U.S. patent application number 17/222461 was filed with the patent office on 2021-07-22 for simplifications of cross-component linear model.
The applicant listed for this patent is BEIJING DAJIA INTERNET INFORMATION TECHNOLOGY CO., LTD.. Invention is credited to Yi-Wen Chen, Xianglin Wang.
Application Number | 20210227235 17/222461 |
Document ID | / |
Family ID | 1000005551534 |
Filed Date | 2021-07-22 |
United States Patent
Application |
20210227235 |
Kind Code |
A1 |
Chen; Yi-Wen ; et
al. |
July 22, 2021 |
SIMPLIFICATIONS OF CROSS-COMPONENT LINEAR MODEL
Abstract
A computing device performs a method of decoding video data by
reconstructing a luma block corresponding to a chroma block;
searching a sub-group of a plurality of reconstructed neighboring
luma samples in a predefined order to identify a maximum luma
sample and a minimum luma sample; computing a down-sampled maximum
luma sample corresponding to the maximum luma sample; computing a
down-sampled minimum luma sample corresponding to the minimum luma
sample; generating a linear model using the down-sampled maximum
luma sample, the down-sampled minimum luma sample, the first
reconstructed chroma sample, and the second reconstructed chroma
sample; computing down-sampled luma samples from luma samples of
the reconstructed luma block, wherein each down-sampled luma sample
corresponds to a chroma sample of the chroma block; and predicting
chroma samples of the chroma block by applying the liner model to
the corresponding down-sampled luma samples.
Inventors: |
Chen; Yi-Wen; (Beijing,
CN) ; Wang; Xianglin; (Beijing, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BEIJING DAJIA INTERNET INFORMATION TECHNOLOGY CO., LTD. |
Beijing |
|
CN |
|
|
Family ID: |
1000005551534 |
Appl. No.: |
17/222461 |
Filed: |
April 5, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2019/055208 |
Oct 8, 2019 |
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17222461 |
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62742806 |
Oct 8, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04N 19/30 20141101;
H04N 19/176 20141101; H04N 19/105 20141101; H04N 19/59 20141101;
H04N 19/132 20141101; H04N 19/186 20141101 |
International
Class: |
H04N 19/186 20060101
H04N019/186; H04N 19/176 20060101 H04N019/176; H04N 19/30 20060101
H04N019/30; H04N 19/105 20060101 H04N019/105; H04N 19/59 20060101
H04N019/59; H04N 19/132 20060101 H04N019/132 |
Claims
1. A method for decoding a video signal, comprising: reconstructing
a luma block corresponding to a chroma block, wherein the luma
block is adjacent to a plurality of reconstructed neighboring luma
samples, and wherein the chroma block is adjacent to a plurality of
reconstructed neighboring chroma samples; computing at least one
down-sampled luma sample from the plurality of reconstructed
neighboring luma samples; identifying, from a sub-group of the at
least one computed down-sampled luma sample, at least one
down-sampled maximum luma sample, wherein the at least one
down-sampled maximum luma sample corresponds to a first
reconstructed chroma sample of the plurality of reconstructed
neighboring chroma samples; identifying, from a sub-group of the at
least one computed down-sampled luma sample, at least one
down-sampled minimum luma sample, wherein the at least one
down-sampled minimum luma sample corresponds to a second
reconstructed chroma sample of the plurality of reconstructed
neighboring chroma samples; generating a linear model based on the
at least one down-sampled maximum luma sample, the at least one
down-sampled minimum luma sample, the first reconstructed chroma
sample, and the second reconstructed chroma sample; computing
down-sampled luma samples from luma samples of the reconstructed
luma block, wherein each down-sampled luma sample corresponds to a
chroma sample of the chroma block; and predicting chroma samples of
the chroma block by applying the linear model to the corresponding
computed down-sampled luma samples.
2. The method of claim 1, wherein the chroma block and the luma
block are encoded using a 4:2:0 or 4:2:2 chroma sub-sampling
scheme, and wherein the chroma block and the luma block have
different resolutions.
3. The method of claim 1, wherein the chroma block and the luma
block are encoded using a 4:4:4 chroma full sampling scheme, and
wherein the chroma block and the luma block have the same
resolution.
4. The method of claim 1, wherein the plurality of reconstructed
neighboring luma samples includes luma samples located above the
reconstructed luma block and/or luma samples to left of the
reconstructed luma block.
5. The method of claim 1, wherein the computing down-sampled luma
samples from luma samples of the reconstructed luma block comprises
performing a weighted average of six neighboring luma samples to
the luma sample.
6. The method of claim 1, wherein the generating the linear model
comprises fitting a linear equation through at least one data point
associated with the at least one down-sampled maximum luma sample
and the first reconstructed chroma sample and at least one data
point associated with the at least one down-sampled minimum luma
sample and the second reconstructed chroma sample.
7. A non-transitory computer readable storage medium storing a
plurality of programs for execution by a computing device having
one or more processors, wherein the plurality of programs, when
executed by the one or more processors, cause the computing device
to perform operations including: reconstructing a luma block
corresponding to a chroma block, wherein the luma block is adjacent
to a plurality of reconstructed neighboring luma samples, and
wherein the chroma block is adjacent to a plurality of
reconstructed neighboring chroma samples; computing at least one
down-sampled luma sample from the plurality of reconstructed
neighboring luma samples; identifying, from a sub-group of the at
least one computed down-sampled luma sample, at least one
down-sampled maximum luma sample, wherein the at least one
down-sampled maximum luma sample corresponds to a first
reconstructed chroma sample of the plurality of reconstructed
neighboring chroma samples; identifying, from a sub-group of the at
least one computed down-sampled luma sample, at least one
down-sampled minimum luma sample, wherein the at least one
down-sampled minimum luma sample corresponds to a second
reconstructed chroma sample of the plurality of reconstructed
neighboring chroma samples; generating a linear model based on the
at least one down-sampled maximum luma sample, the at least one
down-sampled minimum luma sample, the first reconstructed chroma
sample, and the second reconstructed chroma sample; computing
down-sampled luma samples from luma samples of the reconstructed
luma block, wherein each down-sampled luma sample corresponds to a
chroma sample of the chroma block; and predicting chroma samples of
the chroma block by applying the linear model to the corresponding
computed down-sampled luma samples.
8. The non-transitory computer readable storage medium of claim 7,
wherein the chroma block and the luma block are encoded using a
4:2:0 or 4:2:2 chroma sub-sampling scheme, and wherein the chroma
block and the luma block have different resolutions.
9. The non-transitory computer readable storage medium of claim 7,
wherein the chroma block and the luma block are encoded using a
4:4:4 chroma full sampling scheme, and wherein the chroma block and
the luma block have the same resolution.
10. The non-transitory computer readable storage medium of claim 7,
wherein the plurality of reconstructed neighboring luma samples
includes luma samples located above the reconstructed luma block
and/or luma samples to left of the reconstructed luma block.
11. The non-transitory computer readable storage medium of claim 7,
wherein the computing down-sampled luma samples from luma samples
of the reconstructed luma block comprises performing a weighted
average of six neighboring luma samples to the luma sample.
12. The non-transitory computer readable storage medium of claim 7,
wherein the generating the linear model comprises fitting a linear
equation through at least one data point associated with the at
least one down-sampled maximum luma sample and the first
reconstructed chroma sample and at least one data point associated
with the at least one down-sampled minimum luma sample and the
second reconstructed chroma sample.
13. A method for decoding a video signal, comprising:
reconstructing a luma block corresponding to a chroma block,
wherein the luma block is adjacent to a plurality of reconstructed
neighboring luma samples, and wherein the chroma block is adjacent
to a plurality of reconstructed neighboring chroma samples;
computing one or more down-sampled luma samples from the plurality
of reconstructed neighboring luma samples; searching a sub-group of
the one or more down-sampled luma samples in a predefined order to
identify at least one down-sampled maximum luma sample and at least
one down-sampled minimum luma sample, respectively, wherein the at
least one down-sampled maximum luma sample corresponds to a first
reconstructed chroma sample of the plurality of reconstructed
neighboring chroma samples and the at least one down-sampled
minimum luma sample corresponds to a second reconstructed chroma
sample of the plurality of reconstructed neighboring chroma
samples; generating a linear model using the at least one
down-sampled maximum luma sample, the at least one down-sampled
minimum luma sample, the first reconstructed chroma sample, and the
second reconstructed chroma sample; computing down-sampled luma
samples from luma samples of the reconstructed luma block, wherein
each down-sampled luma sample corresponds to a chroma sample of the
chroma block; and predicting chroma samples of the chroma block by
applying the linear model to the corresponding computed
down-sampled luma samples.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of PCT Patent
Application No. PCT/US2019/055208, filed on Oct. 8, 2019, which
claims the benefit of U.S. Provisional Patent Application No.
62/742,806, filed Oct. 8, 2018, all of which are incorporated
herein by reference in their entirety.
TECHNICAL FIELD
[0002] The present application generally relates to video data
encoding and decoding, and in particular, to method and system of
reconstructing a chroma block using a cross-component linear model
during video data encoding and decoding.
BACKGROUND
[0003] Digital video is supported by a variety of electronic
devices, such as digital televisions, laptop or desktop computers,
tablet computers, digital cameras, digital recording devices,
digital media players, video gaming consoles, smart phones, video
teleconferencing devices, video streaming devices, etc. The
electronic devices transmit, receive, encode, decode, and/or store
digital video data by implementing video compression/decompression
standards as defined by MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4,
Part 10, Advanced Video Coding (AVC), High Efficiency Video Coding
(HEVC), and Versatile Video Coding (VVC) standard. Video
compression typically includes performing spatial (intra frame)
prediction and/or temporal (inter frame) prediction to reduce or
remove redundancy inherent in the video data. For block-based video
coding, a video frame is partitioned into one or more slices, each
slice having multiple video blocks, which may also be referred to
as coding tree units (CTUs). Each CTU may contain one coding unit
(CU) or recursively split into smaller CUs until the predefined
minimum CU size is reached. Each CU (also named leaf CU) contains
one or multiple transform units (TUs) and each CU also contains one
or multiple prediction units (PUs). Each CU can be coded in either
intra, inter or IBC modes. Video blocks in an intra coded (I) slice
of a video frame are encoded using spatial prediction with respect
to reference samples in neighbor blocks within the same video
frame. Video blocks in an inter coded (P or B) slice of a video
frame may use spatial prediction with respect to reference samples
in neighbor blocks within the same video frame or temporal
prediction with respect to reference samples in other previous
and/or future reference video frames.
