
quad-tree split into one or multiple coding units (CUs). For
example, a CTU of 64.times.64 pixels can be split into one CU of
64.times.64 pixels, or 4 CUs of 32.times.32 pixels, or 16 CUs of
16.times.16 pixels. In an example, each CU is analyzed to determine
a prediction type for the CU, such as an inter prediction type or
an intra prediction type. The CU is split into one or more
prediction units (PUs) depending on the temporal and/or spatial
predictability. Generally, each PU includes a luma prediction block
(PB), and two chroma PBs. In an embodiment, a prediction operation
in coding (encoding/decoding) is performed in the unit of a
prediction block. Using a luma prediction block as an example of a
prediction block, the prediction block includes a matrix of values
(e.g., luma values) for pixels, such as 8.times.8 pixels,
16.times.16 pixels, 8.times.16 pixels, 16.times.8 pixels, and the
like.
[0114] FIG. 6 shows a diagram of a video encoder (603) according to
another embodiment of the disclosure. The video encoder (603) is
configured to receive a processing block (e.g., a prediction block)
of sample values within a current video picture in a sequence of
video pictures, and encode the processing block into a coded
picture that is part of a coded video sequence. In an example, the
video encoder (603) is used in the place of the video encoder (303)
in the FIG. 3 example.
[0115] In an HEVC example, the video encoder (603) receives a
matrix of sample values for a processing block, such as a
prediction block of 8.times.8 samples, and the like. The video
encoder (603) determines whether the processing block is best coded
using intra mode, inter mode, or bi-prediction mode using, for
example, rate-distortion optimization. When the processing block is
to be coded in intra mode, the video encoder (603) may use an intra
prediction technique to encode the processing block into the coded
picture; and when the processing block is to be coded in inter mode
or bi-prediction mode, the video encoder (603) may use an inter
prediction or bi-prediction technique, respectively, to encode the
processing block into the coded picture. In certain video coding
technologies, merge mode can be an inter picture prediction submode
where the motion vector is derived from one or more motion vector
predictors without the benefit of a coded motion vector component
outside the predictors. In certain other video coding technologies,
a motion vector component applicable to the subject block may be
present. In an example, the video encoder (603) includes other
components, such as a mode decision module (not shown) to determine
the mode of the processing blocks.
[0116] In the FIG. 6 example, the video encoder (603) includes the
inter encoder (630), an intra encoder (622), a residue calculator
(623), a switch (626), a residue encoder (624), a general
controller (621), and an entropy encoder (625) coupled together as
shown in FIG. 6.
[0117] The inter encoder (630) is configured to receive the samples
of the current block (e.g., a processing block), compare the block
to one or more reference blocks in reference pictures (e.g., blocks
in previous pictures and later pictures), generate inter prediction
information (e.g., description of redundant information according
to inter encoding technique, motion vectors, merge mode
information), and calculate inter prediction results (e.g.,
predicted block) based on the inter prediction information using
any suitable technique. In some examples, the reference pictures
are decoded reference pictures that are decoded based on the
encoded video information.
[0118] The intra encoder (622) is configured to receive the samples
of the current block (e.g., a processing block), in some cases
compare the block to blocks already coded in the same picture,
generate quantized coefficients after transform, and in some cases
also intra prediction information (e.g., an intra prediction
direction information according to one or more intra encoding
techniques). In an example, the intra encoder (622) also calculates
intra prediction results (e.g., predicted block) based on the intra
prediction information and reference blocks in the same
picture.
[0119] The general controller (621) is configured to determine
general control data and control other components of the video
encoder (603) based on the general control data. In an example, the
general controller (621) determines the mode of the block, and
provides a control signal to the switch (626) based on the mode.
For example, when the mode is the intra mode, the general
controller (621) controls the switch (626) to select the intra mode
result for use by the residue calculator (623), and controls the
entropy encoder (625) to select the intra prediction information
and include the intra prediction information in the bitstream; and
when the mode is the inter mode, the general controller (621)
controls the switch (626) to select the inter prediction result for
use by the residue calculator (623), and controls the entropy
encoder (625) to select the inter prediction information and
include the inter prediction information in the bitstream.
[0120] The residue calculator (623) is configured to calculate a
difference (residue data) between the received block and prediction
results selected from the intra encoder (622) or the inter encoder
(630). The residue encoder (624) is configured to operate based on
the residue data to encode the residue data to generate the
transform coefficients. In an example, the residue encoder (624) is
configured to convert the residue data from a spatial domain to a
frequency domain, and generate the transform coefficients. The
transform coefficients are then subject to quantization processing
to obtain quantized transform coefficients. In various embodiments,
the video encoder (603) also includes a residue decoder (628). The
residue decoder (628) is configured to perform inverse-transform,
and generate the decoded residue data. The decoded residue data can
be suitably used by the intra encoder (622) and the inter encoder
(630). For example, the inter encoder (630) can generate decoded
blocks based on the decoded residue data and inter prediction
information, and the intra encoder (622) can generate decoded
blocks based on the decoded residue data and the intra prediction
information. The decoded blocks are suitably processed to generate
decoded pictures and the decoded pictures can be buffered in a
memory circuit (not shown) and used as reference pictures in some
examples.
[0121] The entropy encoder (625) is configured to format the
bitstream to include the encoded block. The entropy encoder (625)
is configured to include various information according to a
suitable standard, such as the HEVC standard. In an example, the
entropy encoder (625) is configured to include the general control
data, the selected prediction information (e.g., intra prediction
information or inter prediction information), the residue
information, and other suitable information in the bitstream. Note
that, according to the disclosed subject matter, when coding a
block in the merge submode of either inter mode or bi-prediction
mode, there is no residue information.
