U.S. patent application number 13/964295 was filed with the patent office on 2013-12-12 for method and apparatus for image data transformation.
This patent application is currently assigned to Dolby Laboratories Licensing Corporation. The applicant listed for this patent is Dolby Laboratories Licensing Corporation. Invention is credited to Anders Ballestad, Andrey Kostin.
Application Number | 20130328907 13/964295 |
Document ID | / |
Family ID | 45895475 |
Filed Date | 2013-12-12 |
United States Patent
Application |
20130328907 |
Kind Code |
A1 |
Ballestad; Anders ; et
al. |
December 12, 2013 |
Method and Apparatus for Image Data Transformation
Abstract
Image data is transformed for display on a target display. A
sigmoidal transfer function provides a free parameter controlling
min-tone contrast. The transfer function may be dynamically
adjusted to accommodate changing ambient lighting conditions. The
transformation may be selected so as to automatically adapt image
data for display on a target display in a way that substantially
preserves creative intent embodied in the image data. The image
data may be video data.
Inventors: |
Ballestad; Anders;
(Vancouver, CA) ; Kostin; Andrey; (Ithaca,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dolby Laboratories Licensing Corporation |
San Francisco |
CA |
US |
|
|
Assignee: |
Dolby Laboratories Licensing
Corporation
San Francisco
CA
|
Family ID: |
45895475 |
Appl. No.: |
13/964295 |
Filed: |
August 12, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US2012/029189 |
Mar 15, 2012 |
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13964295 |
|
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61567784 |
Dec 7, 2011 |
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61453107 |
Mar 15, 2011 |
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Current U.S.
Class: |
345/590 |
Current CPC
Class: |
G09G 2320/066 20130101;
G06T 11/001 20130101; G09G 2320/0276 20130101; H04N 1/6027
20130101; H04N 9/67 20130101; G09G 2320/0666 20130101; H04N 1/62
20130101; H04N 9/68 20130101; H04N 1/6088 20130101; G09G 5/02
20130101 |
Class at
Publication: |
345/590 |
International
Class: |
G06T 11/00 20060101
G06T011/00 |
Claims
1. A method to improve the display management of images, the method
comprising: receiving first information data for a reference
display, the first information data comprising a black point level,
a white point level, and a mid-point level for the reference
display; accessing second information data for a target display,
the second information data comprising a black point level, a white
point level, and a mid-point level for the target display;
determining a transfer function to map pixel values of an input
image signal to corresponding pixel values of an output image
signal using the first and second information data, wherein the
transfer function comprises three anchor points, wherein the first
anchor point is determined using the black point levels of the
reference and target displays, the second anchor point is
determined using the white point levels of the reference and target
displays, and the third anchor point is determined using the
mid-point levels of the reference and target displays; and mapping
the input image signal to the output image signal using the
determined transfer function.
2. The method of claim 1, wherein determining the transfer function
further comprises applying a free parameter, wherein the free
parameter adjusts the slope of the transfer function at the third
anchor point.
3. The method of claim 1, wherein the first information data is
received as part of metadata of the input image signal, wherein the
metadata is transmitted by an image encoder to a decoder.
4. The method of claim 1, wherein the first anchor point has
horizontal and vertical coordinates respectively equal to the black
point levels of the reference display and the target display, the
second anchor point has horizontal and vertical coordinates
respectively equal to the white point levels of the reference
display and the target display, and the third anchor point has
horizontal and vertical coordinates respectively equal to the
mid-point levels of the reference display and the target
display.
5. The method of claim 1, wherein the transfer function comprises a
transformation according to: V ' = C 1 + C 2 V n 1 + C 3 V n
##EQU00014## wherein V denotes an input pixel value, V' denotes an
output pixel value, C.sub.1, C.sub.2, and C.sub.3 are parameters
determined using the three anchor points, and n denotes a free
parameter.
6. The method of claim 1, wherein the pixel values of the input
image comprise color values for two or more color components, and
the transfer function is determined for each one of the two or more
color components.
7. A method to improve the display management of images, the method
comprising: receiving reference display information, the reference
display information characterizing performance of a reference
display by three luminance levels of the reference display;
accessing target display information, the target display
information characterizing performance of a target display by three
luminance levels of the target display; determining a transfer
function to map pixel values of an input image signal encoded on
the reference display to corresponding pixel values for the target
display using the reference display information and the target
display information, wherein the transfer function comprises three
anchor points, each of the three anchor points being determined by
at least one of the three luminance levels of the reference display
and at least one of the three luminance levels of the target
display; and mapping the input image signal to the output image
signal using the determined transfer function.
8. The method of claim 7, wherein the three luminance levels
characterizing the performance of the reference display or the
target display comprise a black-point level value, a white-point
level value, and a mid-point level value.
9. The method of claim 8, wherein the first anchor point has
horizontal and vertical coordinates respectively equal to the black
point levels of the reference display and the target display, the
second anchor point has horizontal and vertical coordinates
respectively equal to the white point levels of the reference
display and the target display, and the third anchor point has
horizontal and vertical coordinates respectively equal to the
mid-point levels of the reference display and the target
display.
10. The method of claim 7, wherein the reference display
information is received as part of metadata of the input image
signal, wherein the metadata is transmitted by an image encoder to
a decoder.
11. The method of claim 7, wherein the transfer function comprises
a transformation according to: V ' = C 1 + C 2 V n 1 + C 3 V n
##EQU00015## wherein V denotes an input pixel value, V' denotes an
output pixel value, C.sub.1, C.sub.2, and C.sub.3 are parameters
determined using the reference and target display information and n
denotes a free parameter.
12. The method of claim 11, wherein the free parameter (n) is
determined according to the slope of the transfer function in a
mid-range region.
13. The method of claim 11, wherein the free parameter (n) is
determined according to a desired mid-range contrast, independently
of the determined anchor points.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International Patent
Application No. PCT/US2012/029189, filed on 15 Mar. 2012 which
claims the benefit of the filing date of U.S. Provisional Patent
Application Ser. No. 61/453,107 filed on Mar. 15, 2011 and U.S.
Provisional Patent Application Ser. No. 61/567,784 filed on Dec. 7,
2011, all of which are hereby incorporated by reference.