[0004] Spatial or temporal prediction based on a reference block
that has been previously encoded, e.g., a neighbor block, results
in a predictive block for a current video block to be coded. The
process of finding the reference block may be accomplished by block
matching algorithm. Residual data representing pixel differences
between the current block to be coded and the predictive block is
referred to as a residual block or prediction errors. An
inter-coded block is encoded according to a motion vector that
points to a reference block in a reference frame forming the
predictive block, and the residual block. The process of
determining the motion vector is typically referred to as motion
estimation. An intra coded block is encoded according to an intra
prediction mode and the residual block. For further compression,
the residual block is transformed from the pixel domain to a
transform domain, e.g., frequency domain, resulting in residual
transform coefficients, which may then be quantized. The quantized
transform coefficients, initially arranged in a two-dimensional
array, may be scanned to produce a one-dimensional vector of
transform coefficients, and then entropy encoded into a video
bitstream to achieve even more compression.
[0005] The encoded video bitstream is then saved in a
computer-readable storage medium (e.g., flash memory) to be
accessed by another electronic device with digital video capability
or directly transmitted to the electronic device wired or
wirelessly. The electronic device then performs video decompression
(which is an opposite process to the video compression described
above) by, e.g., parsing the encoded video bitstream to obtain
syntax elements from the bitstream and reconstructing the digital
video data to its original format from the encoded video bitstream
based at least in part on the syntax elements obtained from the
bitstream, and renders the reconstructed digital video data on a
display of the electronic device.
[0006] With digital video quality going from high definition, to
4K.times.2K or even 8K.times.4K, the amount of vide data to be
encoded/decoded grows exponentially. It is a constant challenge in
terms of how the video data can be encoded/decoded more efficiently
while maintaining the image quality of the decoded video data.
SUMMARY
[0007] The present application describes implementations related to
video data encoding and decoding and, more particularly, to system
and method of reconstructing a chroma block using a cross-component
linear model during video data encoding and decoding.
[0008] According to a first aspect of the present application, a
method of decoding video data is performed at a computing device
having one or more processors and memory storing a plurality of
programs to be executed by the one or more processors. A computing
device performs a method of reconstructing a luma block
corresponding to a chroma block; searching a sub-group of a
plurality of reconstructed neighboring luma samples in a predefined
order to identify a maximum luma sample and a minimum luma sample;
computing a down-sampled maximum luma sample corresponding to the
maximum luma sample; computing a down-sampled minimum luma sample
corresponding to the minimum luma sample; generating a linear model
using the down-sampled maximum luma sample, the down-sampled
minimum luma sample, the first reconstructed chroma sample, and the
second reconstructed chroma sample; computing down-sampled luma
samples from luma samples of the reconstructed luma block, wherein
each down-sampled luma sample corresponds to a chroma sample of the
chroma block; and predicting chroma samples of the chroma block by
applying the liner model to the corresponding down-sampled luma
samples.
[0009] According to a second aspect of the present application, a
computing device includes one or more processors, memory and a
plurality of programs stored in the memory. The programs, when
executed by the one or more processors, cause the computing device
to perform operations as described above.
[0010] According to a third aspect of the present application, a
non-transitory computer readable storage medium stores a plurality
of programs for execution by a computing device having one or more
processors. The programs, when executed by the one or more
processors, cause the computing device to perform operations as
described above.
BRIEF DESCRIPTION OF DRAWINGS
[0011] The accompanying drawings, which are included to provide a
further understanding of the implementations and are incorporated
herein and constitute a part of the specification, illustrate the
described implementations and together with the description serve
to explain the underlying principles. Like reference numerals refer
to corresponding parts.
[0012] FIG. 1 is a block diagram illustrating an exemplary video
encoding and decoding system in accordance with some
implementations of the present disclosure.
[0013] FIG. 2 is a block diagram illustrating an exemplary video
encoder in accordance with some implementations of the present
disclosure.
[0014] FIG. 3 is a block diagram illustrating an exemplary video
decoder in accordance with some implementations of the present
disclosure.
[0015] FIGS. 4A-4D are block diagrams illustrating how a frame is
recursively quad-tree partitioned into multiple video blocks of
different sizes in accordance with some implementations of the
present disclosure.
[0016] FIG. 5A is a block diagram illustrating spatially
neighboring and temporally collocated block positions of a current
CU to be encoded in accordance with some implementations of the
present disclosure.
[0017] FIG. 5B is a block diagram illustrating multi-threaded
encoding of multiple rows of CTUs of a picture using wavefront
parallel processing in accordance with some implementations of the
present disclosure.
[0018] FIGS. 6A and 6B are block diagrams illustrating an exemplary
reconstructed luma block and an exemplary associated chroma block,
respectively, in accordance with some implementations of the
present disclosure.
[0019] FIGS. 7A-7E illustrate various ways of using the
cross-component linear model to derive a linear model between luma
values and chroma values in accordance with some implementations of
the present disclosure.
[0020] FIG. 8 is a flowchart illustrating an exemplary process by
which a video codec implements the techniques of using a
cross-component linear model to reconstruct chroma samples for a
chroma block based on reconstructed luma samples from a luma block
in accordance with some implementations of the present
disclosure.
DETAILED DESCRIPTION
[0021] Reference will now be made in detail to specific
implementations, examples of which are illustrated in the
accompanying drawings. In the following detailed description,
numerous non-limiting specific details are set forth in order to
assist in understanding the subject matter presented herein. But it
will be apparent to one of ordinary skill in the art that various
alternatives may be used without departing from the scope of claims
and the subject matter may be practiced without these specific
details. For example, it will be apparent to one of ordinary skill
in the art that the subject matter presented herein can be
implemented on many types of electronic devices with digital video
capabilities.
[0022] FIG. 1 is a block diagram illustrating an exemplary system
10 for encoding and decoding video blocks in parallel in accordance
with some implementations of the present disclosure. As shown in
FIG. 1, system 10 includes a source device 12 that generates and
encodes video data to be decoded at a later time by a destination
device 14. Source device 12 and destination device 14 may comprise
any of a wide variety of electronic devices, including desktop or
laptop computers, tablet computers, smart phones, set-top boxes,
digital televisions, cameras, display devices, digital media
players, video gaming consoles, video streaming device, or the
like. In some implementations, source device 12 and destination
device 14 are equipped with wireless communication
capabilities.
[0023] In some implementations, destination device 14 may receive
the encoded video data to be decoded via a link 16. Link 16 may
comprise any type of communication medium or device capable of
moving the encoded video data from source device 12 to destination
device 14. In one example, link 16 may comprise a communication
medium to enable source device 12 to transmit the encoded video
data directly to destination device 14 in real-time. The encoded
video data may be modulated according to a communication standard,
such as a wireless communication protocol, and transmitted to
destination device 14. The communication medium may comprise any
wireless or wired communication medium, such as a radio frequency
(RF) spectrum or one or more physical transmission lines. The
communication medium may form part of a packet-based network, such
as a local area network, a wide-area network, or a global network
such as the Internet. The communication medium may include routers,
switches, base stations, or any other equipment that may be useful
to facilitate communication from source device 12 to destination
device 14.
[0024] In some other implementations, the encoded video data may be
transmitted from output interface 22 to a storage device 32.
Subsequently, the encoded video data in storage device 32 may be
accessed by destination device 14 via input interface 28. Storage
device 32 may include any of a variety of distributed or locally
accessed data storage media such as a hard drive, Blu-ray discs,
DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or
any other suitable digital storage media for storing encoded video
data. In a further example, storage device 32 may correspond to a
file server or another intermediate storage device that may hold
the encoded video data generated by source device 12. Destination
device 14 may access the stored video data from storage device 32
via streaming or downloading. The file server may be any type of
computer capable of storing encoded video data and transmitting the
encoded video data to destination device 14. Exemplary file servers
include a web server (e.g., for a website), an FTP server, network
attached storage (NAS) devices, or a local disk drive. Destination
device 14 may access the encoded video data through any standard
data connection, including a wireless channel (e.g., a Wi-Fi
connection), a wired connection (e.g., DSL, cable modem, etc.), or
a combination of both that is suitable for accessing encoded video
data stored on a file server. The transmission of encoded video
data from storage device 32 may be a streaming transmission, a
download transmission, or a combination of both.
[0025] As shown in FIG. 1, source device 12 includes a video source
18, a video encoder 20 and an output interface 22. Video source 18
may include a source such as a video capture device, e.g., a video
camera, a video archive containing previously captured video, a
video feed interface to receive video from a video content
provider, and/or a computer graphics system for generating computer
graphics data as the source video, or a combination of such
sources. As one example, if video source 18 is a video camera of a
security surveillance system, source device 12 and destination
device 14 may form camera phones or video phones. However, the
implementations described in the present application may be
applicable to video coding in general, and may be applied to
wireless and/or wired applications.
[0026] The captured, pre-captured, or computer-generated video may
be encoded by video encoder 20. The encoded video data may be
transmitted directly to destination device 14 via output interface
22 of source device 12. The encoded video data may also (or
alternatively) be stored onto storage device 32 for later access by
destination device 14 or other devices, for decoding and/or
playback. Output interface 22 may further include a modem and/or a
transmitter.
[0027] Destination device 14 includes an input interface 28, a
video decoder 30, and a display device 34. Input interface 28 may
include a receiver and/or a modem and receive the encoded video
data over link 16. The encoded video data communicated over link
16, or provided on storage device 32, may include a variety of
syntax elements generated by video encoder 20 for use by video
decoder 30 in decoding the video data. Such syntax elements may be
included within the encoded video data transmitted on a
communication medium, stored on a storage medium, or stored a file
server.
[0028] In some implementations, destination device 14 may include a
display device 34, which can be an integrated display device and an
external display device that is configured to communicate with
destination device 14. Display device 34 displays the decoded video
data to a user, and may comprise any of a variety of display
devices such as a liquid crystal display (LCD), a plasma display,
an organic light emitting diode (OLED) display, or another type of
display device.
[0029] Video encoder 20 and video decoder 30 may operate according
to proprietary or industry standards, such as VVC, HEVC, MPEG-4,
Part 10, Advanced Video Coding (AVC), or extensions of such
standards. It should be understood that the present application is
not limited to a specific video coding/decoding standard and may be
applicable to other video coding/decoding standards. It is
generally contemplated that video encoder 20 of source device 12
may be configured to encode video data according to any of these
current or future standards. Similarly, it is also generally
contemplated that video decoder 30 of destination device 14 may be
configured to decode video data according to any of these current
or future standards.
[0030] Video encoder 20 and video decoder 30 each may be
implemented as any of a variety of suitable encoder circuitry, such
as one or more microprocessors, digital signal processors (DSPs),
application specific integrated circuits (ASICs), field
programmable gate arrays (FPGAs), discrete logic, software,
hardware, firmware or any combinations thereof. When implemented
partially in software, an electronic device may store instructions
for the software in a suitable, non-transitory computer-readable
medium and execute the instructions in hardware using one or more
processors to perform the video coding/decoding operations
disclosed in the present disclosure. Each of video encoder 20 and
video decoder 30 may be included in one or more encoders or
decoders, either of which may be integrated as part of a combined
encoder/decoder (CODEC) in a respective device.
