WO2015058877A1 - Procédé et appareil pour la mise en correspondance des couleurs d'une image selon le contenu de cette image - Google Patents

Procédé et appareil pour la mise en correspondance des couleurs d'une image selon le contenu de cette image Download PDF

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Publication number
WO2015058877A1
WO2015058877A1 PCT/EP2014/067431 EP2014067431W WO2015058877A1 WO 2015058877 A1 WO2015058877 A1 WO 2015058877A1 EP 2014067431 W EP2014067431 W EP 2014067431W WO 2015058877 A1 WO2015058877 A1 WO 2015058877A1
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WIPO (PCT)
Prior art keywords
color
image
mapping
change
gamut
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PCT/EP2014/067431
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English (en)
Inventor
Jurgen Stauder
Jonathan Kervec
Emmanuel Jolly
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Thomson Licensing
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
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Publication of WO2015058877A1 publication Critical patent/WO2015058877A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/56—Processing of colour picture signals
    • H04N1/60—Colour correction or control
    • H04N1/6058—Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Definitions

  • the invention relates to a mapping method of the colors of an image and to the corresponding apparatus.
  • the invention concerns notably a method and a system modifying the color gamut of images depending on the content of those images.
  • Images are usually artistically produced for specific reference or target display devices. Images include photos, video content, stereo images, feature films, graphics content, multiple view content and other pictorial and textual content that is intended to be reproduced on display devices.
  • a photographer captures and post processes i.e. produces an appealing photo
  • the director of photography uses color grading equipment for the processing of the video in order to arrange colors, contrast, saturation, black level, white level, white temperature and hues in the images of the video, and a cinema projector will be likely used to reproduce the video production.
  • Today, studios use generally a ITU-R BT.709 compliant monitor to adjust colors in video content when producing a video content for bluray discs.
  • legacy content prepared with legacy reference displays that have limited color capabilities with respect to more recent target displays.
  • a huge amount of content (TV broadcast, DVD, blue ray) is produced for displays according to the ITU-R BT.709 standard while today's consumer target displays, for example LCD displays, have often a larger color gamut that defined by this standard taken as a reference.
  • luminance levels and contrast ratios evolve in recent displays.
  • legacy content can be efficiently adapted to target displays with better color capabilities than the reference display used for their production and/or transmission, notably adapted to target displays with wide color gamut, higher contrast ratio, higher luminance levels and higher bit depth.
  • a color aspect is defined for each color region of an image, based on a luminance histogram specific to this color region. From the definition of this color aspect, a color information can be determined for each color region of an image, namely the luminance histogram itself of this region (fig.4 of US7903178). Then, for each color aspect, namely for each luminance histogram specific to a color region, there is a specific rule that is adapted to determine an amount of change of this color aspect (i.e. amount of change of the histogram) in function of the color information (i.e. the original luminance histogram) representing this color aspect of the image.
  • an amount of change of this color aspect i.e. amount of change of the histogram
  • This rule is predefined and is for instance based on a greater amount of change for the luminance histogram of the "dark region” than for the luminance histograms of the other color regions (see col.4, lines 56-58).
  • the application of the rule specific to each color region of the image leads to a global color correction based on the content itself. Such a color correction is equivalent to a color mapping.
  • This method of color mapping has several drawbacks.
  • a drawback of this method is that it requires large computational effort, notably because it is applied to color coordinates of psycho-visual color spaces such as L,a,b coordinates of CIELAB such as standardized by the CIE in 1976. This is due to the highly non-linear characteristics of these color spaces. Notably in video applications, real time processing might be required and calculation of L,a,b coordinates might not be possible.
  • a color aspect is defined for each color, namely a saturation expansion coefficient which is calculated from xy coordinates of this color (col. 9, lines 1 -16).
  • a color information is determined for each color of an image, namely the value of the corresponding saturation expansion coefficient.
  • the application of the rule specific to each color of the image leads to a color mapping of the content.
  • An object of the invention is to avoid the aforementioned drawbacks
  • the subject of the invention is a method of mapping the colors of an image, a series of color mapping operators being predetermined which differentiate one from another by different amount of change of at least one color aspect of an image, and, for each of at least one color aspect of an image, a rule being set that is adapted to determine an amount of change of said color aspect in function of a color information representing said color aspect of an image, comprising:
  • the selection of at least one color mapping operator such that the combination of said at least one color mapping operator provides an amount of change of said color aspect corresponding to the determined amount of change
  • the series of predetermined color mapping operators and the rules adapted to determine an amount of change of the color aspects are generally stored in the apparatus used to implement this color mapping method, or sent to this apparatus before implementing this method. As these operators are predeternnined and as these rules are set in advance, the computational effort needed to implement the color mapping method is advantageously limited.