[0122] FIG. 7 shows a diagram of a video decoder (710) according to
another embodiment of the disclosure. The video decoder (710) is
configured to receive coded pictures that are part of a coded video
sequence, and decode the coded pictures to generate reconstructed
pictures. In an example, the video decoder (710) is used in the
place of the video decoder (310) in the FIG. 3 example.
[0123] In the FIG. 7 example, the video decoder (710) includes an
entropy decoder (771), an inter decoder (780), a residue decoder
(773), a reconstruction module (774), and an intra decoder (772)
coupled together as shown in FIG. 7.
[0124] The entropy decoder (771) can be configured to reconstruct,
from the coded picture, certain symbols that represent the syntax
elements of which the coded picture is made up. Such symbols can
include, for example, the mode in which a block is coded (such as,
for example, intra mode, inter mode, bi-predicted mode, the latter
two in merge submode or another submode), prediction information
(such as, for example, intra prediction information or inter
prediction information) that can identify certain sample or
metadata that is used for prediction by the intra decoder (772) or
the inter decoder (780), respectively, residual information in the
form of, for example, quantized transform coefficients, and the
like. In an example, when the prediction mode is inter or
bi-predicted mode, the inter prediction information is provided to
the inter decoder (780); and when the prediction type is the intra
prediction type, the intra prediction information is provided to
the intra decoder (772). The residual information can be subject to
inverse quantization and is provided to the residue decoder
(773).
[0125] The inter decoder (780) is configured to receive the inter
prediction information, and generate inter prediction results based
on the inter prediction information.
[0126] The intra decoder (772) is configured to receive the intra
prediction information, and generate prediction results based on
the intra prediction information.
[0127] The residue decoder (773) is configured to perform inverse
quantization to extract de-quantized transform coefficients, and
process the de-quantized transform coefficients to convert the
residual from the frequency domain to the spatial domain. The
residue decoder (773) may also require certain control information
(to include the Quantizer Parameter (QP)), and that information may
be provided by the entropy decoder (771) (data path not depicted as
this may be low volume control information only).
[0128] The reconstruction module (774) is configured to combine, in
the spatial domain, the residual as output by the residue decoder
(773) and the prediction results (as output by the inter or intra
prediction modules as the case may be) to form a reconstructed
block, that may be part of the reconstructed picture, which in turn
may be part of the reconstructed video. It is noted that other
suitable operations, such as a deblocking operation and the like,
can be performed to improve the visual quality.
[0129] It is noted that the video encoders (303), (503), and (603),
and the video decoders (310), (410), and (710) can be implemented
using any suitable technique. In an embodiment, the video encoders
(303), (503), and (603), and the video decoders (310), (410), and
(710) can be implemented using one or more integrated circuits. In
another embodiment, the video encoders (303), (503), and (603), and
the video decoders (310), (410), and (710) can be implemented using
one or more processors that execute software instructions.
[0130] When constructing a motion vector predictor (MVP) candidate
list to predict a current block, a motion vector predictor index
(MVP_idx) can be signaled to select one of a plurality of MVP
candidates in the MVP candidate list. Depending on the prediction
mode, a motion vector difference (MVD) may be signaled, such as in
AMVP mode. In addition, for AMVP mode with bi-directional
prediction in which two MVP candidate lists are constructed, two
MVDs can be signaled for the two MVP candidate lists, respectively.
However, signaling MVDs may eliminate the possibility of using
merge candidates as MVP candidates, and thus may degrade coding
performance. Aspects of this disclosure includes techniques to
improve coding performance for the MVP candidate list construction,
for example in AMVP mode, and enable more flexible MV prediction
capabilities. For example, inter-prediction is performed with two
or more MVP candidates generated from AMVP or merge mode.
[0131] According to aspects of the disclosure, an MVP_idx is
signaled for an MVP candidate list and is compared with a threshold
T. In an embodiment, if N is a maximum allowed number of MVP
candidates included in an MVP candidate list, for example, N=5, 6,
or 7, an MVP_idx for the MVP candidate list can range from 0 to N-1
(inclusive), and a threshold T can be a non-negative integer
ranging from 0 to N (inclusive). In addition, for AMVP mode with
bi-directional prediction in which two MVP candidate lists are
constructed and two MVDs are respectively signaled for the two MVP
candidate lists, the two MVP candidate lists may share the same
value of T or have different values of T. The threshold T can be a
predefined number (e.g., 1, 2, or 3) which may or may not be
signaled in some embodiments, or can be varied and be signaled in
the coded video sequence (e.g., sequence, picture parameter, slice,
or tile header) in other embodiments.
[0132] According to aspects of the disclosure, when the signaled
MVP_idx is smaller than the threshold T, an MVD is signaled for the
MVP candidate list. The MVD corresponds to an MVP candidate that is
selected in the MVP candidate list according to the MVP_idx. That
is, the current block is predicted based on both of the MVP
candidate selected by the MVP_idx and the MVD corresponding to the
selected MVP candidate. When the signaled MVP_idx is equal to or
larger than the threshold T, no MVD is signaled for the MVP
candidate list. That is, the MVP candidate selected by the MVP_idx
is a merge candidate that can be derived without the MVD.