[0031] FIG. 2 is a block diagram illustrating an exemplary video
encoder 20 in accordance with some implementations described in the
present application. Video encoder 20 may perform intra and inter
predictive coding of video blocks within video frames. Intra
predictive coding relies on spatial prediction to reduce or remove
spatial redundancy in video data within a given video frame or
picture. Inter predictive coding relies on temporal prediction to
reduce or remove temporal redundancy in video data within adjacent
video frames or pictures of a video sequence.
[0032] As shown in FIG. 2, video encoder 20 includes video data
memory 40, prediction processing unit 41, decoded picture buffer
(DPB) 64, summer 50, transform processing unit 52, quantization
unit 54, and entropy encoding unit 56. Prediction processing unit
41 further includes motion estimation unit 42, motion compensation
unit 44, partition unit 45, intra prediction processing unit 46,
and intra block copy (BC) unit 48. In some implementations, video
encoder 20 also includes inverse quantization unit 58, inverse
transform processing unit 60, and summer 62 for video block
reconstruction. A deblocking filter (not shown) may be positioned
between summer 62 and DPB 64 to filter block boundaries to remove
blockiness artifacts from reconstructed video. An in loop filter
(not shown) may also be used in addition to the deblocking filter
to filter the output of summer 62. Video encoder 20 may take the
form of a fixed or programmable hardware unit or may be divided
among one or more of the illustrated fixed or programmable hardware
units.
[0033] Video data memory 40 may store video data to be encoded by
the components of video encoder 20. The video data in video data
memory 40 may be obtained, for example, from video source 18. DPB
64 is a buffer that stores reference video data for use in encoding
video data by video encoder 20 (e.g., in intra or inter predictive
coding modes). Video data memory 40 and DPB 64 may be formed by any
of a variety of memory devices. In various examples, video data
memory 40 may be on-chip with other components of video encoder 20,
or off-chip relative to those components.
[0034] As shown in FIG. 2, after receiving video data, partition
unit 45 within prediction processing unit 41 partitions the video
data into video blocks. This partitioning may also include
partitioning a video frame into slices, tiles, or other larger
coding units (CUs) according to a predefined splitting structures
such as quad-tree structure associated with the video data. The
video frame may be divided into multiple video blocks (or sets of
video blocks referred to as tiles). Prediction processing unit 41
may select one of a plurality of possible predictive coding modes,
such as one of a plurality of intra predictive coding modes or one
of a plurality of inter predictive coding modes, for the current
video block based on error results (e.g., coding rate and the level
of distortion). Prediction processing unit 41 may provide the
resulting intra or inter prediction coded block to summer 50 to
generate a residual block and to summer 62 to reconstruct the
encoded block for use as part of a reference frame subsequently.
Prediction processing unit 41 also provides syntax elements, such
as motion vectors, intra-mode indicators, partition information,
and other such syntax information, to entropy encoding unit 56.
[0035] In order to select an appropriate intra predictive coding
mode for the current video block, intra prediction processing unit
46 within prediction processing unit 41 may perform intra
predictive coding of the current video block relative to one or
more neighbor blocks in the same frame as the current block to be
coded to provide spatial prediction. Motion estimation unit 42 and
motion compensation unit 44 within prediction processing unit 41
perform inter predictive coding of the current video block relative
to one or more predictive blocks in one or more reference frames to
provide temporal prediction. Video encoder 20 may perform multiple
coding passes, e.g., to select an appropriate coding mode for each
block of video data.
[0036] In some implementations, motion estimation unit 42
determines the inter prediction mode for a current video frame by
generating a motion vector, which indicates the displacement of a
prediction unit (PU) of a video block within the current video
frame relative to a predictive block within a reference video
frame, according to a predetermined pattern within a sequence of
video frames. Motion estimation, performed by motion estimation
unit 42, is the process of generating motion vectors, which
estimate motion for video blocks. A motion vector, for example, may
indicate the displacement of a PU of a video block within a current
video frame or picture relative to a predictive block within a
reference frame (or other coded unit) relative to the current block
being coded within the current frame (or other coded unit). The
predetermined pattern may designate video frames in the sequence as
P frames or B frames. Intra BC unit 48 may determine vectors, e.g.,
block vectors, for intra BC coding in a manner similar to the
determination of motion vectors by motion estimation unit 42 for
inter prediction, or may utilize motion estimation unit 42 to
determine the block vector.
[0037] A predictive block is a block of a reference frame that is
deemed as closely matching the PU of the video block to be coded in
terms of pixel difference, which may be determined by sum of
absolute difference (SAD), sum of square difference (SSD), or other
difference metrics. In some implementations, video encoder 20 may
calculate values for sub-integer pixel positions of reference
frames stored in DPB 64. For example, video encoder 20 may
interpolate values of one-quarter pixel positions, one-eighth pixel
positions, or other fractional pixel positions of the reference
frame. Therefore, motion estimation unit 42 may perform a motion
search relative to the full pixel positions and fractional pixel
positions and output a motion vector with fractional pixel
precision.
[0038] Motion estimation unit 42 calculates a motion vector for a
PU of a video block in an inter prediction coded frame by comparing
the position of the PU to the position of a predictive block of a
reference frame selected from a first reference frame list (List 0)
or a second reference frame list (List 1), each of which identifies
one or more reference frames stored in DPB 64. Motion estimation
unit 42 sends the calculated motion vector to motion compensation
unit 44 and then to entropy encoding unit 56.
[0039] Motion compensation, performed by motion compensation unit
44, may involve fetching or generating the predictive block based
on the motion vector determined by motion estimation unit 42. Upon
receiving the motion vector for the PU of the current video block,
motion compensation unit 44 may locate a predictive block to which
the motion vector points in one of the reference frame lists,
retrieve the predictive block from DPB 64, and forward the
predictive block to summer 50. Summer 50 then forms a residual
video block of pixel difference values by subtracting pixel values
of the predictive block provided by motion compensation unit 44
from the pixel values of the current video block being coded. The
pixel difference values forming the residual vide block may include
luma or chroma difference components or both. Motion compensation
unit 44 may also generate syntax elements associated with the video
blocks of a video frame for use by video decoder 30 in decoding the
video blocks of the video frame. The syntax elements may include,
for example, syntax elements defining the motion vector used to
identify the predictive block, any flags indicating the prediction
mode, or any other syntax information described herein. Note that
motion estimation unit 42 and motion compensation unit 44 may be
highly integrated, but are illustrated separately for conceptual
purposes.
[0040] In some implementations, intra BC unit 48 may generate
vectors and fetch predictive blocks in a manner similar to that
described above in connection with motion estimation unit 42 and
motion compensation unit 44, but with the predictive blocks being
in the same frame as the current block being coded and with the
vectors being referred to as block vectors as opposed to motion
vectors. In particular, intra BC unit 48 may determine an
intra-prediction mode to use to encode a current block. In some
examples, intra BC unit 48 may encode a current block using various
intra-prediction modes, e.g., during separate encoding passes, and
test their performance through rate-distortion analysis. Next,
intra BC unit 48 may select, among the various tested
intra-prediction modes, an appropriate intra-prediction mode to use
and generate an intra-mode indicator accordingly. For example,
intra BC unit 48 may calculate rate-distortion values using a
rate-distortion analysis for the various tested intra-prediction
modes, and select the intra-prediction mode having the best
rate-distortion characteristics among the tested modes as the
appropriate intra-prediction mode to use. Rate-distortion analysis
generally determines an amount of distortion (or error) between an
encoded block and an original, unencoded block that was encoded to
produce the encoded block, as well as a bitrate (i.e., a number of
bits) used to produce the encoded block. Intra BC unit 48 may
calculate ratios from the distortions and rates for the various
encoded blocks to determine which intra-prediction mode exhibits
the best rate-distortion value for the block.
[0041] In other examples, intra BC unit 48 may use motion
estimation unit 42 and motion compensation unit 44, in whole or in
part, to perform such functions for Intra BC prediction according
to the implementations described herein. In either case, for Intra
block copy, a predictive block may be a block that is deemed as
closely matching the block to be coded, in terms of pixel
difference, which may be determined by sum of absolute difference
(SAD), sum of squared difference (SSD), or other difference
metrics, and identification of the predictive block may include
calculation of values for sub-integer pixel positions.
[0042] Whether the predictive block is from the same frame
according to intra prediction, or a different frame according to
inter prediction, video encoder 20 may form a residual video block
by subtracting pixel values of the predictive block from the pixel
values of the current video block being coded, forming pixel
difference values. The pixel difference values forming the residual
video block may include both luma and chroma component
differences.
[0043] Intra prediction processing unit 46 may intra-predict a
current video block, as an alternative to the inter-prediction
performed by motion estimation unit 42 and motion compensation unit
44, or the intra block copy prediction performed by intra BC unit
48, as described above. In particular, intra prediction processing
unit 46 may determine an intra prediction mode to use to encode a
current block. To do so, intra prediction processing unit 46 may
encode a current block using various intra prediction modes, e.g.,
during separate encoding passes, and intra prediction processing
unit 46 (or a mode select unit, in some examples) may select an
appropriate intra prediction mode to use from the tested intra
prediction modes. Intra prediction processing unit 46 may provide
information indicative of the selected intra-prediction mode for
the block to entropy encoding unit 56. Entropy encoding unit 56 may
encode the information indicating the selected intra-prediction
mode in the bitstream.
[0044] After prediction processing unit 41 determines the
predictive block for the current video block via either inter
prediction or intra prediction, summer 50 forms a residual video
block by subtracting the predictive block from the current video
block. The residual video data in the residual block may be
included in one or more transform units (TUs) and is provided to
transform processing unit 52. Transform processing unit 52
transforms the residual video data into residual transform
coefficients using a transform, such as a discrete cosine transform
(DCT) or a conceptually similar transform.
[0045] Transform processing unit 52 may send the resulting
transform coefficients to quantization unit 54. Quantization unit
54 quantizes the transform coefficients to further reduce bit rate.
The quantization process may also reduce the bit depth associated
with some or all of the coefficients. The degree of quantization
may be modified by adjusting a quantization parameter. In some
examples, quantization unit 54 may then perform a scan of a matrix
including the quantized transform coefficients. Alternatively,
entropy encoding unit 56 may perform the scan.
[0046] Following quantization, entropy encoding unit 56 entropy
encodes the quantized transform coefficients into a video bitstream
using, e.g., context adaptive variable length coding (CAVLC),
context adaptive binary arithmetic coding (CABAC), syntax-based
context-adaptive binary arithmetic coding (SBAC), probability
interval partitioning entropy (PIPE) coding or another entropy
encoding methodology or technique. The encoded bitstream may then
be transmitted to video decoder 30, or archived in storage device
32 for later transmission to or retrieval by video decoder 30.
Entropy encoding unit 56 may also entropy encode the motion vectors
and the other syntax elements for the current video frame being
coded.