[0002] The present application is also related to International
Patent Application No. PCT/US2012/027267, filed on 1 Mar. 2012,
incorporated herein by reference.
TECHNICAL FIELD
[0003] The invention relates to displaying and processing images.
The invention relates specifically to methods and apparatus
involving tone and/or gamut mapping. Methods and apparatus as
described herein may be applied to provide quality images on
destination displays while preserving creative intent. The
invention may be embodied, for example, in electronic displays such
as televisions, computer monitors, media players, video capable
portable telephones and other portable devices, specialized
displays such as virtual reality displays, advertising displays,
and the like as well as to upstream image processing equipment such
as set top boxes, access points and the like.
BACKGROUND OF THE INVENTION
[0004] Patent publications in the general field of this invention
include the following: [0005] US20010050757; [0006] US20020075136;
[0007] US20020080245; [0008] US20070127093; [0009] US20080094515;
[0010] US20080170031; [0011] US20080186707; [0012] US20090201309;
[0013] US20090267876; [0014] US20100007599; [0015] US20100118008;
[0016] US7158673; [0017] US6989859; [0018] US5276779; and [0019]
JP2002092655.
[0020] The creator of a video production or other image (for
example a director, colorist, or the like) may set tones and colors
of pixels in the image so that, when viewed, the image has a
desired appearance which agrees with the creator's creative intent.
For example, a creator may wish some scenes to have a darker, more
oppressive, feel than others. The creator may wish certain features
depicted in a scene to stand out or to be less prominent. Adjusting
tones and colors of pixels in an image may include performing color
grading (or `color timing`) on the source video data. Color grading
may be performed using a hardware/software system that permits a
user to change the video data in various ways to achieve a desired
appearance.
[0021] Various display technologies are now available. For example,
there are plasma displays, LCD displays backlit by a variety of
types of light sources such as LEDs of various types, fluorescent
lamps or high-intensity incandescent lamps, CRT-based displays,
digital cinema displays, etc. A particular display combines display
hardware with video signal processing components that receive video
signals and drive display hardware to display video content of the
video signals.
[0022] Different displays may vary significantly with respect to
features such as: [0023] the color gamut that can be reproduced by
the display; [0024] the maximum brightness achievable; [0025]
contrast ratio; [0026] resolution; [0027] acceptable input signal
formats; [0028] color depth; [0029] white level; [0030] black
level; [0031] white point; [0032] grey steps; [0033] etc.
Consequently, the same image content may appear different when
played back on different displays. Image content that matches a
creator's creative intent when displayed on some displays may
depart from the creator's creative intent in one or more ways when
viewed on other displays.
[0034] Some current displays can outperform displays that were
state-of-the-art at a time when certain content was created in one
or more aspects. For example, new displays may be capable of
providing images that have brighter highlights, greater contrast,
and/or wider color gamuts than older displays. It can be desirable
to take advantage of these improved capabilities without departing
significantly from the creative intent embodied in the content
being viewed.
[0035] It may be desirable to play video content created to take
advantage of high-performance displays on legacy displays or
displays that have lower capabilities. It would be desirable to
provide methods and apparatus for adapting the appearance of video
and other images displayed on different displays to preserve as
much as possible a creative intent embodied in the image data.
[0036] The perception of color and luminance can be affected by
ambient conditions. Video or other images presented under theater
conditions (low ambient lighting) may be perceived by viewers
significantly differently than the same video or other images would
be perceived when viewed under conditions with significant ambient
light. Further, the characteristics (such as the color temperature)
of ambient light can affect a viewer's perception of video content.
It would be desirable to display video or other images taking into
account the environment in which the content is being viewed to
preserve as much as possible a creative intent embodied in the
video or other images.
[0037] There is a need to provide viewers of images (including
still and/or video images) with viewing experiences that exploit
the capabilities of displays on which they view the images. There
remains a need for apparatus and methods which may be applied to
adjust image data such that video or other image content encoded in
the image data has a desired appearance when played.
SUMMARY OF THE INVENTION
[0038] The invention has a range of aspects. These include, without
limitation, apparatus which comprises gamut transformation
functionality; method for gamut transformation, methods for
adapting display of image content to take account of ambient
lighting conditions; program products comprising computer readable
code which, when executed by a data processor causes the data
processor to execute a method according to the invention.
[0039] One non-limiting aspect provides apparatus comprising a
pixel coordinate mapping unit that is configured to transform image
data according to a transfer function. The transfer function is
characterized by a plurality of anchor points and a free parameter.
The transfer function has a mid-range slope controlled by the free
parameter. Transformations at the anchor points are not affected by
the free parameter. Such apparatus can be useful for transforming
color-graded content for display on a particular target display,
for example.
[0040] In some embodiments the apparatus comprises or receives a
signal from an ambient light sensor and a circuit connected to
receive an ambient-lighting signal from the ambient light sensor is
configured to control one or more of the free parameter and a
coordinate of one of the anchor points based at least in part on
the ambient-lighting signal.
[0041] In some embodiments the image data comprises sets of pixel
values for pixels in an image. The sets of pixel values comprise
color values for each of a plurality of primary colors (for
example, values corresponding to red, green and blue primary
colors). The apparatus comprises a plurality of the pixel
coordinate mapping units each connected to transform a
corresponding one of the color values. The parameters for the
transfer functions in the different coordinate mapping units may be
the same or different. With properly selected different parameters
the transformations may perform color correction as well as gamut
translation.
[0042] Another aspect of the invention comprises methods for
mapping image data for display on a target display. The methods
comprise transforming pixel values of the image data to
corresponding transformed pixel values according to a transfer
function. The transfer function is characterized by a plurality of
anchor points and a free parameter. The transfer function has a
mid-range slope controlled by the free parameter. Transformations
of pixel values corresponding to the anchor points are not affected
by the free parameter. In some embodiments, one or more of the free
parameter and the location of a mid-range anchor point are
automatically changed to account for ambient lighting and/or
adaptation of viewers' visual systems.
[0043] Another aspect comprises methods for mapping image data for
display on a target display by combining a global tone-mapping
transformation with a local multi-scale tone mapping operation.