[0133] According to embodiments of the disclosure, a reference
index (ref_idx) can be signaled to select a reference picture in a
current reference list of the current block. The signaled ref_idx
can be a target ref_idx, similar to that in an HEVC AMVP
process.
[0134] In an embodiment, whether the ref_idx is signaled depends on
a number of the reference pictures in the current reference list.
In an example, when more than one reference picture is available in
the current reference list, the ref_idx is always signaled.
[0135] In an embodiment, whether the ref_idx is signaled depends on
the availability of the MVD. In an example, the ref_idx is signaled
only when the MVD is signaled.
[0136] In an embodiment, whether the ref_idx is signaled depends on
a comparison between the MVP_idx and the threshold T. In an
example, when the MVP_idx is smaller than the threshold T, the
ref_idx is not signaled.
[0137] According to embodiments of the disclosure, when the ref_idx
is not signaled, there is no target ref_idx so that motion vector
scaling may not need to be performed for the selected MVP candidate
indicated by the MVP_idx. In such embodiments, the ref_idx can be
inferred based on the method used in HEVC merge mode. That is, the
reference picture for the current inter-prediction direction will
be the one associated with the selected MVP candidate indicated by
MVP_idx.
[0138] According to aspects of the disclosure, the MVP candidate
list construction process in AMVP mode is modified such that more
candidates than those in HEVC AMVP mode may be included in the MVP
candidate list. For example, at least one MVP candidate is
associated with a zero MVD and at least one MVP candidate is
associated with a non-zero MVD. In addition, pruning may be used to
avoid duplicated candidates.
[0139] In an embodiment, MVP candidates are first derived, for
example, based on the method used in HEVC AMVP mode, and are then
concatenated with additional MVP candidates. The additional MVP
candidates can be based on one or a combination of an HEVC merge
mode, an SbTMVP method, an HMVP method, a Pairwise Average MVP
method, and/or a Multi-hypothesis MVP method. The additional MVP
candidates can be added for each MVP candidate list.
[0140] In an embodiment, during the MVP candidate list
construction, an MVP candidate is considered as invalid if an
inter-prediction hypothesis of the MVP candidate conflicts with an
inter-prediction hypothesis of the current MVP candidate list. That
is, the inter-prediction hypothesis of the MVP candidate is
different from that of the current MVP candidate list. In an
example, if the inter-prediction hypothesis of the current MVP
candidate list is uni-prediction and the current reference list is
L1, and an MVP candidate does not have a valid L1 predictor, then
the MVP candidate is considered as invalid. In another example, if
the current reference list of the current MVP candidate list is L0,
and one MVP candidate has both valid L0 and L1 predictors, only the
L0 predictor is kept. That is, the predictor with the same
inter-prediction hypothesis as the current MVP candidate list is
considered as valid.
[0141] In an embodiment, during the MVP candidate list
construction, when a ref_idx of an MVP candidate is different from
the target ref_idx, motion vector scaling, for example as used in
an HEVC TMVP process, may be used to scale the MVP candidate to the
target ref_idx, or may be discarded.
[0142] In an embodiment, during the MVP candidate list
construction, the MVP candidate list is not padded with zero MVs.
As described above, the MVP candidate list may include merge
candidates and/or other candidates (e.g., HMVP candidates, SbTMVP
candidates, etc.) besides AMVP candidates. In HEVC merge mode, zero
MVs may be padded when the merge list is not full. Thus, the merge
candidates may include zero MVs. However, in this embodiment, when
additional MVP candidates, for example based on an HEVC merge mode,
are to be included into the current MVP candidate list, zero MVs
are not added.
[0143] In an embodiment, during the MVP candidate list
construction, when two or more MVP candidates are identical in the
MVP candidate list, only the first MVP candidate remains and others
are discarded in order to simplify the construction process. One of
the other MVP candidates can remain and the first MVP candidate can
be discarded in another embodiment.
[0144] In an embodiment, during the MVP candidate list
construction, when a number of MVP candidates in the MVP candidate
list is greater than a predefined number (e.g., N), only the first
N candidates are kept and the rest candidates are discarded. For
example, if the predefined number N is ten, then only the first ten
MVP candidates are kept and the MVP candidates after the first ten
MVP candidates are discarded.
[0145] In an embodiment, during the MVP candidate list
construction, when a number of MVP candidates in an MVP candidate
list is below a predefined number (e.g., N), zero MVs can be used
to pad to the end of the MVP candidate list such that the MVP
candidate list contains N candidates. Padding can be performed for
each MVP candidate list.
[0146] According aspects of the disclosure, some constraints can
apply to the MVP candidate list construction process. In an
embodiment, when the current block has one reference picture list,
the MVP_idx is limited to being smaller than the threshold T.
[0147] In an embodiment, whether the MVP_idx can be allowed to be
equal to or larger than the threshold T is related to a block area
of the current block. In an example, when the block area is above
an area threshold, the MVP_idx is allowed to be equal to or larger
than the threshold T. In another example, when the block area is
below the area threshold, the MVP_idx is not allowed to be equal to
or larger than the threshold T. The area threshold, for example,
can be 64, 128, 356, 512, or 1024, etc.
[0148] In an embodiment, for non-merge mode, at least one MVP
candidate corresponding to an MVP_idx that is smaller than the
threshold T is included in the MVP candidate list.
[0149] In an embodiment, for bi-directional prediction in which two
sets of MVP_idx are signaled, if the first signaled MVP_idx is
equal to or larger than the threshold T, the second signaled
MVP_idx is limited to be smaller than the threshold T, or
vice-versa.