[0047] Inverse quantization unit 58 and inverse transform
processing unit 60 apply inverse quantization and inverse
transformation, respectively, to reconstruct the residual video
block in the pixel domain for generating a reference block for
prediction of other video blocks. As noted above, motion
compensation unit 44 may generate a motion compensated predictive
block from one or more reference blocks of the frames stored in DPB
64. Motion compensation unit 44 may also apply one or more
interpolation filters to the predictive block to calculate
sub-integer pixel values for use in motion estimation.
[0048] Summer 62 adds the reconstructed residual block to the
motion compensated predictive block produced by motion compensation
unit 44 to produce a reference block for storage in DPB 64. The
reference block may then be used by intra BC unit 48, motion
estimation unit 42 and motion compensation unit 44 as a predictive
block to inter predict another video block in a subsequent video
frame.
[0049] FIG. 3 is a block diagram illustrating an exemplary video
decoder 30 in accordance with some implementations of the present
application. Video decoder 30 includes video data memory 79,
entropy decoding unit 80, prediction processing unit 81, inverse
quantization unit 86, inverse transform processing unit 88, summer
90, and DPB 92. Prediction processing unit 81 further includes
motion compensation unit 82, intra prediction unit 84, and intra BC
unit 85. Video decoder 30 may perform a decoding process generally
reciprocal to the encoding process described above with respect to
video encoder 20 in connection with FIG. 2. For example, motion
compensation unit 82 may generate prediction data based on motion
vectors received from entropy decoding unit 80, while
intra-prediction unit 84 may generate prediction data based on
intra-prediction mode indicators received from entropy decoding
unit 80.
[0050] In some examples, a unit of video decoder 30 may be tasked
to perform the implementations of the present application. Also, in
some examples, the implementations of the present disclosure may be
divided among one or more of the units of video decoder 30. For
example, intra BC unit 85 may perform the implementations of the
present application, alone, or in combination with other units of
video decoder 30, such as motion compensation unit 82, intra
prediction unit 84, and entropy decoding unit 80. In some examples,
video decoder 30 may not include intra BC unit 85 and the
functionality of intra BC unit 85 may be performed by other
components of prediction processing unit 81, such as motion
compensation unit 82.
[0051] Video data memory 79 may store video data, such as an
encoded video bitstream, to be decoded by the other components of
video decoder 30. The video data stored in video data memory 79 may
be obtained, for example, from storage device 32, from a local
video source, such as a camera, via wired or wireless network
communication of video data, or by accessing physical data storage
media (e.g., a flash drive or hard disk). Video data memory 79 may
include a coded picture buffer (CPB) that stores encoded video data
from an encoded video bitstream. Decoded picture buffer (DPB) 92 of
video decoder 30 stores reference video data for use in decoding
video data by video decoder 30 (e.g., in intra or inter predictive
coding modes). Video data memory 79 and DPB 92 may be formed by any
of a variety of memory devices, such as dynamic random access
memory (DRAM), including synchronous DRAM (SDRAM),
magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types
of memory devices. For illustrative purpose, video data memory 79
and DPB 92 are depicted as two distinct components of video decoder
30 in FIG. 3. But it will be apparent to one skilled in the art
that video data memory 79 and DPB 92 may be provided by the same
memory device or separate memory devices. In some examples, video
data memory 79 may be on-chip with other components of video
decoder 30, or off-chip relative to those components.
[0052] During the decoding process, video decoder 30 receives an
encoded video bitstream that represents video blocks of an encoded
video frame and associated syntax elements. Video decoder 30 may
receive the syntax elements at the video frame level and/or the
video block level. Entropy decoding unit 80 of video decoder 30
entropy decodes the bitstream to generate quantized coefficients,
motion vectors or intra-prediction mode indicators, and other
syntax elements. Entropy decoding unit 80 then forwards the motion
vectors and other syntax elements to prediction processing unit
81.
[0053] When the video frame is coded as an intra predictive coded
(I) frame or for intra coded predictive blocks in other types of
frames, intra prediction unit 84 of prediction processing unit 81
may generate prediction data for a video block of the current video
frame based on a signaled intra prediction mode and reference data
from previously decoded blocks of the current frame.
[0054] When the video frame is coded as an inter-predictive coded
(i.e., B or P) frame, motion compensation unit 82 of prediction
processing unit 81 produces one or more predictive blocks for a
video block of the current video frame based on the motion vectors
and other syntax elements received from entropy decoding unit 80.
Each of the predictive blocks may be produced from a reference
frame within one of the reference frame lists. Video decoder 30 may
construct the reference frame lists, List 0 and List 1, using
default construction techniques based on reference frames stored in
DPB 92.
[0055] In some examples, when the video block is coded according to
the intra BC mode described herein, intra BC unit 85 of prediction
processing unit 81 produces predictive blocks for the current video
block based on block vectors and other syntax elements received
from entropy decoding unit 80. The predictive blocks may be within
a reconstructed region of the same picture as the current video
block defined by video encoder 20.
[0056] Motion compensation unit 82 and/or intra BC unit 85
determines prediction information for a video block of the current
video frame by parsing the motion vectors and other syntax
elements, and then uses the prediction information to produce the
predictive blocks for the current video block being decoded. For
example, motion compensation unit 82 uses some of the received
syntax elements to determine a prediction mode (e.g., intra or
inter prediction) used to code video blocks of the video frame, an
inter prediction frame type (e.g., B or P), construction
information for one or more of the reference frame lists for the
frame, motion vectors for each inter predictive encoded video block
of the frame, inter prediction status for each inter predictive
coded video block of the frame, and other information to decode the
video blocks in the current video frame.
[0057] Similarly, intra BC unit 85 may use some of the received
syntax elements, e.g., a flag, to determine that the current video
block was predicted using the intra BC mode, construction
information of which video blocks of the frame are within the
reconstructed region and should be stored in DPB 92, block vectors
for each intra BC predicted video block of the frame, intra BC
prediction status for each intra BC predicted video block of the
frame, and other information to decode the video blocks in the
current video frame.
[0058] Motion compensation unit 82 may also perform interpolation
using the interpolation filters as used by video encoder 20 during
encoding of the video blocks to calculate interpolated values for
sub-integer pixels of reference blocks. In this case, motion
compensation unit 82 may determine the interpolation filters used
by video encoder 20 from the received syntax elements and use the
interpolation filters to produce predictive blocks.
[0059] Inverse quantization unit 86 inverse quantizes the quantized
transform coefficients provided in the bitstream and entropy
decoded by entropy decoding unit 80 using the same quantization
parameter calculated by video encoder 20 for each video block in
the video frame to determine a degree of quantization. Inverse
transform processing unit 88 applies an inverse transform, e.g., an
inverse DCT, an inverse integer transform, or a conceptually
similar inverse transform process, to the transform coefficients in
order to reconstruct the residual blocks in the pixel domain.
[0060] After motion compensation unit 82 or intra BC unit 85
generates the predictive block for the current video block based on
the vectors and other syntax elements, summer 90 reconstructs
decoded video block for the current video block by summing the
residual block from inverse transform processing unit 88 and a
corresponding predictive block generated by motion compensation
unit 82 and intra BC unit 85. An in-loop filter (not pictured) may
be positioned between summer 90 and DPB 92 to further process the
decoded video block. The decoded video blocks in a given frame are
then stored in DPB 92, which stores reference frames used for
subsequent motion compensation of next video blocks. DPB 92, or a
memory device separate from DPB 92, may also store decoded video
for later presentation on a display device, such as display device
34 of FIG. 1.
[0061] In a typical video coding process, a video sequence
typically includes an ordered set of frames or pictures. Each frame
may include three sample arrays, denoted SL, SCb, and SCr. SL is a
two-dimensional array of luma samples. SCb is a two-dimensional
array of Cb chroma samples. SCr is a two-dimensional array of Cr
chroma samples. In other instances, a frame may be monochrome and
therefore includes only one two-dimensional array of luma
samples.
[0062] As shown in FIG. 4A, video encoder 20 (or more specifically
partition unit 45) generates an encoded representation of a frame
by first partitioning the frame into a set of coding tree units
(CTUs). A video frame may include an integer number of CTUs ordered
consecutively in a raster scan order from left to right and from
top to bottom. Each CTU is a largest logical coding unit and the
width and height of the CTU are signaled by the video encoder 20 in
a sequence parameter set, such that all the CTUs in a video
sequence have the same size being one of 128.times.128,
64.times.64, 32.times.32, and 16.times.16. But it should be noted
that the present application is not necessarily limited to a
particular size. As shown in FIG. 4B, each CTU may comprise one
coding tree block (CTB) of luma samples, two corresponding coding
tree blocks of chroma samples, and syntax elements used to code the
samples of the coding tree blocks. The syntax elements describe
properties of different types of units of a coded block of pixels
and how the video sequence can be reconstructed at the video
decoder 30, including inter or intra prediction, intra prediction
mode, motion vectors, and other parameters. In monochrome pictures
or pictures having three separate color planes, a CTU may comprise
a single coding tree block and syntax elements used to code the
samples of the coding tree block. A coding tree block may be an
N.times.N block of samples.
[0063] To achieve a better performance, video encoder 20 may
recursively perform tree partitioning such as binary-tree
partitioning, quad-tree partitioning or a combination of both on
the coding tree blocks of the CTU and divide the CTU into smaller
coding units (CUs). As depicted in FIG. 4C, the 64.times.64 CTU 400
is first divided into four smaller CU, each having a block size of
32.times.32. Among the four smaller CUs, CU 410 and CU 420 are each
divided into four CUs of 16.times.16 by block size. The two
16.times.16 CUs 430 and 440 are each further divided into four CUs
of 8.times.8 by block size. FIG. 4D depicts a quad-tree data
structure illustrating the end result of the partition process of
the CTU 400 as depicted in FIG. 4C, each leaf node of the quad-tree
corresponding to one CU of a respective size ranging from
32.times.32 to 8.times.8. Like the CTU depicted in FIG. 4B, each CU
may comprise a coding block (CB) of luma samples and two
corresponding coding blocks of chroma samples of a frame of the
same size, and syntax elements used to code the samples of the
coding blocks. In monochrome pictures or pictures having three
separate color planes, a CU may comprise a single coding block and
syntax structures used to code the samples of the coding block.
[0064] In some implementations, video encoder 20 may further
partition a coding block of a CU into one or more M.times.N
prediction blocks (PB). A prediction block is a rectangular (square
or non-square) block of samples on which the same prediction, inter
or intra, is applied. A prediction unit (PU) of a CU may comprise a
prediction block of luma samples, two corresponding prediction
blocks of chroma samples, and syntax elements used to predict the
prediction blocks. In monochrome pictures or pictures having three
separate color planes, a PU may comprise a single prediction block
and syntax structures used to predict the prediction block. Video
encoder 20 may generate predictive luma, Cb, and Cr blocks for
luma, Cb, and Cr prediction blocks of each PU of the CU.