[0044] Another aspect provides color manipulation apparatus. The
apparatus may comprise a workstation for modifying still or video
images for example. The apparatus may be applied for modifying
color values in image data. The color manipulation apparatus
comprises a first memory or input for source image data and a
second memory or output for modified image data. A pixel coordinate
mapping unit is connected to access the first memory or input and
configured to transform the source image data according to a
transfer function characterized by a plurality of anchor points and
a free parameter. The transfer function has a mid-range slope
controlled by the free parameter, wherein transformations at the
anchor points are not affected by the free parameter to yield the
modified image data and to provide the modified image data to the
second memory or output. A user input is configured to accept from
a user a value for the free parameter. A display connected to
display the modified image data. A user may adjust the value of the
free parameter to obtain a desired appearance of the image
displayed on the display.
[0045] Further aspects of the invention and features of specific
embodiments of the invention are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] The accompanying drawings illustrate non-limiting
embodiments of the invention.
[0047] FIG. 1 is a schematic depiction of a video distribution
pipeline.
[0048] FIG. 2 shows apparatus according to an example embodiment of
the invention.
[0049] FIG. 3 illustrates an example transfer function.
[0050] FIG. 4 is a flow chart illustrating a method which uses
information about a target display and input image data to
determine appropriate values for parameters defining a transfer
function.
[0051] FIG. 5 is a flow chart illustrating a method for processing
image data according to an example embodiment.
[0052] FIG. 6 depicts a flow chart illustrating a method that
combines a global tone-mapping operator with a local multi-scale
tone-mapping operator according to an example embodiment.
DESCRIPTION OF THE INVENTION
[0053] Throughout the following description, specific details are
set forth in order to provide a more thorough understanding of the
invention. However, the invention may be practiced without these
particulars. In other instances, well known elements have not been
shown or described in detail to avoid unnecessarily obscuring the
invention. Accordingly, the specification and drawings are to be
regarded in an illustrative, rather than a restrictive, sense.
[0054] FIG. 1 shows schematically a video distribution pipeline 20.
Raw video data 22 is acquired and edited at an editing suite 23 to
provide a raw video production 24. Tones and/or colors in the raw
video production are adjusted at a color timing station 26 by a
colorist (e.g. a human who uses tools provided by the color timing
station by way of a suitable user interface) to achieve a color
timed video production 27. Color timing station 26 includes a
professional monitor 30 on which the colorist views the video
production. Using tools and controls provided by color timing
station 26, the colorist adjusts tones and/or colors of all or
parts of the images which make up the video production to achieve
an overall appearance which, when viewed on display 30, matches the
colorist's artistic intent.
[0055] If all viewers of color timed video production 27 watched
the video production on a display identical to display 30 under
ambient conditions identical to those experienced by the colorist
then, excepting individual variations in the human perception of
images, the viewers would all see the video production exactly as
intended by the colorist (i.e. in a manner true to the colorist's
artistic intent). Given the very wide range of displays that are in
use, it is unrealistic to expect that viewers will all have the
same display or even that displays on which different viewers will
view a video production will have similar characteristics such as
maximum brightness, black level and color gamut.
[0056] One aspect of the invention provides mapping methods and
apparatus that may be applied automatically to map tones and/or
colors from image data such as, for example, color timed video
production 27 for display on a particular destination display in a
manner that closely replicates the viewing experience of the
colorist.
[0057] In some embodiments, the mapping methods and apparatus
provide direct control over one or more of: [0058] average image
brightness (adaptation point); [0059] mid-tone local contrast;
[0060] color saturation; [0061] level at which input black is
displayed; and level at which input white is displayed.
[0062] These parameters affect the viewing experience.
[0063] FIG. 2 shows apparatus 40 according to an example embodiment
of the invention. In this example, apparatus 40 has an input 42 for
receiving video data 43 to be displayed on a screen 44 of a target
display 41 for viewing by a viewer V. Video data 43 may comprise
color-timed video data embodying the intent of a creator. Apparatus
40 comprises a color space translator 46 that translates pixel
values for video data 43 into a color space that is native to
target display 41. In the illustrated example embodiment, the
native color space of target display 41 is an RGB color space,
which specifies colors in terms of the intensities of primary
colors of target display 41.
[0064] Color space translator 46 may comprise, for example, a
matrix multiplier that multiplies a vector of pixel values in video
data 43 by a 3.times.3 matrix to yield a vector of display 41
native color space values (e.g., RGB values). The transfer matrix
may be specified taking into account the primaries and white point
of target display 41. In some embodiments, color space translator
46 may be configured to apply a color space transformation matrix
without scaling for peak luminance. As explained below, this may
make selection of parameters for subsequently image processing
operations more intuitive.
[0065] In the following example, pixel values in video data 43 are
represented in an XYZ color space and color space translator 46
performs a translation from XYZ color space into positive RGB
values. The invention is not limited to color data presented in an
XYZ color space. Video data 43 may be presented in any suitable
color space.
[0066] Negative RGB values may result for translations of
combinations of pixel values that are out-of-gamut (e.g. colors
that cannot be reproduced using any available combination of the
primary colors used by the display). Any negative RGB values
generated by color space translator 46 may be clipped to a low
non-negative value. In the alternative, out-of-gamut pixel values
may be mapped to in-gamut pixel values prior to the translation
(e.g., according to a mapping within the color space of video data
43). This may be performed by a separate mapping unit or by a
component of color space translator 46 for example.
[0067] After being processed by color space translator 46 video
data 43 comprises values 48R, 48G, and 48B which respectively
correspond to red, green and blue (RGB) primaries of target display
41.
[0068] Each of values 48R, 48G, and 48B is independently mapped to
a new value by a mapping unit 50. Mapping units 50R, 50G, and 50B
are shown. Each mapping unit maps a corresponding input value
received from color space translator 46 to a transformed value. In
the illustrated embodiment, the transformed values are indicated by
48R' 48G' and 48B' respectively.
[0069] Each mapping unit 50 maps its input value to an output value
according to a transfer function 55. Advantageously, transfer
function(s) 55 may be characterized by a plurality of fixed points,
which may be referred to as `anchor points`, and a free parameter
that adjusts a slope of the transfer function in a mid-range
region. This slope corresponds to mid-range contrast. Adjustment of
the free parameter provides a means for controlling mid-range
contrast. The transfer function may be linear or approach linearity
in the mid-range region.