  • the amount of change of the color aspect - named "requested amount" - which is determined by the application of the rule in function of a color information representing the color aspect of an image may be distinguished in definition from the definition of the amounts of change of color aspect that differentiate one from the color mapping operators of the series.
  • color mapping operators of said series are predetermined to change the color gamut of said image.
  • said color information comprises lightness, brightness, colorfulness, saturation, contrast, white temperature, black level and/or hue.
  • said color information is semantic, being at least one of shape, motion, texture, illumination or reflectance.
  • Content can be adapted to any type of display device without producing a specific version for specific displays.
  • a subject of the invention is also a color mapping apparatus for the mapping the colors of an image, comprising,
  • a module configured to receive and/or to store a series of predetermined color mapping operators which differentiate one from another by different amount of change of at least one color aspect of an image, and, to receive and/or to store, for each of at least one color aspect of an image, a predetermined rule that is adapted to determine an amount of change of said color aspect in function of a color information representing said color aspect of an image,
  • - a module configured to determine, for each of at least one color aspect, color information representing said color aspect of said image to map
  • a module configured to apply the rule related to said color aspect to the determined color information in order to determine an amount of change of said color aspect
  • a module configured to select, within said series, at least one color mapping operator such that the combination of said at least one color mapping operator provides an amount of change of said color aspect corresponding to the determined amount of change
  • - a module configured to apply the combination of said selected at least one color mapping operator to said image to map.
  • a color mapping apparatus which comprises notably :
  • a module configured to store a series of predetermined color mapping operators and to store, for a color aspect of an image, a predetermined rule that is adapted to determine an amount of change of this color aspect in function of a color information representing this color aspect of an image,
  • - a module configured to determine a color information representing the color aspect of an image to map
  • a module configured to apply the predetermined rule to a determined color information in order to determine an amount of change of this color aspect corresponding to this color information
  • - a module configured to select, within the series above, at least one color mapping operator such that the combination of this at least one color mapping operator provides an amount of change of the color aspect corresponding to the determined amount of change, - a module configured to apply the combination of these selected at least one color mapping operator to said image to map.
  • each module of the color mapping apparatus corresponds to a piece of such hardware and/or software.
  • the non-limiting embodiment of mapping the colors of a video content according to the invention is described.
  • the implementation of this embodiment is shown in Figure 1 .
  • the video content to map has been produced using a video production reference display with limited color capabilities.
  • This implementation aims a color gamut expansion of the video content so that the mapped video content can be watched on a target reproduction display with better color capabilities.
  • colors of the video content include for instance hue, lightness, saturation, white temperature, white level, black level and/or contrast.
  • the color aspect that will be changed through this implementation of the embodiment is color saturation.
  • the invention is not limited to color gamut expansion; there are for instance other applications of the invention that require gamut compression, such as HDTV to SDTV conversion.
  • the mapping method according to the invention is applied image by image, i.e. frame by frame to the video content.
  • the image information that represents the color aspect to be changed through the color mapping and that is used for the application of the rule described below is extracted in real time from each frame or image of the video.
  • color information related to the presence of saturated colors in the frames are represented by RGB coordinates encoded according to the standard ITU-R BT.709.
  • Another possibility to implement the method according to the invention is to apply the above method to a group of frames instead of each frame.
  • Qjnean is calculated over the group of frames.
  • the color information of each frame may be received over a transmission channel from a specific provider of image information.
  • a set or series of 1 1 gamut change operators is predetermined as follows. These operators are based on color management (CMM).
  • the color characteristics of the video production reference display compliant with the ITU-R BT.709 standard and the aimed target reproduction display are measured, mathematically modeled and then compensated in a manner known per se using a color transformation.
  • All gamut change operators of the series are also adapted to map colors from the color gamut of the video production reference display into the color gamut of the target reproduction display.
  • gamut mapping operators can be applied to any color values that are defined within a source color gamut in order to transform these color values such that they are included in a target color gamut.
  • the source color gamut can be linked to a capture device such as camera or scanner. It can be linked to a reference display device such as the video production reference display. It can also be linked to a predefined color gamut for example according to a standard such as ITU-R BT.709 as stated above.
  • the target color gamut can be linked to a targeted display device such as the mentioned target reproduction display.
  • It can be linked also to a predefined gamut for transmission, compression or storage purpose, for example the standard ITU-R BT.2020. It can be linked to a medium such as film or paper prints.
  • a source display with a source color gamut or reference color gamut and a target display having a target color gamut.
  • the gamut change operators of the series aim at the reproduction of a color on the target display such that this color as reproduced is identical or as close as possible to the color as displayed on the reference display or source display.
  • the colors can be measured using the XYZ values of the CIE.