[0150] In an embodiment, for non-merge mode, at least one MVD is
signaled. For example, in bi-prediction, if an MVD for L0 is not
signaled, an MVD for L1 will be signaled, or vice-versa.
[0151] FIG. 8 shows a flowchart outlining an exemplary process
(800) according to some embodiments of the disclosure. In various
embodiments, the process (800) is executed by processing circuitry,
such as the processing circuitry in the terminal devices (210),
(220), (230) and (240), the processing circuitry that performs
functions of the video encoder (303), the processing circuitry that
performs functions of the video decoder (310), the processing
circuitry that performs functions of the video decoder (410), the
processing circuitry that performs functions of the intra
prediction module (452), the processing circuitry that performs
functions of the video encoder (503), the processing circuitry that
performs functions of the predictor (535), the processing circuitry
that performs functions of the intra encoder (622), the processing
circuitry that performs functions of the intra decoder (772), and
the like. In some embodiments, the process (800) is implemented in
software instructions, thus when the processing circuitry executes
the software instructions, the processing circuitry performs the
process (800).
[0152] The process (800) may generally start at step (S801), where
the process (800) decodes prediction information for a current
block in a current picture that is a part of a coded video
sequence. The prediction information indicates a motion vector
predictor index (MVP_idx) for selecting a motion vector predictor
(MVP) candidate in a motion vector predictor candidate list. After
decoding the prediction information, the process proceeds to step
(S802).
[0153] At (S802), the process (800) determines whether the MVP_idx
is smaller than a threshold T. When the MVP_idx is determined to be
smaller than the threshold T, the process (800) proceeds to step
(S803). Otherwise, the process (800) proceeds to step (S805).
[0154] At (S803), the process (800) decodes a motion vector
difference (MVD) corresponding to the MVP candidate. After decoding
the MVD, the process (800) proceeds to step (S804).
[0155] At (S804), the process (800) reconstructs the current block
based on the MVP candidate and the MVD.
[0156] At (S805), the process (800) reconstructs the current block
based on the MVP candidate without the MVD since the MVD is not
signaled in the coded video sequence.
[0157] In an embodiment, the process (800) determines a reference
picture in a reference picture list for the current block according
to one of a reference index to the reference picture in the
reference picture list and a reference picture associated with the
MVP candidate.
[0158] In an embodiment, the threshold T is a preset number. In
another embodiment, the threshold T is signaled in the coded video
sequence.
[0159] In an embodiment, the process (800) determines whether an
inter-prediction hypothesis of a new MVP candidate is different
from an inter-prediction hypothesis of the MVP candidate list.
[0160] In an embodiment, the MVP_idx is smaller than the threshold
T when the current block has one reference picture list.
[0161] In an embodiment, the MVP candidate list includes an MVP
candidate associated with a non-zero MVD and an MVP candidate
associated with a zero MVD.
[0162] After reconstructing the current block, the process (800)
terminates.
[0163] According to aspects of the disclosure, techniques are
provided to improve MVD coding When coding/decoding an MVD, some
syntax elements associated with the MVD are coded/decoded. MVD
coding, however, uses contexts to code syntax elements such as
abs_mvd_greater0_flag and abvs_mvd_greater1_flag. The efficiency of
these contexts is not high. Further, multiple MVD coding modules
can occur during the decoding of a coding block, such as in
bi-directional predication and/or affine mode with control points.
The correlations among these MVDS have not been utilized.
[0164] FIG. 9A shows a syntax table for coding an MVD for list 0
(L0) or bi-prediction according to some embodiments of the
disclosure. When an inter prediction mode (inter_pred_idc) of a
current block is not list 1 (PRED_L1), that is, list 0 (PRED_L0) or
bi-prediction (PRED_BI) is used for the current block, and the
maximum reference index for list 0 (num_ref_idx_10_active_minus1)
is above 0, the list 0 reference picture index (ref_idx_10) is
specified and an MVD coding (mvd_coding) is performed for the
current block, where the array indices x0, y0 specify the location
(x0, y0) of the top-left luma sample of the current block relative
to the top-left luma sample of the current picture. It is noted
that a syntax table for coding an MVD for list 1 (L1) can be
defined in a similar way.
[0165] FIG. 9B shows a syntax table for an MVD coding according to
some embodiments of the disclosure. Specifically, the syntax
element abs_mvd_greater0_flag[compIdx] specifies whether the
absolute value of a motion vector component difference is greater
than 0, where compIdx=0 and 1 for x component (axis) and y
component (axis), respectively. The syntax element
abs_mvd_greater1_flag[compIdx] specifies whether the absolute value
of a motion vector component difference is greater than 1. When
abs_mvd_greater1_flag[compIdx] is not present, it can be inferred
to be equal to one of the values such as 0. The syntax element
abs_mvd_minus2[compIdx] plus 2 specifies the absolute value of a
motion vector component difference. When abs_mvd_minus2[compIdx] is
not present, it can be inferred to be equal to one of the values
such as -1. The syntax element mvd_sign_flag[compIdx] specifies the
sign of a motion vector component difference as follows: if
mvd_sign_flag[compIdx]=0, the corresponding motion vector component
difference has a positive value; otherwise (e.g.,
mvd_sign_flag[compIdx]=1), the corresponding motion vector
component difference has a negative value. When
mvd_sign_flag[compIdx] is not present, it can be inferred to be
equal to one of the values such as 0.