[0070] FIG. 3 shows an example transfer function. In FIG. 3, input
values are indicated on the horizontal axis and output values are
indicated on the vertical axis. Each axis has a logarithmic scale.
Transfer function 55 is characterized by a maximum value 56A for
output values, a minimum value 56B for output values and a
substantially linear mid-tone region 56C. The transfer functions
55R, 55G and 55B applied to red, blue, and green channel signals by
mapping units 50A, 50B and 50C may be identical or different.
Mapping units 50A, 50B and 50C may be completely independent or may
share hardware and/or software components.
[0071] In an example embodiment, transfer function 55 is given by
the following equation:
V ' = C 1 + C 2 V n 1 + C 3 V n ( 1 ) ##EQU00001##
where C.sub.1, C.sub.2, and C.sub.3 are constants, V is the input
value for the color channel, V' is the output value for the color
channel, and n is a parameter. The transfer function of Equation
(1) is an example of a parameterized sigmoidal tone curve
function.
[0072] Other parameterized transfer functions may be used in the
alternative. In some embodiments the transfer function includes
parameters that provide control over one or more of low-end slope,
high-end slope, and "sharpness" of the roll-off at the top and
bottom ends of the transfer function.
[0073] One method for establishing values for the parameters in
Equation (1) in a specific case is illustrated by the method 70 of
FIG. 4. Method 70 uses information about the target display and
information about a display used in color-timing or approving the
input video data (a `color-timing display`) to determine
appropriate values for the parameters of Equation (1). Block 71
identifies three luminance anchor points on curve 55. A first
anchor point 57A has horizontal and vertical coordinates
respectively equal to black levels of the color-timing display and
the target display. In some embodiments information about the
color-timing display is inferred from the input signal. For
example, a black level for the color-timing display may be inferred
from the input signal by taking a small percentile (e.g. the 0.1
percentile) of the luminance channel in the input signal. A black
level for the target display is the black level for the target
display.
[0074] A second anchor point 57B has as its horizontal coordinate a
white level for the color-timing display and, as a vertical
coordinate, a white point for the target display. For example, a
white point for the color-timing display may be inferred from the
input signal as the maximum value of any color channel in the input
signal.
[0075] The position of a middle anchor point 57C affects the
overall brightness of a displayed image (e.g. the `key` of the
image). Appropriate selection of mid-tone anchor point 57C
facilitates the input image being perceived as being appropriately
bright on the target display.
[0076] The horizontal location of point 57C may be set in various
ways; these include the following: [0077] calculating the geometric
mean of the input luminance; [0078] selecting a fixed value that
would be perceived in the color-grading environment as being a
suitable middle value. For example, in some embodiments this value
could be set to a level such as 10.
[0079] The vertical value for point 57C may be based on a luminance
level corresponding to middle grey for the target display. For
example, in a display that can produce luminance values between 1
cd/m.sup.2 and 400 cd/m.sup.2, middle grey is approximately 20
cd/m.sup.2 (which is logarithmically half-way between 1 and 400
cd/m.sup.2). An appropriate value for point 57C may therefore be a
value corresponding to middle grey (about 20 cd/m.sup.2 in this
example). In embodiments where color space translator 46 is
configured to apply a color space transformation matrix without
scaling for peak luminance, a value of 20 will correspond to a
middle grey of 20 cd/m.sup.2.
[0080] In some embodiments, the mid-tone anchor point 57C is
selected so as to make the ratio of the coordinate of the mid-tone
anchor point to the coordinate of the white anchor point equal,
within a desired factor, for both the input and output of the
transfer function.
[0081] In some embodiments, different transfer functions for each
of the RGB coordinates may be used to provide a transformation such
that the white point of the video data is transformed to match the
white point of the target display and/or target viewing
environment. One way to achieve this is to express the white point
of the input video data in terms of chromaticity coordinates (such
as, for example, CIE x,y chromaticity coordinates) and to convert
to scaled XYZ values given by the following equations:
X = x y ( 2 ) Y = 1 ( 3 ) Z = 1 - x - y y ( 4 ) ##EQU00002##
These XYZ values may subsequently be converted to the RGB color
space for the target display to yield a white point for the input
data which may be denoted as (R, G, B).sub.wp,in. In cases where
the source and target white points are the same, both white points
should be (111) in the normalized RGB coordinates. Coordinates for
anchor points 57A, 57B, 57C for the red, green and blue channels
can then be obtained by multiplying luminance anchor values by the
white point values as follows:
(R,G,B).sub.min,in=Y.sub.min,in(R,G,B).sub.wp,in (5)
(R,G,B).sub.max,in=Y.sub.max,in(R,G,B).sub.wp,in (5)
(R,G,B).sub.mid,in=Y.sub.mid,in(R,G,B).sub.wp,in (5)
(R,G,B).sub.min,out=Y.sub.min,out(R,G,B).sub.wp,out (5)
(R,G,B).sub.mid,out=Y.sub.mid,out(R,G,B).sub.wp,out (5)
(R,G,B).sub.max,out=Y.sub.max,out(R,G,B).sub.wp,out (5)
where the subscript `in` denotes the input image data, the
subscript `out` denotes the output data (i.e. the data being passed
on for display); (Y.sub.max,in, Y.sub.max,out) are the unadjusted
coordinates for anchor point 57B; (Y.sub.min,in, Y.sub.min,out) are
the unadjusted coordinates for anchor point 57A; and (Y.sub.mid,in,
Y.sub.mid,out) are the unadjusted coordinates for anchor point 57C;
and (R, G, B).sub.wp,out are the RGB coordinates of the white point
of the target display.
[0082] Equations (5) through (10) provide a set of three anchor
points for each color channel. For example, anchor point 57A for
the red color channel is given by (R.sub.max,in, R.sub.max,out);
anchor point 57B for the red color channel is given by
(R.sub.min,in, R.sub.min,out); and anchor point 57C for the red
color channel is given by (R.sub.mid,in, R.sub.mid,out). Where the
white points for the input video data and target display are not
the same, the sets of anchor points will be different, this results
in a different transfer function for each color channel.