[0065] Video encoder 20 may use intra prediction or inter
prediction to generate the predictive blocks for a PU. If video
encoder 20 uses intra prediction to generate the predictive blocks
of a PU, video encoder 20 may generate the predictive blocks of the
PU based on decoded samples of the frame associated with the PU. If
video encoder 20 uses inter prediction to generate the predictive
blocks of a PU, video encoder 20 may generate the predictive blocks
of the PU based on decoded samples of one or more frames other than
the frame associated with the PU.
[0066] After video encoder 20 generates predictive luma, Cb, and Cr
blocks for one or more PUs of a CU, video encoder 20 may generate a
luma residual block for the CU by subtracting the CU's predictive
luma blocks from its original luma coding block such that each
sample in the CU's luma residual block indicates a difference
between a luma sample in one of the CU's predictive luma blocks and
a corresponding sample in the CU's original luma coding block.
Similarly, video encoder 20 may generate a Cb residual block and a
Cr residual block for the CU, respectively, such that each sample
in the CU's Cb residual block indicates a difference between a Cb
sample in one of the CU's predictive Cb blocks and a corresponding
sample in the CU's original Cb coding block and each sample in the
CU's Cr residual block may indicate a difference between a Cr
sample in one of the CU's predictive Cr blocks and a corresponding
sample in the CU's original Cr coding block.
[0067] Furthermore, as illustrated in FIG. 4C, video encoder 20 may
use quad-tree partitioning to decompose the luma, Cb, and Cr
residual blocks of a CU into one or more luma, Cb, and Cr transform
blocks. A transform block is a rectangular (square or non-square)
block of samples on which the same transform is applied. A
transform unit (TU) of a CU may comprise a transform block of luma
samples, two corresponding transform blocks of chroma samples, and
syntax elements used to transform the transform block samples.
Thus, each TU of a CU may be associated with a luma transform
block, a Cb transform block, and a Cr transform block. In some
examples, the luma transform block associated with the TU may be a
sub-block of the CU's luma residual block. The Cb transform block
may be a sub-block of the CU's Cb residual block. The Cr transform
block may be a sub-block of the CU's Cr residual block. In
monochrome pictures or pictures having three separate color planes,
a TU may comprise a single transform block and syntax structures
used to transform the samples of the transform block.
[0068] Video encoder 20 may apply one or more transforms to a luma
transform block of a TU to generate a luma coefficient block for
the TU. A coefficient block may be a two-dimensional array of
transform coefficients. A transform coefficient may be a scalar
quantity. Video encoder 20 may apply one or more transforms to a Cb
transform block of a TU to generate a Cb coefficient block for the
TU. Video encoder 20 may apply one or more transforms to a Cr
transform block of a TU to generate a Cr coefficient block for the
TU.
[0069] After generating a coefficient block (e.g., a luma
coefficient block, a Cb coefficient block or a Cr coefficient
block), video encoder 20 may quantize the coefficient block.
Quantization generally refers to a process in which transform
coefficients are quantized to possibly reduce the amount of data
used to represent the transform coefficients, providing further
compression. After video encoder 20 quantizes a coefficient block,
video encoder 20 may entropy encode syntax elements indicating the
quantized transform coefficients. For example, video encoder 20 may
perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the
syntax elements indicating the quantized transform coefficients.
Finally, video encoder 20 may output a bitstream that includes a
sequence of bits that forms a representation of coded frames and
associated data, which is either saved in storage device 32 or
transmitted to destination device 14.
[0070] After receiving a bitstream generated by video encoder 20,
video decoder 30 may parse the bitstream to obtain syntax elements
from the bitstream. Video decoder 30 may reconstruct the frames of
the video data based at least in part on the syntax elements
obtained from the bitstream. The process of reconstructing the
video data is generally reciprocal to the encoding process
performed by video encoder 20. For example, video decoder 30 may
perform inverse transforms on the coefficient blocks associated
with TUs of a current CU to reconstruct residual blocks associated
with the TUs of the current CU. Video decoder 30 also reconstructs
the coding blocks of the current CU by adding the samples of the
predictive blocks for PUs of the current CU to corresponding
samples of the transform blocks of the TUs of the current CU. After
reconstructing the coding blocks for each CU of a frame, video
decoder 30 may reconstruct the frame.
[0071] As noted above, video coding achieves video compression
using primarily two modes, i.e., intra-frame prediction (or
intra-prediction) and inter-frame prediction (or inter-prediction).
It is noted that IBC could be regarded as either intra-frame
prediction or a third mode. Between the two modes, inter-frame
prediction contributes more to the coding efficiency than
intra-frame prediction because of the use of motion vectors for
predicting a current video block from a reference video block.
[0072] But with the ever improving video data capturing technology
and more refined video block size for preserving details in the
video data, the amount of data required for representing motion
vectors for a current frame also increases substantially. One way
of overcoming this challenge is to benefit from the fact that not
only a group of neighboring CUs in both the spatial and temporal
domains have similar video data for predicting purpose but the
motion vectors between these neighboring CUs are also similar.
Therefore, it is possible to use the motion information of
spatially neighboring CUs and/or temporally collocated CUs as an
approximation of the motion information (e.g., motion vector) of a
current CU by exploring their spatial and temporal correlation,
which is also referred to as "motion vector predictor" (MVP) of the
current CU.
[0073] Instead of encoding, into the video bitstream, an actual
motion vector of the current CU determined by motion estimation
unit 42 as described above in connection with FIG. 2, the motion
vector predictor of the current CU is subtracted from the actual
motion vector of the current CU to produce a motion vector
difference (MVD) for the current CU. By doing so, there is no need
to encode the motion vector determined by motion estimation unit 42
for each CU of a frame into the video bitstream and the amount of
data used for representing motion information in the video
bitstream can be significantly decreased.
[0074] Like the process of choosing a predictive block in a
reference frame during inter-frame prediction of a code block, a
set of rules need to be adopted by both video encoder 20 and video
decoder 30 for constructing a motion vector candidate list (also
known as a "merge list") for a current CU using those potential
candidate motion vectors associated with spatially neighboring CUs
and/or temporally collocated CUs of the current CU and then
selecting one member from the motion vector candidate list as a
motion vector predictor for the current CU. By doing so, there is
no need to transmit the motion vector candidate list itself between
video encoder 20 and video decoder 30 and an index of the selected
motion vector predictor within the motion vector candidate list is
sufficient for video encoder 20 and video decoder 30 to use the
same motion vector predictor within the motion vector candidate
list for encoding and decoding the current CU.
[0075] In some implementations, each inter-prediction CU has three
motion vector prediction modes including inter (which is also
referred to as "advanced motion vector prediction" (AMVP)), skip,
and merge for constructing the motion vector candidate list. Under
each mode, one or more motion vector candidates may be added to the
motion vector candidate list according to the algorithms described
below. Ultimately one of them in the candidate list is used as the
best motion vector predictor of the inter-prediction CU to be
encoded into the video bitstream by video encoder 20 or decoded
from the video bitstream by video decoder 30. To find the best
motion vector predictor from the candidate list, a motion vector
competition (MVC) scheme is introduced to select a motion vector
from a given candidate set of motion vectors, i.e., the motion
vector candidate list, that includes spatial and temporal motion
vector candidates.
[0076] In addition to deriving motion vector predictor candidates
from spatially neighboring or temporally collocated CUs, the motion
vector predictor candidates can also be derived from the so-called
"history-based motion vector prediction" (HMVP) table. The HMVP
table houses a predefined number of motion vector predictors, each
having been used for encoding/decoding a particular CU of the same
row of CTUs (or sometimes the same CTU). Because of the
spatial/temporal proximity of these CUs, there is a high likelihood
that one of the motion vector predictors in the HMVP table may be
reused for encoding/decoding different CUs within the same row of
CTUs. Therefore, it is possible to achieve a higher code efficiency
by including the HMVP table in the process of constructing the
motion vector candidate list.
[0077] In some implementations, the HMVP table has a fixed length
(e.g., 5) and is managed in a quasi-First-In-First-Out (FIFO)
manner. For example, a motion vector is reconstructed for a CU when
decoding one inter-coded block of the CU. The HMVP table is updated
on-the-fly with the reconstructed motion vector because such motion
vector could be the motion vector predictor of a subsequent CU.
When updating the HMVP table, there are two scenarios: (i) the
reconstructed motion vector is different from other existing motion
vectors in the HMVP table or (ii) the reconstructed motion vector
is the same as one of the existing motion vectors in the HMVP
table. For the first scenario, the reconstructed motion vector is
added to the HMVP table as the newest one if the HMVP table is not
full. If the HMVP table is already full, the oldest motion vector
in the HMVP table needs to be removed from the HMVP table first
before the reconstructed motion vector is added as the newest one.
In other words, the HMVP table in this case is similar to a FIFO
buffer such that the motion information located at the head of the
FIFO buffer and associated with another previously inter-coded
block is shifted out of the buffer so that the reconstructed motion
vector is appended to the tail of the FIFO buffer as the newest
member in the HMVP table. For the second scenario, the existing
motion vector in the HMVP table that is substantially identical to
the reconstructed motion vector is removed from the HMVP table
before the reconstructed motion vector is added to the HMVP table
as the newest one. If the HMVP table is also maintained in the form
of a FIFO buffer, the motion vector predictors after the identical
motion vector in the HMVP table are shifted forward by one element
to occupy the space left by the removed motion vector and the
reconstructed motion vector is then appended to the tail of the
FIFO buffer as the newest member in the HMVP table.
[0078] The motion vectors in the HMVP table could be added to the
motion vector candidate lists under different prediction modes such
as AMVP, merge, skip, etc. It has been found that the motion
information of previously inter-coded blocks stored in the HMVP
table even not adjacent to the current block can be utilized for
more efficient motion vector prediction.
[0079] After one MVP candidate is selected within the given
candidate set of motion vectors for a current CU, video encoder 20
may generate one or more syntax elements for the corresponding MVP
candidate and encode them into the video bitstream such that video
decoder 30 can retrieve the MVP candidate from the video bitstream
using the syntax elements. Depending on the specific mode used for
constructing the motion vectors candidate set, different modes
(e.g., AMVP, merge, skip, etc.) have different sets of syntax
elements. For the AMVP mode, the syntax elements include inter
prediction indicators (List 0, List 1, or bi-directional
prediction), reference indices, motion vector candidate indices,
motion vector prediction residual signal, etc. For the skip mode
and the merge mode, only merge indices are encoded into the
bitstream because the current CU inherits the other syntax elements
including the inter prediction indicators, reference indices, and
motion vectors from a neighboring CU referred by the coded merge
index. In the case of a skip coded CU, the motion vector prediction
residual signal is also omitted.