[0166] According to aspects of the disclosure, syntax elements of
the MVD can be context coded. For example, the syntax elements
abs_mvd_greater0_flag (e.g., 1 bin) and abs_mvd_greater1_flag
(e.g., 1 bin) can be context coded, for example using Context-Based
Adaptive Binary Arithmetic Coding (CABAC). The syntax element
abs_mvd_minus2 can be coded, for example using first-order
Exponential-Golomb binarization with equal probability. The syntax
element mvd_sign_flag (e.g., 1 bin) can be coded with equal
probability, for example using bypass coding. However, as noted
above, the efficiency of the context coding is not high. In
addition, the MVD coding module (i.e., the MVD syntax table) may be
invoked multiple times during the decoding of a coded block, such
as in bi-directional prediction where two MVDs may be decoded,
and/or in affine mode with multiple control points, where each
control point may correspond to a respective MVD.
[0167] According to aspects of the disclosure, a number of syntax
elements that can be context coded is limited. According to some
embodiments, the number of syntax elements that can be context
coded in MVD coding can be limited to 1. For example, in FIG. 9B,
the MVD coding includes four syntax elements, but no more than one
syntax element is context coded. It is noted that
abs_mvd_greater0_flag[0] and abs_mvd_greater_flag[1] are regarded
as one syntax element in this application, and this rule is also
valid for abs_mvd_greater_flag, abs_mvd_minus2, and
mvd_sign_flag.
[0168] In an embodiment, all the syntax elements (e.g.,
abs_mvd_greater0_flag, abs_mvd_greater1_flag, abs_mvd_minus2, and
mvd_sign_flag) in MVD coding are coded without context-based model.
All the syntax elements may be coded with equal probability, for
example, using bypass coding.
[0169] Compared to other two syntax elements, the syntax elements
abs_mvd_greater0_flag and abs_mvd_greater1_flag may be invoked more
often. Accordingly, one of them can be context coded and the other
one can be bypass coded.
[0170] In an embodiment, only the syntax element
abs_mvd_greater0_flag is context coded. Other syntax elements
abs_mvd_greater1_flag, abs_mvd_minus2, and mvd_sign_flag may be
coded with equal probability.
[0171] In an embodiment, only the syntax element abs mid greater
flag is context coded. Other syntax elements abs_mvd_greater0_flag,
abs_mvd_minus2, and mvd_sign_flag may be coded with equal
probability.
[0172] FIG. 10 shows a syntax table for an MVD coding according to
an embodiment of the disclosure. In this table, three syntax
elements abs_mvd_greater0_flag, abs_mvd_minus1, and mi sign flag,
are used for MVD coding and may be context coded. The syntax
element abs_mvd_minus1 plus one specifies the absolute value of a
motion vector component difference, and may be binarized with a
K-th order Exponential-Golomb binarization, where K may be 0, 2, or
3. K may be 1 in some embodiments.
[0173] According to some embodiments of the disclosure, a syntax
element for MVD coding has a first component and a second
component. The second component of the syntax element can be
decoded based on the first component of the syntax element. In
other words, one component of an MVD syntax element can be used as
the context for coding another component of the same MVD syntax
element.
[0174] In an embodiment, when the first component
abs_mvd_greater0_flag[1-N] is signaled before the second component
abs_mvd_greater0_flag[N], the value of the first component
abs_mvd_greater0_flag[1-N] can be used to derive the context value
for the second component abs_mvd_greater0_flag[N], where N can be 0
or 1.
[0175] In an embodiment, when the first component
abs_mvd_greater0_flag[1-N] is signaled before the second component
abs_mvd_greater0_flag[N] and the first component
abs_mvd_greater0_flag[1-N] is signaled using a value that indicates
a zero MVD for the first component, then bypass coding is used to
signal the second component abs_mvd_greater0_flag[N], where N can
be 0 or 1.
[0176] The above embodiments can also apply to other MVD syntax
elements, such as abs_mvd_greater1_flag and/or abs_mvd_minus1.
[0177] According to some embodiments of the disclosure, when the
prediction information indicates a bi-directional prediction mode
for the current block, i.e., the current block has a first
prediction list and a second prediction list, an MVD of the second
prediction list can be decoded based on a corresponding MVD of the
first prediction list.
[0178] In an embodiment, an MVD syntax element of a previously
decoded prediction list can be used to determine the same syntax
element of a later decoded prediction list. For example, when an
MVD syntax element abs_mvd_greater0_flag[N] of the first prediction
list is signaled before a corresponding MVD syntax element
abs_mvd_greater0_flag[N] of the second prediction list, the value
of the MVD syntax element abs_mvd_greater0_flag[N] of the first
prediction list can be used to derive the context value for the
corresponding MVD syntax element abs_mvd_greater0_flag[N] of the
second prediction list, where N can be 0 or 1.
[0179] In an embodiment, bypass coding can be used to signal an MVD
syntax element of a later coded prediction list when the same
syntax element of an earlier decoded prediction list indicates a
zero MVD. For example, when an MVD syntax element
abs_mvd_greater0_flag[N] of the first prediction list is signaled
before a corresponding syntax element abs_mvd_greater0_flag[N] of
the second prediction list, and the syntax element
abs_mvd_greater0_flag[N] of the first prediction list is signaled
using a value that indicates a zero MVD for the syntax element
abs_mvd_greater0_flag[N] of the first prediction list, then bypass
coding is used to signal the corresponding syntax element
abs_mvd_greater0_flag[N] of the second prediction list, where N can
be 0 or 1.
[0180] The above embodiments can also apply to other MVD syntax
elements, such as abs_mvd_greater1_flag and/or abs_mvd_minus1.