[0083] The transfer function for each color channel of the form
provided by Equation (1) may be obtained from the coordinates of
the corresponding anchor points by performing the computation:
( c 1 c 2 c 3 ) = 1 x 3 y 3 ( x 1 - x 2 ) + x 2 y 2 ( x 3 - x 1 ) +
x 1 y 1 ( x 2 - x 3 ) ( x 2 x 3 ( y 2 - y 3 ) x 1 x 3 ( y 3 - y 1 )
x 1 x 2 ( y 1 - y 2 ) ( x 3 y 3 - x 2 y 2 ) ( x 1 y 1 - x 3 y 3 ) (
x 2 y 2 - x 1 y 1 ) ( x 3 - x 2 ) ( x 1 - x 3 ) ( x 2 - x 1 ) ) ( y
1 y 2 y 3 ) ( 11 ) ##EQU00003##
in which x.sub.1, x.sub.2 and x.sub.3 are given by:
( x 1 x 2 x 3 ) = ( V min , in n V mid , in n V max , in n ) ( 12 )
##EQU00004##
and y.sub.1, y.sub.2 and y.sub.3 are given by:
( y 1 y 2 y 3 ) = ( V min , out V mid , out V max , out ) ( 13 )
##EQU00005##
[0084] One feature of the transfer functions described above is
that n remains a free parameter. This permits the mid-tone contrast
to be set to any desired level. It should be noted that the log-log
slope at the mid-tone anchor point will differ slightly from the
value of n if the mid-tone anchor point is not centered in the
input and output ranges. However, the mid-tone contrast can be set
by adjusting the value for n. A good starting point for the
mid-tone contrast parameter, n, is 1. This value for n ensures that
the mapped scene has substantially similar mid-range local contrast
on the target display and in the original scene.
[0085] With transfer functions as given above, the display linear
luminance values for each of the red, green and blue color channels
can be expressed as follows:
R out = c 1 R + c 2 R R in n 1 + c 3 R R in n ( 14 ) G out = c 1 G
+ c 2 G G in n 1 + c 3 G G in n ( 15 ) B out = c 1 B + c 2 B B in n
1 + c 3 B B in n ( 16 ) ##EQU00006##
These values may be used to drive the target display to display the
image. In some embodiments, these values may be corrected for the
target display's response to linear input values (e.g., normalized)
before being used to drive the target display.
[0086] In some embodiments, normalized drive values (R.sub.norm,
G.sub.norm, B.sub.norm) for the target display are computed using
the following relationships:
R norm = R out - R out , min R out , max - R out , min ( 17 ) G
norm = G out - G out , min G out , max - G out , min ( 18 ) B norm
= B out - B out , min B out , max - B out , min ( 19 )
##EQU00007##
Normalized values may be scaled to the range of driving signals for
the target display (e.g., to the range 0-255 for an 8-bit target
display).
[0087] Optionally, image colors may be enhanced by increasing color
saturation. This may be done, for example, using the following
relationships:
R ' = aR norm + bG norm + cB norm a + b ( G norm R norm ) S + c ( B
norm R norm ) S ( 20 ) G ' = R ' ( G norm R norm ) S ( 21 ) B ' = R
' ( B norm R norm ) S ( 22 ) ##EQU00008##
Values for a, b and c in Equation (20) may be defined with
reference to the elements of an inverse transform matrix M
corresponding to the inverse color space translator 46 ([X, Y,
Z].sup.T=M*[R, G, B]), specifically a may be given by: a=M(2,1), b
may be given by: b=M(2,2), and c may be given by: c=M(2,3). In
Equations (20), (21) and (22), S is a free parameter. Values for S
greater than 1 will cause the color saturation to be increased.
Values for S less than 1 will cause the color saturation to be
decrease (i.e. will cause colors to become more de-saturated).
[0088] Where necessary or desired, the normalized drive values may
be gamma corrected. This may be done, for example, according to the
following relationships:
R.sub.corrected=R.sub.norm.sup.1/.gamma. (23)
G.sub.corrected=G'.sub.norm.sup.1/.gamma. (24)
B.sub.corrected=B.sub.norm.sup.1/.gamma. (25)
where .gamma. is the display response. .gamma. is approximately 2.2
in some target displays. Where normalized drive values are
re-saturated, gamma correction may be performed on re-saturated
drive values (R', G' and B').
[0089] In some embodiments, image colors are re-saturated to
restore, at least approximately, the saturation lost as a result of
tonal compression. Where tonal compression is not constant across
the range of tones in an image, different levels of tonal
compression applied to different tones results in different colors
being de-saturated to different degrees. In general, the greater
the amount of tonal compression, the greater the amount of
de-saturation. The amount of tonal compression may be quantified by
the log-log slope of the tone-curve. As an illustrative example,
the sigmoidal tone curve function plotted as curve 55 in FIG. 3 has
a steeper log-log slope substantially in the substantially linear
mid-tone region 56C than it does in the vicinity of maximum value
56A and minimum value 56B. Accordingly, tone compression going from
the input (horizontal coordinate) to output (vertical coordinate)
is greater in the vicinity of values 56A and 56B as compared with
substantially linear mid-tone region 56C.