[0080] FIG. 5A is a block diagram illustrating spatially
neighboring and temporally collocated block positions of a current
CU to be encoded/decoded in accordance with some implementations of
the present disclosure. For a given mode, a motion vector
prediction (MVP) candidate list is constructed by first checking
the availability of motion vectors associated with the spatially
left and above neighboring block positions, and the availability of
motion vectors associated with temporally collocated block
positions and then the motion vectors in the HMVP table. During the
process of constructing the MVP candidate list, some redundant MVP
candidates are removed from the candidate list and, if necessary,
zero-valued motion vector is added to make the candidate list to
have a fixed length (note that different modes may have different
fixed lengths). After the construction of the MVP candidate list,
video encoder 20 can select the best motion vector predictor from
the candidate list and encode the corresponding index indicating
the chosen candidate into the video bitstream.
[0081] Using FIG. 5A as an example and assuming that the candidate
list has a fixed length of two, the motion vector predictor (MVP)
candidate list for the current CU may be constructed by performing
the following steps in order under the AMVP mode: [0082] 1)
Selection of MVP candidates from spatially neighboring CUs [0083]
a) Derive up to one non-scaled MVP candidate from one of the two
left spatial neighbour CUs starting with A0 and ending with A1;
[0084] b) If no non-scaled MVP candidate from left is available in
the previous step, derive up to one scaled MVP candidate from one
of the two left spatial neighbour CUs starting with A0 and ending
with A1; [0085] c) Derive up to one non-scaled MVP candidate from
one of the three above spatial neighbour CUs starting with B0, then
B1, and ending with B2; [0086] d) If neither A0 nor A1 is available
or if they are coded in intra modes, derive up to one scaled MVP
candidate from one of the three above spatial neighbour CUs
starting with B0, then B1, and ending with B2; [0087] 2) If two MVP
candidates are found in the previous steps and they are identical,
remove one of the two candidates from the MVP candidate list;
[0088] 3) Selection of MVP candidates from temporally collocated
CUs [0089] a) If the MVP candidate list after the previous step
does not include two MVP candidates, derive up to one MVP candidate
from the temporal collocated CUs (e.g., T0) [0090] 4) Selection of
MVP candidates from the HMVP table [0091] a) If the MVP candidate
list after the previous step does not include two MVP candidates,
derive up to two history-based MVP from the HMVP table; and [0092]
5) If the MVP candidate list after the previous step does not
include two MVP candidates, add up to two zero-valued MVPs to the
MVP candidate list.
[0093] Since there are only two candidates in the AMVP-mode MVP
candidate list constructed above, an associated syntax element like
a binary flag is encoded into the bitstream to indicate that which
of the two MVP candidates within the candidate list is used for
decoding the current CU.
[0094] In some implementations, the MVP candidate list for the
current CU under the skip or merge mode may be constructed by
performing a similar set of steps in order like the ones above. It
is noted that one special kind of merge candidate called "pair-wise
merge candidate" is also included into the MVP candidate list for
the skip or merge mode. The pair-wise merge candidate is generated
by averaging the MVs of the two previously derived merge-mode
motion vector candidates. The size of the merge MVP candidate list
(e.g., from 1 to 6) is signaled in a slice header of the current
CU. For each CU in the merge mode, an index of the best merge
candidate is encoded using truncated unary binarization (TU). The
first bin of the merge index is coded with context and bypass
coding is used for other bins.
[0095] As mentioned above, the history-based MVPs can be added to
either the AMVP-mode MVP candidate list or the merge MVP candidate
list after the spatial MVP and temporal MVP. The motion information
of a previously inter-coded CU is stored in the HMVP table and used
as a MVP candidate for the current CU. The HMVP table is maintained
during the encoding/decoding process. Whenever there is a
non-sub-block inter-coded CU, the associated motion vector
information is added to the last entry of the HMVP table as a new
candidate while the motion vector information stored in the first
entry of the HMVP table is removed from therein (if the HMVP table
is already full and there is no identical duplicate of the
associated motion vector information in the table). Alternatively,
the identical duplicate of the associated motion vector information
is removed from the table before the associated motion vector
information is added to the last entry of the HMVP table.
[0096] As noted above, intra block copy (IBC) can significantly
improve the coding efficiency of screen content materials. Since
IBC mode is implemented as a block-level coding mode, block
matching (BM) is performed at video encoder 20 to find an optimal
block vector for each CU. Here, a block vector is used to indicate
the displacement from the current block to a reference block, which
has already been reconstructed within the current picture. An IBC
mode is treated as the third prediction mode other than the intra
or inter prediction modes.
[0097] At the CU level, the IBC mode can be signaled as IBC AMVP
mode or IBC skip/merge mode as follows: [0098] IBC AMVP mode: a
block vector difference (BVD) between the actual block vector of a
CU and a block vector predictor of the CU selected from block
vector candidates of the CU is encoded in the same way as a motion
vector difference is encoded under the AMVP mode described above.
The block vector prediction method uses two block vector candidates
as predictors, one from left neighbor and the other one from above
neighbor (if IBC coded). When either neighbor is not available, a
default block vector will be used as a block vector predictor. A
binary flag is signaled to indicate the block vector predictor
index. The IBC AMVP candidate list consists of spatial and HMVP
candidates. [0099] IBC skip/merge mode: a merge candidate index is
used to indicate which of the block vector candidates in the merge
candidate list (also known as a "merge list") from neighboring IBC
coded blocks is used to predict the block vector for the current
block. The IBC merge candidate list consists of spatial, HMVP, and
pairwise candidates.
[0100] Another approach of improving the coding efficiency adopted
by the state-of-art coding standard is to introduce the parallel
processing to the video encoding/decoding process using, e.g., a
multi-core processor. For example, wavefront parallel processing
(WPP) has already been introduced into HEVC as a feature of
encoding or decoding of multiple rows CTUs in parallel using
multiple threads.
[0101] FIG. 5B is a block diagram illustrating multi-threaded
encoding of multiple rows of CTUs of a picture using wavefront
parallel processing (WPP) in accordance with some implementations
of the present disclosure. When WPP is enabled, it is possible to
process multiple rows of CTUs in parallel in a wavefront fashion,
where there may be a delay of two CTUs between the start of two
neighboring wavefronts. For example, to code the picture 500 using
WPP, a video coder, such as video encoder 20 and video decoder 30,
may divide the coding tree units (CTUs) of the picture 500 into a
plurality of wavefronts, each wavefront corresponding to a
respective row of CTUs in the picture. The video coder may start
coding a top wavefront, e.g., using a first coder core or thread.
After the video coder has coded two or more CTUs of the top
wavefront, the video coder may start coding a second-to-top
wavefront in parallel with coding the top wavefront, e.g., using a
second, parallel coder core or thread. After the video coder has
coded two or more CTUs of the second-to-top wavefront, the video
coder may start coding a third-to-top wavefront in parallel with
coding the higher wavefronts, e.g., using a third, parallel coder
core or thread. This pattern may continue down the wavefronts in
the picture 500. In the present disclosure, a set of CTUs that a
video coder is concurrently coding, using WPP, is referred to as a
"CTU group." Thus, when the video coder uses WPP to code a picture,
each CTU of the CTU group may belong to a unique wavefront of the
picture and the CTU may be offset from a CTU in a respective, above
wavefront by at least two columns of CTUs of the picture.
[0102] The video coder may initialize a context for a current
wavefront for performing context adaptive binary arithmetic coding
(CABAC) of the current wavefront based on data of the first two
blocks of the above wavefront, as well as one or more elements of a
slice header for a slice including the first code block of the
current wavefront. The video coder may perform CABAC initialization
of a subsequent wavefront (or CTU row) using the context states
after coding two CTUs of a CTU row above the subsequent CTU row. In
other words, before beginning coding of a current wavefront, a
video coder (or more specifically, a thread of the video coder) may
code at least two blocks of a wavefront above the current
wavefront, assuming the current wavefront is not the top row of
CTUs of a picture. The video coder may then initialize a CABAC
context for the current wavefront after coding at least two blocks
of a wavefront above the current wavefront. In this example, each
CTU row of the picture 500 is a separated partition and has an
associated thread (WPP Thread 1, WPP Thread 2, . . . ) such that
the number of CTU rows in the picture 500 can be encoded in
parallel.
[0103] Because the current implementation of the HMVP table uses a
global motion vector (MV) buffer to store previously reconstructed
motion vectors, this HMVP table cannot be implemented on the
WPP-enabled parallel encoding scheme described above in connection
with FIG. 5B. In particular, the fact that the global MV buffer is
shared by all the threads of the encoding/decoding process of a
video coder prevents the WPP threads after the first WPP thread
(i.e., WPP Thread 1) from being started since these WPP threads
have to wait for the HMVP table update from the last CTU (i.e.,
rightmost CTU) of the first WPP thread (i.e., the first CTU row) to
be completed.
[0104] To overcome the problem, it is proposed that the global MV
buffer shared by the WPP threads be replaced with multiple CTU
row-dedicated buffers such that each wavefront of CTU row has its
own buffer for storing an HMVP table corresponding to the CTU row
being processed by a corresponding WPP thread when WPP is enabled
at the video coder. It is noted that each CTU row having its own
HMVP table is equivalent to resetting the HMVP table before coding
a first CU of the CTU row. The HMVP table reset is to flush out all
the motion vectors in the HMVP table resulting from coding of
another CTU row. In one implementation, the reset operation is to
set the size of the available motion vector predictors in the HMVP
table to be zero. In yet another implementation, the reset
operations could be to set the reference index of all the entries
in the HMVP table to be an invalid value such as -1. By doing so,
the construction of MVP candidate list for a current CTU within a
particular wavefront, regardless of which one of the three modes,
AMVP, merge, and skip, is dependent upon an HMVP table associated
with a WPP thread processing the particular wavefront. There is no
inter-dependency between different wavefronts other than the
two-CTU delay described above and the construction of motion vector
candidate lists associated with different wavefronts can proceed in
parallel like the WPP process depicted in FIG. 5B. In other words,
at the beginning of processing a particular wavefront, the HMVP
table is reset to be empty without affecting the coding of another
wavefront of CTUs by another WPP thread. In some cases, the HMVP
table can be reset to be empty before the coding of each individual
CTU. In this case, the motion vectors in the HMVP table are limited
to a particular CTU and there is probably a higher chance of a
motion vector within the HMVP table being selected as a motion
vector of a current CU within the particular CTU.
[0105] FIGS. 6A and 6B are block diagrams illustrating an exemplary
reconstructed luma block 602 and an exemplary associated chroma
block 620, respectively, in accordance with some implementations of
the present disclosure. In this example, luma samples of
reconstructed luma block 602 (e.g., luma sample 604), top
neighboring luma group 606 (e.g., luma sample 608), and left
neighboring luma group 610 (e.g., luma sample 613) have been
predicted during a video coding process. Chroma samples of chroma
block 620 are to be predicted, while chroma samples of top
neighboring chroma group 624 (e.g., chroma sample 626) and left
neighboring chroma group 628 (e.g., chroma sample 630) have been
predicted during the video coding process. In some embodiments,
chroma samples of chroma block 620 can be predicted by applying a
cross component linear model (CCLM) to the corresponding
down-sampled luma samples of reconstructed luma block 602.