[0181] According to some embodiments of the disclosure, when the
prediction information indicates an affine mode with at least a
first control point and a second control point for the current
block, an MVD of the second control point can be decoded based on a
corresponding MVD of the first control point.
[0182] In an embodiment, an MVD syntax element of a previously
decoded control point can be used to determine the same syntax
element of a later decoded control point. For example, when an MVD
syntax element abs_mvd_greater0_flag[N] of the first control point
is signaled before a corresponding syntax element
abs_mvd_greater0_flag[N] of the second control point, the value of
the syntax element abs_mvd_greater0_flag[N] of the first control
point can be used to derive the context value for the corresponding
syntax element abs_mvd_greater0_flag[N] of the second control
point, where N can be 0 or 1.
[0183] In an embodiment, bypass coding can be used to signal an MVD
syntax element of a later coded control point when the same syntax
element of an earlier coded control point indicates a zero MVD. For
example, when an MVD syntax element abs_mvd_greater0_flag[N] of the
first control point is signaled before a corresponding syntax
element abs_mvd_greater0_flag[N] of the second control point, and
the syntax element abs_mvd_greater0_flag[N] of the first control
point is signaled using a value that indicates a zero MVD for the
syntax element abs_mvd_greater0_flag[N] of the first control point,
then bypass coding is used to signal the corresponding syntax
element abs_mvd_greater0_flag[N] of the second control point, where
N can be 0 or 1.
[0184] In an embodiment, a previously signaled control point is set
to be the first control point for a plurality of remaining control
point(s). For example, one previously signaled control point is
used to determine each of the plurality of remaining control
points.
[0185] The above embodiments can also apply to other MVD syntax
elements, such as abs_mvd_greater1_flag and/or abs_mvd_minus1.
[0186] According to some embodiments of the disclosure, when the
prediction information indicates an affine mode with bi-directional
prediction for the current block, an MVD of each control point in a
first prediction list can be decoded based on an MVD of the same
control point in a second prediction list. For example, when an MVD
of a control point in list 0 is signaled first, an MVD of the same
control point in list 1 can be predicted/context coded
accordingly.
[0187] FIG. 11 shows a flowchart outlining an exemplary process
(1100) according to some embodiments of the disclosure. In various
embodiments, the process (1100) is executed by processing
circuitry, such as the processing circuitry in the terminal devices
(210), (220), (230) and (240), the processing circuitry that
performs functions of the video encoder (303), the processing
circuitry that performs functions of the video decoder (310), the
processing circuitry that performs functions of the video decoder
(410), the processing circuitry that performs functions of the
intra prediction module (452), the processing circuitry that
performs functions of the video encoder (503), the processing
circuitry that performs functions of the predictor (535), the
processing circuitry that performs functions of the intra encoder
(622), the processing circuitry that performs functions of the
intra decoder (772), and the like. In some embodiments, the process
(1100) is implemented in software instructions, thus when the
processing circuitry executes the software instructions, the
processing circuitry performs the process (1100).
[0188] The process (1100) may generally start at step (S1101),
where the process (1100) decodes prediction information for a
current block in a current picture that is a part of a coded video
sequence. The prediction information indicates a first motion
vector difference (MVD) corresponding to a first motion vector
predictor candidate in a first motion vector predictor candidate
list and a plurality of syntax elements associated with the first
MVD. No more than one of the plurality of syntax elements is
context coded, according to some embodiments. In other embodiments,
the number of context coded syntax elements is limited to be less
than or equal to a predetermined number. For example, the number of
content coded syntax elements can be limited to less than 4. In
this case, the number of content coded syntax elements can be
limited to no more than 1 for example when the total number of
syntax elements is 4, or the number of content coded syntax
elements may not need to be limited when the total number of syntax
elements is 3 Then the process (1100) proceeds to step (S1102).
[0189] At step (S1102), the process (1100) decodes the first MVD
according to the plurality of syntax elements. The first MVD may be
decoded based on one or a combination of the processes or
constraints described above. Then the process (1100) proceeds to
step (S1103).
[0190] At step (S1103), the process (1100) reconstructs the current
block based on the first MVD and the first motion vector predictor
candidate.
[0191] In an embodiment, a total number of the plurality of syntax
elements associated with the first MVD is smaller than four.
[0192] In an embodiment, one of the plurality of syntax elements
associated with the first MVD has a first component and a second
component, and the second component is decoded based on the first
component.
[0193] In an embodiment, when the prediction information indicates
a bi-directional prediction mode for the current block, the process
(1000) decodes a second MVD corresponding to a second motion vector
predictor candidate in a second motion vector predictor candidate
list based on the first MVD. The process (1100) further
reconstructs the current block based on the first motion vector
predictor candidate, the first MVD, the second motion vector
predictor candidate, and the second MVD.
[0194] In an embodiment, when the prediction information indicates
a first affine mode for the current block and the first motion
vector predictor candidate corresponds to a first control point of
the current block, the process (1100) decodes a third MVD
corresponding to a third motion vector predictor candidate based on
the first MVD. The third motion vector predictor candidate
corresponds to a second control point of the current block. The
process (1100) further reconstructs the current block based on the
first motion vector predictor candidate, the first MVD, the third
motion vector predictor candidate, and the third MVD.
[0195] In an embodiment, when the prediction information indicates
a second affine mode with bi-directional prediction for the current
block, the process (1100) decodes a fourth MVD corresponding to a
fourth motion vector predictor candidate in a fourth motion vector
predictor candidate list based on the first MVD. The fourth motion
vector predictor candidate and the first motion vector predictor
candidate correspond to a same control point of the current block.