[0090] Applying a global re-saturation technique may re-saturate
all pixels without regard to the amount of de-saturation caused by
tonal compression. Some embodiments re-saturate transformed image
data pixels according to the amount of tonal compression of the
transformed image data pixels. Given that the amount of tonal
compression corresponds to the log-log slope of the tone-curve, the
amount of tonal compression for an input value L.sub.in may be
determined as the derivative of the transfer function
L.sub.out=f(L.sub.in) at the input value L.sub.in. The log-log
slope of this transfer function can be determined by setting
L.sub.in=e.sup.x and L.sub.out=e.sup.y and solving for dy/dx, which
represents the log-log slope. For a tone curve according to
Equation (1) above,
y may be expressed as:
y=log(c.sub.1+c.sub.2e.sup.nx)-log(1+c.sub.3e.sup.nx) (26)
and the log-log slope c(L.sub.in) at any point on the tone curve
may be calculated as the derivative of y with respect to x at
L.sub.in:
c ( L in ) = y x = nL in n ( c 2 - c 1 c 3 ) ( c 1 + c 2 L in n ) (
1 + c 3 L in n ) ( 27 ) ##EQU00009##
[0091] For color channels R, G, and B, re-saturated drive values
(B.sub.re-sat, G.sub.re-sat, B.sub.re-sat) may be determined in
terms of the normalized driving values as follows:
R re - sat = R norm ( C in L in ) f ( c ) - c ( 28 ) G re - sat = G
norm ( C in L in ) f ( c ) - c ( 29 ) B re - sat = B norm ( C in L
in ) f ( c ) - c ( 30 ) ##EQU00010##
where f(c) is given as:
f ( c ) = ( 1 + k 1 ) c k 2 1 + k 1 c k 2 ( 31 ) ##EQU00011##
and k.sub.1 and k.sub.2 are constants. In some embodiments
k.sub.1=1.6774. In some embodiments, k.sub.1=1.677. In some
embodiments, k.sub.1=1.68. In some embodiments (including without
limitation some embodiments in which k.sub.1=1.6774, k.sub.1=1.677
or k.sub.1=1.68) k.sub.2=0.9925. In some embodiments (including
without limitation some embodiments in which k.sub.1=1.6774,
k.sub.1=1.677 or k.sub.1=1.68) k.sub.2=0.992. In some embodiments
(including without limitation some embodiments in which
k.sub.1=1.6774, k.sub.1=1.677 or k.sub.1=1.68) k.sub.2=0.99. It
will be appreciated that acceptable results may be obtained using
other values of k.sub.1 and k.sub.2. It will also be appreciated
that re-saturated drive values, R.sub.re-sat, G.sub.re-sat and
B.sub.re-set could be calculated based on the display linear
luminance values for each of the red, green and blue color channels
(R.sub.out, G.sub.out and B.sub.out).
[0092] It will be appreciated that the above-described technique
for tonal compression-dependent re-saturation may be practiced in a
manner that is parameter free (automatic).
[0093] FIG. 5 is a flow chart illustrating a method 80 according to
a further example embodiment. Method 80 incorporates a number of
optional steps. In block 81 method 80 converts image data into a
color space of the target display. In the illustrated example the
target display has red- green- and blue-primaries and the color
space is an RGB color space. Block 81 may comprise performing a
transformation that takes into account the white point and
primaries of the target display. Block 81 is optional in the event
that the image data is already in the native color space of the
target display.
[0094] Block 82 determines chromatic white points for the source
and target. The white points may, for example, be represented as
chromaticity coordinates in any suitable color space and converted
to the native color space of the target display.
[0095] Block 83 establishes initial black level and white level
anchor points for a transfer function. The initial anchor points
may be set based on black and white levels for the source and for
the target display.
[0096] Block 84 establishes an initial mid-tone anchor point for
the transfer function. The mid-tone anchor point may be determined
through analysis of the source image data (e.g. by determining a
geometric mean of the luminance of the source image data) and
determining characteristics of the target display (or
characteristics of the target display and the current viewing
environment at the target display).
[0097] Block 85 adjusts the anchor points based on the white points
determined in block 82 (applying, for example Equations (5) to
(10)).
[0098] Block 86 maps the image data using transfer functions
specified by the adjusted anchor points determined in block 85.
[0099] Block 87 computes drive values for the target display based
on the mapped image data from block 86.
[0100] Optional block 88 adjusts color saturation (block 88 may,
for example, apply Equations (20) to (22) or (28) to (30)).
[0101] Block 89 gamma corrects the drive values.
[0102] The drive values produced through application of method 80
may be applied to drive the target display to display images and/or
stored or transmitted for later display on the target display.
[0103] Apparatus and methods as described herein may be used to
optimize a target display for specific ambient viewing conditions.
Transfer functions of the general type described above may be
shifted dynamically to accommodate changes in ambient lighting and
the resulting changing adaptation level of the human visual system
(HVS). The ideal luminance mid-point for the target display may be
a function of the ambient light. The vertical component of the
mid-tone anchor point may be selected based upon the ambient
lighting conditions.
[0104] In some embodiments, fixing the middle anchor point 57C is
done based in part on ambient lighting or on an estimated
adaptation of viewers' eyes (which itself may be based at least in
part on measured ambient lighting or on a combination of measured
ambient lighting and past display content) as well as
characteristics of the target display. For example, the vertical
coordinate of point 57C may be adjusted based upon ambient lighting
in the vicinity of the target display. For example, the vertical
coordinate could be reduced to a lower luminance value if the
display is in dark ambient lighting conditions (or the viewer's
eyes are estimated to be dark-adapted) and the value could be
increased to a higher value where the target display is in an
environment having high ambient lighting (or the viewers' eyes are
estimated to be adapted to brighter conditions).
[0105] In some embodiments, the amount of saturation adjustment
(e.g., according to Equations (20), (21) and (22) and Equations
(28), (29) and (30)) is based in part on ambient lighting or on an
estimated adaptation of viewers' eyes (which itself may be based at
least in part on measured ambient lighting or on a combination of
measured ambient lighting and past display content) as well as
characteristics of the target display. For example, the parameter S
may be adjusted based upon ambient lighting in the vicinity of the
target display or on a combination of measured ambient lighting and
past displayed content. For example, the value of the parameter S
could be set relatively lower if the display is in dark ambient
lighting conditions (or the viewer's eyes are estimated to be
dark-adapted) and the value could be set relatively higher where
the target display is in an environment having high ambient
lighting (or the viewers' eyes are estimated to be adapted to
brighter conditions). Some embodiments provide a resaturation
control unit that receives a signal from an ambient light sensor
and/or signals containing past image content and/or signals
indicative of the overall brightness of past displayed content. The
resaturation control unit may be configured to set new values for a
parameter (for example the parameter S) that affects an amount of
resaturation based on the received signal(s).
[0106] In some embodiments, spectral characteristics of the ambient
lighting are taken into account. For example, the location of
points 57C in the transfer functions for each color channel may be
separately set based in part on the amount of ambient lighting in a
spectral range corresponding to the color channel.
[0107] Additionally or in the alternative, the slope of the
transfer functions may be controlled based on ambient lighting (or
estimates of the adaptation of viewers' eyes). Where ambient light
is brighter, reflections from a display surface tend to raise the
black level. This effectively reduces the range of the target
display. Under high ambient lighting conditions (viewers' eyes are
estimated to be light-adapted) the slope of the transfer curve in
the mid-tone region may be reduced to provide an enhanced viewing
experience under the ambient conditions. For example, for low
(dark) ambient lighting conditions, the perception of contrast
decreases. This can result in an image appearing "flat". Thus the
slope of the mid-tone part of the transfer function may be
increased from a slope of 1:1, to a greater slope, such as a slope
up to 1:1.5 or so (for example, a slope of 1.3) to increase the
contrast level for dark-adapted eyes. This may be done by changing
the value of the free parameter n where transfer functions of the
type illustrated by Equation (1) are applied. The slope may be
controlled in response to an input from an ambient light
sensor.