Derivation and application of the CCLM is provided below in
connection with FIGS. 7A-7E.
[0106] In some embodiments, reconstructed luma block 602 and chroma
block 620 each represent a different component of a portion of a
reconstructed video frame. For example, in the YCbCr color space,
an image is represented by a luma component (Y), a blue-difference
chroma component (Cb), and a red-difference chroma component (Cr).
Reconstructed luma block 602 represents the luma component (i.e.,
brightness) of a portion of the video frame, and chroma block 620
represents a chroma component (i.e., color) of the same portion of
the video frame. A luma sample (e.g., luma sample 604) of
reconstructed luma block 602 has a luma value representing the
brightness at a particular pixel of the video frame, and a chroma
sample (e.g., chroma sample 622) has a chroma value representing
the color at a particular pixel of the video frame.
[0107] In some embodiments, reconstructed luma block 602 is a
2M.times.2N block with 2M luma samples across the block width and
2N luma samples across the block height. M and N can be the same
value (e.g., reconstructed luma block 602 is a square block) or
different values (e.g., reconstructed luma block 602 is a
non-square block).
[0108] Chroma subsampling is a common compression technique as
human visual system is less sensitive to the color difference than
to brightness difference. As a result, reconstructed luma block 602
and chroma block 620 may represent the same portion of a video
frame but are encoded with different resolutions. For example, the
video frame may have been encoded using a chroma subsampling scheme
(e.g., 4:2:0 or 4:2:2) to encode for chroma information than for
luma information with less resolution. As illustrated in FIGS. 6A
and 6B, reconstructed luma block 602 is encoded with a resolution
of 2M.times.2N, while chroma block 620 is encoded with a smaller
resolution of M.times.N. In practice, chroma block 620 can have
other resolution such as 2M.times.2N (e.g., 4:4:4 full sampling),
2M.times.N (e.g., 4:4:0 sub-sampling), M.times.2N (e.g., 4:2:2
sub-sampling), and 1/2M.times.2N (e.g., 4:1:1 sub-sampling).
[0109] Reconstructed luma block 602 is next to top neighboring luma
group 606 and left neighboring luma group 610. The size of the top
neighboring luma group and left neighboring luma group can be
explicitly signaled or dependent upon the size of reconstructed
luma block 602. For example, top neighboring luma group 606 can
have a width of 2M samples (e.g., same as the width of
reconstructed luma block 602) or 4M samples (e.g., double that of
the width of reconstructed luma block 602), and a height of 2
samples. Left neighboring luma group 610 can have a width of 2
samples, with a height of 2N or 4N samples. In some embodiments,
top neighboring luma group 606 and left neighboring luma group 610
are each a portion of another luma block or blocks of the same
video frame that have been reconstructed.
[0110] Chroma block 620 is next to top neighboring chroma group 624
and left neighboring group 628. The size of top neighboring chroma
group 624 and left neighboring group 628 can be explicitly signaled
or dependent upon the size of chroma block 620. For example, top
neighboring chroma group 624 can have a size of 1.times.M, and left
neighboring chroma group 628 can have a size of N.times.1.
[0111] In some embodiments, chroma values (e.g., chroma values for
chroma block 620) can be predicted based on luma values of
reconstructed luma samples (e.g., luma sample 604). For example,
under the assumption that there exists a linear or quasi-linear
relation between luma values and corresponding chroma values of a
video frame, a video codec can predict chroma values based on
corresponding reconstructed luma values using the CCLM. By doing
so, the video codec can save a significant amount of time and
bandwidth for encoding the chroma values, transmitting the encoded
chroma values, and decoding the encoded chroma values. To use the
CCLM to predict chroma samples from luma samples, the video codec
(1) derives a linear model between the chroma samples and the luma
samples, and (2) applies the linear model to reconstructed luma
samples that correspond to the chroma samples to be predicted.
[0112] In some embodiments, since luma blocks and chroma blocks are
of different resolutions (e.g., the chroma blocks having been
sub-sampled), the video codec first performs down-sampling on luma
samples to generate down-sampled luma samples (e.g., down-sampled
luma sample 605, 609, and 612) that uniquely correspond to
respective chroma samples. In some embodiments, six neighboring
reconstructed luma samples in both height and width direction of
the video frame are used to generate a down-sampled luma sample
(e.g., weighted-averaging schemes known in the art including
six-tap down-sampling or like). For example, the six reconstructed
luma samples within region 611 (each represented by a small box in
the figure) inside top neighboring luma group are used to generate
down-sampled luma sample 609 through averaging of their
corresponding luma values, and the six reconstructed luma samples
within region 607 (each represented by a small box in the figure)
inside reconstructed luma block 602 are used to generate
down-sampled luma sample 605. Alternatively, a down-sampled luma
sample is generated by identifying a reconstructed luma sample in a
region of interest, or using a different number of reconstructed
luma samples in a region of a different shape.
[0113] In some embodiments, to drive the linear model, the video
codec uses a Max-Min method by identifying maximum and minimum of
down-sampled luma samples (e.g., the down-sampled luma samples
having the maximum and the minimum luma values, respectively) and
the corresponding reconstructed chroma samples, and fitting a
linear model (e.g., Y=.alpha.X+.beta.) through the maximum and
minimum data points (e.g., the maximum data point includes the
down-sampled luma sample having the maximum luma value and the
corresponding reconstructed chroma sample, and the minimum data
point includes the down-sampled luma sample having the minimum luma
value and the corresponding reconstructed chroma sample). After the
linear model is derived, the video codec applies the linear model
to down-sampled luma samples in reconstructed luma block 602 to
generate corresponding chroma samples of chroma block 620. The
video codec can obtain the max and min data points in the following
way:
[0114] 1. In some embodiments, the video codec searches a group of
down-sampled luma samples (e.g., a selected group of down-sampled
luma samples in top neighboring luma group 606 and left neighboring
luma group 610) to identify the maximum down-sampled luma sample
and the minimum down-sampled luma sample. The video codec then
identifies previously-reconstructed chroma samples (e.g.,
reconstructed chroma samples in top neighboring chroma group 624
and left neighboring chroma group 628) corresponding to the maximum
and minimum down-sampled luma samples, as described below in
connection with FIG. 7A and the related description for detail on
this implementation.
[0115] 2. In some embodiments, the video codec searches a group of
reconstructed luma samples (e.g., a selected group of reconstructed
luma samples in top neighboring luma group 606 and left neighboring
luma group 610) to identify (i) a reconstructed luma sample having
the maximum luma value and (ii) a reconstructed luma sample having
the minimum luma value in the selected group of reconstructed luma
samples without performing down-sampling on the selected group of
reconstructed luma samples to identify the maximum and minimum
reconstructed luma samples. The video codec then performs
down-sampling in a region (e.g., a region with six samples using
weighted-averaging schemes known in the art including six-tap
down-sampling or like) associated with the maximum and the minimum
reconstructed luma samples to generate a down-sampled luma sample
as the maximum reconstructed luma sample (which may or may not be
exactly the maximum down-sampled luma sample) and a down-sampled
luma sample as the min reconstructed luma sample (which may or may
not be exactly the minimum down-sampled luma sample). The video
codec then identifies a reconstructed chroma sample (e.g., in top
neighboring chroma group 624 and left neighboring chroma group 628)
corresponding to the down-sampled luma sample identified as the
maximum reconstructed luma sample, and a reconstructed chroma
sample corresponding to the down-sampled luma sample identified as
the minimum reconstructed luma sample, as described below in
connection with FIG. 7B and the related description for detail on
this implementation.
[0116] 3. In some embodiments, the video codec searches a group of
reconstructed chroma samples (e.g., selected chroma samples in top
neighboring chroma group 624 and left neighboring chroma group 628)
to identify the maximum and the minimum reconstructed chroma
samples (e.g., the chroma samples having the maximum and minimum
chroma values, respectively). The video codec then identifies the
down-sampled luma samples (e.g., down-sampled luma samples in top
neighboring luma group 606 and left neighboring luma group 610)
corresponding to the maximum and min reconstructed chroma samples,
as describe below in connection with FIG. 7C and the related
description for detail on this implementation.
[0117] 4. In some embodiments, the video codec searches a group of
down-sampled luma samples (e.g., a selected group of down-sampled
luma samples in top neighboring luma group 606 and left neighboring
luma group 610) to identify a predefined number (e.g., two) of
down-sampled luma samples having the largest luma values) and a
predefined number (e.g., two) of down-sampled luma samples having
the smallest luma values. The video codec then identifies
reconstructed chroma samples in top neighboring chroma group 624
and left neighboring chroma group 628, respectively, each
corresponding to a respective one of the group of maximum
down-sampled luma samples and the group of minimum down-sampled
luma samples, respectively. The video codec then performs a
weighted average of values (e.g., chroma or luma values) within
each of the identified groups of reconstructed chroma and
down-sampled luma samples to generate a maximum averaged chroma
value, a minimum averaged chroma value, a maximum averaged
down-sampled luma value (e.g., from the group of maximum
down-sampled luma samples), and a minimum averaged down-sampled
luma value (e.g., from the group of minimum down-sampled luma
samples), as described below in connection with FIGS. 7D and 7E and
the related description for detail on this implementation.
[0118] FIGS. 7A-7E illustrate various approaches of using the CCLM
to derive a linear model between luma values and chroma values in
accordance with some implementations of the present disclosure. In
particular, each circle data point (e.g., point 702a) on a plot
represents a pair of reconstructed chroma sample in the horizontal
axis and a corresponding reconstructed luma sample in the vertical
axis. For example, a reconstructed chroma sample corresponds to a
reconstructed luma sample if a down-sampled luma sample generated
in part using the reconstructed luma sample (e.g., using
weighted-averaging schemes known in the art including six-tap
down-sampling or like) corresponds to the reconstructed chroma
sample. Each square data point (e.g., point 702b) on the plot
represents a pair of reconstructed chroma sample in the horizontal
axis and a corresponding down-sampled luma sample in the vertical
axis. In some embodiments, since multiple reconstructed luma
samples are used to generate a down-sampled luma sample (e.g.,
using the weighted-averaging schemes known in the art including
six-tap down-sampling or like), a square data point is associated
with multiple circle data points. The dotted rectangle (e.g., the
dotted rectangle 703) indicates that the encompassed square data
point and the circle data points are associated (e.g., the
down-sampled luma sample corresponding to the square data point is
generated from the reconstructed luma samples corresponding to the
circle data points). For illustrative purpose, only one square data
point and one circle data point are shown inside a dotted
rectangle, although in actual practice, each dotted rectangle can
include multiple circle data points and one square data point.