The process (1100) further reconstructs the current block based on
the first motion vector predictor candidate, the first MVD, the
fourth motion vector predictor candidate, and the fourth MVD.
[0196] After reconstructing the current block, the process (1100)
terminates.
[0197] The techniques described above, can be implemented as
computer software using computer-readable instructions and
physically stored in one or more computer-readable media. For
example. FIG. 12 shows a computer system (1200) suitable for
implementing certain embodiments of the disclosed subject
matter.
[0198] The computer software can be coded using any suitable
machine code or computer language, that may be subject to assembly,
compilation, linking, or like mechanisms to create code comprising
instructions that can be executed directly, or through
interpretation, micro-code execution, and the like, by one or more
computer central processing units (CPUs), Graphics Processing Units
(GPUs), and the like.
[0199] The instructions can be executed on various types of
computers or components thereof, including, for example, personal
computers, tablet computers, servers, smartphones, gaming devices,
internet of things devices, and the like.
[0200] The components shown in FIG. 12 for computer system (1200)
are exemplary in nature and are not intended to suggest any
limitation as to the scope of use or functionality of the computer
software implementing embodiments of the present disclosure.
Neither should the configuration of components be interpreted as
having any dependency or requirement relating to any one or
combination of components illustrated in the exemplary embodiment
of a computer system (1200).
[0201] Computer system (1200) may include certain human interface
input devices. Such a human interface input device may be
responsive to input by one or more human users through, for
example, tactile input (such as: keystrokes, swipes, data glove
movements), audio input (such as: voice, clapping), visual input
(such as: gestures), olfactory input (not depicted). The human
interface devices can also be used to capture certain media not
necessarily directly related to conscious input by a human, such as
audio (such as: speech, music, ambient sound), images (such as:
scanned images, photographic images obtain from a still image
camera), video (such as two-dimensional video, three-dimensional
video including stereoscopic video).
[0202] Input human interface devices may include one or more of
(only one of each depicted): keyboard (1201), mouse (1202),
trackpad (1203), touch screen (1210), data-glove (not shown),
joystick (1205), microphone (1206), scanner (1207), camera
(1208).
[0203] Computer system (1200) may also include certain human
interface output devices. Such human interface output devices may
be stimulating the senses of one or more human users through, for
example, tactile output, sound, light, and smell/taste. Such human
interface output devices may include tactile output devices (for
example tactile feedback by the touch-screen (1210), data-glove
(not shown), or joystick (1205), but there can also be tactile
feedback devices that do not serve as input devices), audio output
devices (such as: speakers (1209), headphones (not depicted)),
visual output devices (such as screens (1210) to include CRT
screens, LCD screens, plasma screens, OLED screens, each with or
without touch-screen input capability, each with or without tactile
feedback capability--some of which may be capable to output two
dimensional visual output or more than three dimensional output
through means such as stereographic output; virtual-reality glasses
(not depicted), holographic displays and smoke tanks (not
depicted)), and printers (not depicted). These visual output
devices (such as screens (1210)) can be connected to a system bus
(1248) through a graphics adapter (1250).
[0204] Computer system (1200) can also include human accessible
storage devices and their associated media such as optical media
including CD/DVD ROM/RW (1220) with CD/DVD or the like media
(1221), thumb-drive (1222), removable hard drive or solid state
drive (1223), legacy magnetic media such as tape and floppy disc
(not depicted), specialized ROM/ASIC/PLD based devices such as
security dongles (not depicted), and the like.
[0205] Those skilled in the art should also understand that term
"computer readable media" as used in connection with the presently
disclosed subject matter does not encompass transmission media,
carrier waves, or other transitory signals.
[0206] Computer system (1200) can also include a network interface
(1254) to one or more communication networks (1255). The one or
more communication networks (1255) can for example be wireless,
wireline, optical. The one or more communication networks (1255)
can further be local, wide-area, metropolitan, vehicular and
industrial, real-time, delay-tolerant, and so on. Examples of the
one or more communication networks (1255) include local area
networks such as Ethernet, wireless LANs, cellular networks to
include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wireless
wide area digital networks to include cable TV, satellite TV, and
terrestrial broadcast TV, vehicular and industrial to include
CANBus, and so forth. Certain networks commonly require external
network interface adapters that attached to certain general purpose
data ports or peripheral buses (1249) (such as, for example USB
ports of the computer system (1200)); others are commonly
integrated into the core of the computer system (1200) by
attachment to a system bus as described below (for example Ethernet
interface into a PC computer system or cellular network interface
into a smartphone computer system). Using any of these networks,
computer system (1200) can communicate with other entities. Such
communication can be uni-directional, receive only (for example,
broadcast TV), uni-directional send-only (for example CANbus to
certain CANbus devices), or bi-directional, for example to other
computer systems using local or wide area digital networks. Certain
protocols and protocol stacks can be used on each of those networks
and network interfaces as described above.
[0207] Aforementioned human interface devices, human-accessible
storage devices, and network interfaces can be attached to a core
(1240) of the computer system (1200).