[0108] In some embodiments a light-adaptation circuit is provided
that estimates an adaptation level of the human visual system in
response to inputs which may include a signal from the ambient
light sensor, a signal that represents a weighted average or other
indicator of the brightness of historical image content or the
like. The light-adaptation circuit may be based on a model of the
human visual system, for example. Various algorithms for estimating
the adaptation level of the human visual system are known in the
art. The light adaptation circuit may implement such algorithms in
any suitable manner including software executing on one or more
programmable data processors, fixed logic circuits, or combinations
thereof. Values for the mid-tone contrast and/or the locations of
points 57C in the transfer functions may be automatically
controlled in response to an output from the light-adaptation
circuit.
[0109] In some embodiments, transfer functions are set up once for
a target display. The transfer functions may, for example, be built
into the target display and embodied in the form of one or more
programmable processors executing firmware or other software that
performs mapping according to transfer functions as described
above; lookup tables which implement the transfer functions
described above; hard-wired or configurable logic circuits that are
set up to provide output based on the transfer functions as
described above; or the like.
[0110] In some embodiments, drive values for red, green and blue
channels of the target display are converted to a bit depth that
matches that of the display. For example, the display may use 8-bit
drive values. If the transfer functions are applied using
floating-point or other higher-precision calculations then the
conversion may involve, for example, rounding the drive values to a
closest corresponding 8-bit value.
[0111] In the foregoing embodiments, minimum and maximum luminance
values for the input video data can be made to map respectively to
minimum and maximum brightness values for pixels of the display.
Furthermore, a selected mid-tone point from the input video signal
can be made to map to a selected mid-tone point for the display.
Mid-tone contrast remains a free parameter. Another feature of the
transfer functions described above is that they provide compression
or expansion both for low and high values while preserving local
contrast in a mid-tone range.
[0112] In some embodiments, particular images (a particular video
frame or sequence of video frames, for example), are relatively low
average luminance (low key) whereas other images (e.g. frames or
groups of frames) may be deliberately made to have a relatively
high average luminance (high key). In some embodiments, information
about the intended key of the image is provided in the form of
metadata. The metadata may, for example, be created and associated
with the image data during a color grading operation. For example,
metadata may be embedded in or otherwise associated with a signal
carrying color graded video data. In such embodiments, the key of
the image, as indicated by the metadata, may be used in determining
the mid-tone anchor point(s) used in the transfer functions. Where
the metadata indicates a low key image the vertical coordinate of
the anchor point may be moved to a lower value, thus recreating the
key in the target display.
[0113] Different video content may be color graded for different
reference displays. When following the approach described above, it
can be desirable to map the content differently in any particular
target display depending upon the characteristics of the reference
display on which the color grading was performed. Information
identifying the reference display or its characteristics may, for
example, be carried in metadata embedded in or otherwise associated
with image data. A target display may store parameters for a
plurality of different sets of transfer functions. The different
sets of transfer functions may correspond to and be used for video
data that has been color timed using different reference
displays.
[0114] Another feature of the example transfer functions having the
form provided by Equation (1) is that the same transfer function
may provide either compression or expansion at high and low ends of
the range, depending upon the parameters chosen. For example, in a
case where the target display has a larger luminance range than the
input data then the target display may be configured with transfer
functions that expand the range of the image data to match or more
closely approach that of the target display.
[0115] One advantage of the methods and apparatus according to some
embodiments described herein are that mapping is performed in the
RGB color space of the target display. This can save very
significant amounts of computation and/or reduce the complexity of
hardware required to perform the mapping.
[0116] Mapping may be performed in real time.
[0117] Methods according to some embodiments provide direct control
over each of: 1) the average image brightness ("adaptation point"),
2) the mid-tone local contrast (as set by the tone-curve slope), 3)
the input black maps to the minimum display luminance, and 4) the
input white maps to the maximum display luminance. These variables
have been found to be fundamental for providing images that
recreate creative intent as embodied in original image data. In
example embodiments these variables explicitly correspond to
separate parameters. Such methods consequently provide a simple and
effective way to perform color mapping which takes original image
data (which may, for example comprise high dynamic range (HDR) data
and/or color-graded image data) and maps the original image data
into the limited 3-dimensional gamut of a specified output
display.
[0118] Color mapping methods and apparatus as described herein may
also or in the alternative be used in color grading/content
creation. A colorist may be provided with a filter which implements
transformations as described above. The filter may have controls
which allow the colorist to directly set parameters of the transfer
functions. The colorist may use these controls, for example, to
adjust black level etc. In some embodiments, the controls include
controls that allow direct setting of one or more of: one or more
coordinates for one or more of a white level anchor point, a black
level anchor point and a mid-level anchor point (e.g. points 57A,
57B and 57C respectively) and a mid-level contrast (e.g. the
parameter n). Such controls can allow the colorist to set the white
level, black level and key without significantly affecting the
mid-tone slope and vice versa.
[0119] In some embodiments, the apparatus is set to automatically
determine a starting set of parameters that may be close to what
the colorist intends. These starting parameters may, for example,
be generated based on information characterizing the input video
content (e g minimum and maximum values for pixel color/luminance
coordinates) and information characterizing a target display (e.g.
white level, black level and optionally metadata (e.g. metadata
indicating a key of the image data being processed).
[0120] Video production involves creating different versions for
displays having greater and lower capabilities. For example,
Standard Dynamic Range (SDR) grading may be performed for producing
video for display on legacy displays. A tool as described herein
may be applied to create a SDR version of video automatically. A
colorist may guide operation of the tool to produce optimized
results.
[0121] Furthermore, where a colorist has set the parameters to
provide a version for viewing on a lower capability display then
parameters for use in performing mapping for displays having
intermediate capabilities may be determined from the parameter
values selected by the colorist for the lower capability display.