[0119] In some embodiments, the video codec searches the
down-sampled luma samples (e.g., down-sampled luma samples 609 and
612 of FIG. 6A) to identify down-sampled luma samples having the
maximum and minimum luma values and corresponding reconstructed
chroma samples to derive the linear model. For example, in FIG. 7A,
square data point 702b represents a down-sampled luma sample having
the minimum luma value of the selected group of down-sampled luma
samples and a corresponding reconstructed chroma sample, and square
data point 704b represents a down-sampled luma sample having the
maximum luma value and a corresponding reconstructed chroma sample.
As a result, a line fitting through point 702b and point 704b
represents the linear model to predict chroma samples (e.g., chroma
sample 622 of chroma block 620 in FIG. 6B) from down-sampled luma
samples (e.g., down-sampled luma sample 605 of luma block 602 in
FIG. 6A).
[0120] Generating the down-sampled luma samples is computationally
intensive as most of the down-sampled luma samples are used in a
very little manner in the Max-Min method. In some embodiments,
instead of performing down-sampling of all the luma samples, the
video codec directly searches reconstructed luma samples (e.g.,
reconstructed luma sample 608 and 613 of FIG. 6A) to identify
reconstructed luma samples having the maximum and minimum luma
values. As illustrated in FIG. 7B, circle data point 702a
represents a reconstructed luma sample having the minimum luma
value and a corresponding reconstructed chroma sample, and circle
data point 706a represents a reconstructed luma sample having the
maximum luma value and a corresponding reconstructed chroma sample.
After identifying the minimum and maximum reconstructed luma
samples (circle data point 702a and 706a), the video codec then
performs down-sampling in a region including the minimum and
maximum reconstructed luma samples (e.g., using weighted-averaging
schemes known in the art including six-tap down-sampling or like)
to generate corresponding quasi minimum and maximum down-sampled
luma samples (represented by square data points 702b and 706b in
FIG. 7B, which may or may not be the same as square data points
702b and 704b in FIG. 7A). A line fitting through square data
points 702b and 706b in FIG. 7B represents the linear model to
predict chroma samples from reconstructed luma samples. Compared to
the method used in FIG. 7A, only two down-sampling operations are
performed. The identified min down-sampled luma sample is the same
as the one used in FIG. 7A, while the max down-sampled luma sample
is different from the one used in FIG. 7A.
[0121] In some embodiments, the video codec generates the linear
model using the reconstructed luma samples with the maximum and
minimum luma values, and forgoes performing down-sampling. In FIG.
7C, no down-sampling is performed on reconstructed luma samples,
and a linear model is generated by fitting a line through circle
data point 702a and 706a directly.
[0122] In some embodiments, instead of searching for down-sampled
luma samples (or reconstructed luma samples) with maximum and
minimum luma values, the video codec first searches for
reconstructed chroma samples having the maximum and minimum chroma
values. Once the maximum and minimum reconstructed chroma samples
are identified, the video codec then computes the corresponding
down-sampled luma samples to generate the linear model. In FIG. 7D,
circle data point 708a represents the reconstructed chroma sample
having the minimum chroma value, and circle data point 704a
represents the reconstructed chroma sample having the maximum
chroma value. The video codec then generates a linear model fitting
through square data point 708b (which represents a down-sampled
luma sample generated in part using the reconstructed luma sample
from circle data point 708a) and square data point 704b (which
represents a down-sampled luma sample generated in part using the
reconstructed luma sample from circle data point 704a).
[0123] In some embodiments, the video codec selects a number (e.g.,
two) of down-sampled luma samples having the largest luma values,
and a number (e.g., two) of down-sampled luma samples having the
smallest luma values. The video codec then and searches for the
corresponding maximum and minimum groups of reconstructed chroma
samples. The video codec performs an averaging operation within
each group, and uses the averaged luma and chroma values to
generate a linear model. In FIG. 7E, the largest two down-sampled
luma samples (square data points 710b and 704b) and the smallest
two down-sampled luma samples (square data points 708b and square
data points 702b) are used to generate the linear model.
[0124] FIG. 8 is a flowchart illustrating an exemplary process 700
by which a video codec implements the techniques of using a
cross-component linear model to reconstruct chroma samples for a
chroma block based on reconstructed luma samples from a luma block.
Process 700 can be implemented during either a decoding process or
an encoding process.
[0125] As the first step, the video codec reconstructs a luma block
corresponding to a chroma block (810). The chroma block (e.g.,
chroma block 620 of FIG. 6B) is to be subsequently reconstructed
from the reconstructed luma block and may be of a different
resolution different from that of the reconstructed luma block
(e.g., reconstructed luma block 602 of FIG. 6A). The luma block
corresponds to the chroma block as they represent different
components (e.g., brightness component and color component,
respectively) of the same portion of a video frame. In some
embodiments, the luma block is adjacent to a plurality of
previously-reconstructed neighboring luma samples (e.g.,
reconstructed luma samples in top neighboring luma group 606 and
left neighboring luma group 610 of FIG. 6A), and the chroma block
is adjacent to a plurality of previously-reconstructed neighboring
chroma samples (e.g., reconstructed chroma samples in top
neighboring chroma group 624 and left neighboring chroma group 628
of FIG. 6A). Note that the term "adjacent to" in this application
is not limited to "immediately next to" and also covers the
situation in which the coding block is not immediately next to the
luma/chroma samples. In some embodiments, the video codec predicts
chroma samples in the chroma block by deriving a cross-component
linear model and applies the model to reconstructed luma samples
(or down-sampled reconstructed luma samples in the luma block).
[0126] Next, the video codec searches a sub-group of the plurality
of reconstructed neighboring luma samples in a predefined order to
identify at least one maximum or quasi-maximum luma sample and at
least one minimum or quasi-minimum luma sample (820). In some
embodiments, the sub-group of the plurality of reconstructed
neighboring luma samples covers all the plurality of reconstructed
neighboring luma samples. For example, the video codec can search
all luma samples in the neighboring luma samples or up to a
predefined number of neighboring luma samples. In some embodiments,
the video codec searches the neighboring luma samples according to
the raster scan order, from left to right, from top to down, or in
any combination of these orders. In some embodiments, the
neighboring luma samples include those samples spatially on top of
the luma block (e.g., top neighboring chroma group 624) and those
samples to the left of the luma block (e.g., left neighboring
chroma group 610). The video codec may search only the top
neighboring chroma samples or the left neighboring chroma
samples.
[0127] After identifying the maximum and minimum luma samples, the
video codec computes at least one down-sampled maximum luma sample
and at least one down-sampled minimum luma sample corresponding to
the identified maximum and minimum luma samples, respectively (830
and 840). For example, the video codec may use the six-tap
down-sampling technique to generate a down-sampled luma sample
(e.g., by weighted-averaging) from six neighboring reconstructed
luma samples (e.g., arranged in a 3.times.2 manner or a 2.times.3
manner). The down-sampled maximum luma sample and the down-sampled
minimum luma sample each correspond to a respective reconstructed
chroma sample (e.g., first reconstructed neighboring chroma sample
and second reconstructed neighboring chroma sample, respectively).
For example, the first reconstructed neighboring chroma sample and
the second reconstructed neighboring chroma sample can be of the
plurality of reconstructed neighboring chroma samples.
[0128] In some implementations, the operations 820, 830, and 840
described above are re-arranged in a different order. For example,
the video codec first computes one or more down-sampled luma
samples from the plurality of reconstructed neighboring luma
samples and then searches, among a sub-group of the one or more
computed down-sampled luma samples, to identify at least one
down-sampled maximum luma sample and at least one down-sampled
minimum luma sample, respectively. The down-sampled maximum luma
sample is chosen to correspond to the first reconstructed chroma
sample and the down-sampled minimum luma sample is chosen to
correspond to the second reconstructed chroma sample.
[0129] Next, the video codec generates a linear model using a first
pair of the down-sampled maximum luma sample and the first
reconstructed neighboring chroma sample and a second pair of the
down-sampled minimum luma sample and the second reconstructed
neighboring chroma sample (850). In some embodiments, the video
codec generates the linear model using the max-min method by
identifying two data points (e.g., (first reconstructed neighboring
chroma sample, down-sampled maximum luma sample) and (second
reconstructed neighboring chroma sample, down-sampled minimum luma
sample)) and fits a linear equation through the two data
points.
[0130] After obtaining the linear model, the video codec computes
down-sampled luma samples from luma samples of the reconstructed
luma block (860). Each down-sampled luma sample corresponds to a
chroma sample of the chroma block. For example, the video codec can
compute down-sampled luma samples using the same down-sampling
technique used to compute the down-sampled max and min luma
samples.
[0131] Finally, the video codec predicts chroma samples in the
chroma block by applying the linear model to the corresponding
down-sampled luma samples (870).
[0132] In one or more examples, the functions described may be
implemented in hardware, software, firmware, or any combination
thereof. If implemented in software, the functions may be stored on
or transmitted over, as one or more instructions or code, a
computer-readable medium and executed by a hardware-based
processing unit. Computer-readable media may include
computer-readable storage media, which corresponds to a tangible
medium such as data storage media, or communication media including
any medium that facilitates transfer of a computer program from one
place to another, e.g., according to a communication protocol. In
this manner, computer-readable media generally may correspond to
(1) tangible computer-readable storage media which is
non-transitory or (2) a communication medium such as a signal or
carrier wave. Data storage media may be any available media that
can be accessed by one or more computers or one or more processors
to retrieve instructions, code and/or data structures for
implementation of the implementations described in the present
application. A computer program product may include a
computer-readable medium.
[0133] The terminology used in the description of the
implementations herein is for the purpose of describing particular
implementations only and is not intended to limit the scope of
claims. As used in the description of the implementations and the
appended claims, the singular forms "a," "an," and "the" are
intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will also be understood that the
term "and/or" as used herein refers to and encompasses any and all
possible combinations of one or more of the associated listed
items. It will be further understood that the terms "comprises"
and/or "comprising," when used in this specification, specify the
presence of stated features, elements, and/or components, but do
not preclude the presence or addition of one or more other
features, elements, components, and/or groups thereof.
[0134] It will also be understood that, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. For example, a first
electrode could be termed a second electrode, and, similarly, a
second electrode could be termed a first electrode, without
departing from the scope of the implementations. The first
electrode and the second electrode are both electrodes, but they
are not the same electrode.
[0135] The description of the present application has been
presented for purposes of illustration and description, and is not
intended to be exhaustive or limited to the invention in the form
disclosed. Many modifications, variations, and alternative
implementations will be apparent to those of ordinary skill in the
art having the benefit of the teachings presented in the foregoing
descriptions and the associated drawings. The embodiment was chosen
and described in order to best explain the principles of the
invention, the practical application, and to enable others skilled
in the art to understand the invention for various implementations
and to best utilize the underlying principles and various
implementations with various modifications as are suited to the
particular use contemplated. Therefore, it is to be understood that
the scope of claims is not to be limited to the specific examples
of the implementations disclosed and that modifications and other
implementations are intended to be included within the scope of the
appended claims.
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