[0208] The core (1240) can include one or more Central Processing
Units (CPU) (1241), Graphics Processing Units (GPU) (1242),
specialized programmable processing units in the form of Field
Programmable Gate Areas (FPGA) (1243), hardware accelerators for
certain tasks (1244), and so forth. These devices, along with
Read-only memory (ROM) (1245), Random-access memory (1246),
internal mass storage such as internal non-user accessible hard
drives, SSDs, and the like (1247), may be connected through the
system bus (1248). In some computer systems, the system bus (1248)
can be accessible in the form of one or more physical plugs to
enable extensions by additional CPUs, GPU, and the like. The
peripheral devices can be attached either directly to the core's
system bus (1248), or through a peripheral bus (1249).
Architectures for a peripheral bus include PCI, USB, and the
like.
[0209] CPUs (1241), GPUs (1242), FPGAs (1243), and accelerators
(1244) can execute certain instructions that, in combination, can
make up the aforementioned computer code. That computer code can be
stored in ROM (1245) or RAM (1246). Transitional data can be also
be stored in RAM (1246), whereas permanent data can be stored for
example, in the internal mass storage (1247). Fast storage and
retrieve to any of the memory devices can be enabled through the
use of cache memory, that can be closely associated with one or
more CPU (1241), GPU (1242), mass storage (1247), ROM (1245), RAM
(1246), and the like.
[0210] The computer readable media can have computer code thereon
for performing various computer-implemented operations. The media
and computer code can be those specially designed and constructed
for the purposes of the present disclosure, or they can be of the
kind well known and available to those having skill in the computer
software arts.
[0211] As an example and not by way of limitation, the computer
system having architecture (1200), and specifically the core (1240)
can provide functionality as a result of processor(s)(including
CPUs, GPUs, FPGA, accelerators, and the like) executing software
embodied in one or more tangible, computer-readable media. Such
computer-readable media can be media associated with
user-accessible mass storage as introduced above, as well as
certain storage of the core (1240) that are of non-transitory
nature, such as core-internal mass storage (1247) or ROM (1245).
The software implementing various embodiments of the present
disclosure can be stored in such devices and executed by core
(1240). A computer-readable medium can include one or more memory
devices or chips, according to particular needs. The software can
cause the core (1240) and specifically the processors therein
(including CPU, GPU, FPGA, and the like) to execute particular
processes or particular parts of particular processes described
herein, including defining data structures stored in RAM (1246) and
modifying such data structures according to the processes defined
by the software. In addition or as an alternative, the computer
system can provide functionality as a result of logic hardwired or
otherwise embodied in a circuit (for example: accelerator (1244)),
which can operate in place of or together with software to execute
particular processes or particular parts of particular processes
described herein. Reference to software can encompass logic, and
vice versa, where appropriate. Reference to a computer-readable
media can encompass a circuit (such as an integrated circuit (IC))
storing software for execution, a circuit embodying logic for
execution, or both, where appropriate. The present disclosure
encompasses any suitable combination of hardware and software.
[0212] While this disclosure has described several exemplary
embodiments, there are alterations, permutations, and various
substitute equivalents, which fall within the scope of the
disclosure. It will thus be appreciated that those skilled in the
art will be able to devise numerous systems and methods which,
although not explicitly shown or described herein, embody the
principles of the disclosure and are thus within the spirit and
scope thereof.
APPENDIX A: ACRONYMS
[0213] AMVP: Advanced Motion Vector Prediction [0214] ASIC:
Application-Specific Integrated Circuit [0215] BMS: Benchmark Set
[0216] BS: Boundary Strength [0217] BV: Block Vector [0218] CANBus:
Controller Area Network Bus [0219] CD: Compact Disc [0220] CPR:
Current Picture Referencing [0221] CPUs: Central Processing Units
[0222] CRT: Cathode Ray Tube [0223] CTBs: Coding Tree Blocks [0224]
CTUs: Coding Tree Units [0225] CU: Coding Unit [0226] DPB: Decoder
Picture Buffer [0227] DVD: Digital Video Disc [0228] FPGA: Field
Programmable Gate Areas [0229] GOPs: Groups of Pictures [0230]
GPUs: Graphics Processing Units [0231] GSM: Global System for
Mobile communications [0232] HDR: High Dynamic Range [0233] HEVC:
High Efficiency Video Coding [0234] HRD: Hypothetical Reference
Decoder [0235] IBC: Intra Block Copy [0236] IC: Integrated Circuit
[0237] JEM: Joint Exploration Model [0238] LAN: Local Area Network
[0239] LCD: Liquid-Crystal Display [0240] LIC: Local Illumination
Compensation [0241] LTE: Long-Term Evolution [0242] MR-SAD:
Mean-Removed Sum of Absolute Difference [0243] MR-SATD:
Mean-Removed Sum of Absolute Hadamard-Transformed Difference [0244]
MV: Motion Vector [0245] OLED: Organic Light-Emitting Diode [0246]
PBs: Prediction Blocks [0247] PCI: Peripheral Component
Interconnect [0248] PLD: Programmable Logic Device [0249] PPS:
Picture Parameter Set [0250] PUs: Prediction Units [0251] RAM:
Random Access Memory [0252] ROM: Read-Only Memory [0253] SCC:
Screen Content Coding [0254] SDR: Standard Dynamic Range [0255]
SEI: Supplementary Enhancement Information [0256] SMVP: Spatial
Motion Vector Predictor [0257] SNR: Signal Noise Ratio [0258] SPS:
Sequence Parameter Set [0259] SSD: Solid-state Drive [0260] TMVP:
Temporal Motion Vector Predictor [0261] TUs: Transform Units [0262]
USB: Universal Serial Bus [0263] VUI: Video Usability Information
[0264] VVC: Versatile Video Coding
* * * * *