This may be done, for example, by interpolation of the parameter
values established by the colorist for displays having higher and
lower capabilities than the display having intermediate
capabilities.
[0122] Methods and apparatus described herein are not restricted to
use in connection with professional-level color timing Tools for
color-timing are available to amateurs, and even where color-timing
is performed on a non-calibrated monitor (e.g., a home computer
display, television, etc.) methods and apparatus described herein
may be used to translate content created on the non-calibrated
monitor to another display (e.g., by estimating the capabilities of
the non-calibrated color timing display). Technology as described
herein also has application to signals that are not color
timed.
Combining a Global Tone-Mapping Operator with a Local Multi-Scale
Tone-Mapping Operator
[0123] Color mapping methods and apparatus as described herein may
also be combined with other tone mapping techniques, e.g., local
tone mapping operators (TMOs). FIG. 6 depicts an example embodiment
where a global tone mapping operator as described herein is
combined with a local multi-scale tone mapping operator as the one
described by G. J. Ward in U.S. Provisional Application 61/448,606,
"A Local Multiscale Tone-Mapping Operator," (herein to be referred
to as the "Ward" reference), filed also as International Patent
Application No. PCT/US2012/027267, filed on 1 Mar. 2012,
incorporated herein by reference in its entirety. The example
embodiment combines the predictability and stability of the global
TMO with the ability to preserve highlights and color fidelity when
using a local multi-scale operator (MS TMO).
[0124] As depicted in FIG. 6, method 60 starts in step 62 by
accessing input image or video data. These data may be stored or
transmitted in a variety of color formats, such as YCbCr, RGB, XYZ,
and the like. In step 63, one may extract the luminance component,
e.g., Y, from the input data. Depending on the format of the input
data (e.g., RGB) this step may require a color transformation
(e.g., RGB to XYZ). Step 64 may apply a global tone mapping
operator 55, as described by equation (1), to the Y color component
of the input data. In an example embodiment, the anchor points of
global TMO 55 may be selected so that the luminance range of the
input data is mapped to the range [4*L.sub.dMin, 1/2*L.sub.dMax],
where L.sub.dMin and L.sub.dMax denote the minimum and maximum
luminance of the target display. The scaling factors of 4 and 1/2
are typical, but adjustable.
[0125] In an example embodiment, the output of step 64 may be
denoted as globally tone-mapped luminance data Y.sub.TM. In step
65, one may apply to the Y.sub.TM data a local multi-scale
tone-mapping operator (MS TMO) as described by "Ward". For example,
first, one may compute a global Log ratio image
R L = log Y TM Y , ( 32 ) ##EQU00012##
defined as the logarithm of the global tone-mapped luminance data
divided by the original luminance pixels. Given the global Log
ratio image R.sub.L, as described by Ward, the output of the MS TMO
(e.g., step 65) may be a locally tone-mapped luminance image,
denoted as Y.sub.MS. Using Y.sub.MS, one may compute
X MS = X * Y MS Y , and ( 33 ) Z MS = Z * Y MS Y . ( 34 )
##EQU00013##
[0126] In step 66, the X.sub.MS, Y.sub.MS, and Z.sub.MS data may be
converted back to R.sub.MS, G.sub.MS, and B.sub.MS (RGB.sub.MS)
data with primaries, white, and black levels determined by the
target display. Negative or out of gamut RGB.sub.MS values may be
clipped to very small positive values or may be re-mapped to
in-gamut RGB values using any of the known gamut mapping
algorithms.
[0127] Given in-gamut RGB.sub.MS data from step 66, step 67, may
reapply the global tone mapping operator 55 to all of the color
components to output globally tone-mapped corrected data
RGB.sub.G-MS. The application of the second global tone-mapping
operation guarantees that the output of the MS TMO is in the range
of the target display. Finally, in step 68, before displaying the
image data (step 69), the RGB.sub.G-MS data may be gamma corrected
as needed for the output display.
[0128] Certain implementations of the invention comprise computer
processors which execute software instructions which cause the
processors to perform a method of the invention. For example, one
or more processors in a display, a color grading station, a set top
box, a transcoder or the like may implement image data
transformation methods as described above by executing software
instructions in a program memory accessible to the processors. The
invention may also be provided in the form of a program product.
The program product may comprise any medium which carries a set of
computer-readable signals comprising instructions which, when
executed by a data processor, cause the data processor to execute a
method of the invention. Program products according to the
invention may be in any of a wide variety of forms. The program
product may comprise, for example, physical media such as magnetic
data storage media including floppy diskettes, hard disk drives,
optical data storage media including CD ROMs, DVDs, electronic data
storage media including ROMs, flash RAM, or the like. The
computer-readable signals on the program product may optionally be
compressed or encrypted.
[0129] Where a component (e.g. a software module, processor,
assembly, device, circuit, etc.) is referred to above, unless
otherwise indicated, reference to that component (including a
reference to a "means") should be interpreted as including as
equivalents of that component any component which performs the
function of the described component (i.e., that is functionally
equivalent), including components which are not structurally
equivalent to the disclosed structure which performs the function
in the illustrated exemplary embodiments of the invention.
[0130] Some non-limiting embodiments may (e.g., depending on
circumstances) provide one or more of the following advantages:
[0131] mapping according to a tone mapping curve with a black point
anchor may avoid excessive tonal compression of dark input content;
[0132] mapping according to a tone mapping curve with black point
and/or white point anchors may utilize more of the luminance range
of a target display than a tone mapping according to a curve
without one or both of such anchor points; [0133] color channel
specific mapping functions that maximize luminance range may be
applied in the RGB color space of a target display (e.g., after
conversion from an input color space to the target display RGB
color space); and [0134] white point, brightness and/or
mid-contrast of output video data for a target display may be
adjusted in the transfer function (e.g., instead of before or after
mapping to according to a transfer function). Some embodiments may
not provide any of the above advantages; some embodiments may
provide different advantages (e.g., rather than or supplementary to
the above advantages).
[0135] As will be apparent to those skilled in the art in the light
of the foregoing disclosure, many alterations and modifications are
possible in the practice of this invention without departing from
the spirit or scope thereof. Accordingly, the scope of the
invention is to be construed in accordance with the substance
defined by the following claims.
* * * * *