  • Colors can also be measured using color appearance attributes calculated by a color appearance model, for example the CIECAM-02 of the CIE. Those models use so-called perceptually constant color spaces that have axes for intensity, hue and saturation of colors. A simple version of these spaces is CIELAB defined by the CIE in 1976.
  • each of the 1 1 predetermined gamut change operators are composed of the following operator steps:
  • An appearance attribute calculator calculating Lab color coordinates of a color in CIELAB color space from the XYZ color coordinates of this color
  • a gamut boundary representation means adapted to represent the gamut boundary of the video production reference display that corresponds to ITU- R BT.709 (i.e. the reference color gamut) and to represent the gamut boundary of the target display (i.e. the target color gamut) in CIELAB color space,
  • a color mapper for the implementation of a percentage of a gamut mapping algorithm adapted to map, in the Lab color space, colors from the reference color gamut into the target color gamut resulting into mapped colors, wherein these colors and mapped colors are represented by Lab color coordinates (the implementation of a percentage of a gamut mapping algorithm is explained below),
  • gamut mapping algorithm we use an algorithm close to the CARISMA algorithm proposed by Green and Luo in their paper entitled “Extending the CARISMA gamut mapping model” published in the Journal of Imaging Science and Technology volume 46 number 1 in 2002.
  • the invention is not limited to this gamut mapping algorithm and other algorithms can be used. In general, gamut mapping algorithm is computationally demanding and therefore the color mapper requires usually high computational effort.
  • the original CARISMA algorithm changes notably the hue of a color and applies different mapping methods for different hue sections of the color space, depending on the relative shapes of the reference color gamut and of the target color gamut.
  • An additional hue shift is defined - similar to the CARISMA algorithm - for those hue planes containing either a primary or a secondary color of the reference color gamut - i.e. red, green, blue for primary colors and magenta, cyan, yellow for secondary colors - half way towards the hue of the corresponding primary or secondary color, respectively, of the target gamut.
  • Hue shift of intermediate colors between primary and secondary colors are determined by interpolation between two adjacent primary and/or secondary colors.
  • Each of the 1 1 gamut change operators are characterized by an amount A of modification of the color aspect of an image, here an amount of change of the color saturation of this image.
  • the 1 1 different values of A corresponding to the 1 1 different operators could be 0, 0.1 , 0.2, ... 1 , and represent a percentage 0%, 10%, 20%, 100% of application of the described modified CARISMA gamut mapping method when these operators are applied to a color.
  • the value of A characterizing an operator means how far the aimed color aspect is changed by this operator after a color is mapped by this operator.
  • A describes thus the actual amount of modification of the aimed color aspect by this operator.
  • Each of the other operators with 0 ⁇ A ⁇ 100% is possible.
  • RGB Look Up Tables having LUT entry values and LUT output values.
  • LUT Look Up Tables
  • the described operator steps above defining a gamut operator are applied and the result target RGB coordinates representing a color mapped according to this operator are stored in the LUT as the corresponding LUT output value.
  • the RGB input color coordinates are preferably regularly sampled in the RGB space. Instead, a non-regular sampling might be used in order to account for non-linear encoding of RGB coordinates such as the case in ITU-R BT.709 standard.
  • a rule is set that is adapted to determine a requested amount of change RA of the color aspect (here : color saturation) in function of the color information Qjnean (as defined above) representing the color saturation of an image.
  • mapping method according to the invention is applied image by image, i.e. frame by frame to the video content.
  • RJinal ( (RA-A1 ) R2 + (A2-RA) R1 ) / (A2-A1 ).
  • the green and blue coordinates of the output value are interpolated accordingly. This interpolation can be multi-dimensional if requested amounts of change RA exist for more than one aspect of color and therefore more than one color change rules and therefore more than eleven color change operators and corresponding LUTs exist.
  • RJinal RA R2 + (1 -RA)
  • R1 R1 + RA (R2-R1 ).
  • the above mapping method is applied image after image, such as to get a mapped video content with a color aspect (here, color saturation) which is changed to better fit the color characteristics of the target display with better color capabilities, namely such that colors as reproduced on this target display are as close as possible to colors as displayed on the reference display.
  • a color aspect here, color saturation
  • the Lab color space in which percentages A of the gamut mapping algorithm are performed is defined by the CIE in 1976.
  • L * a * b * space a constant a-b angle is assumed to correspond to identically perceived hue.
  • the L coordinate represents the intensity. Unfortunately, this space was shown to not well represent hues, notably in blue tones.
  • JCh space the JCh color space defined in the CIECAM-02 standard defined by the CIE in 2002.
  • JCh space the h coordinate is assumed to correspond to perceived hue by the human eye and the J coordinate is assumed to correspond to perceived intensity.
  • JCh space was shown to better represent hues and intensity than Lab.
  • all these color spaces require highly computational effort for the appearance attribute calculator and for the inverse appearance attribute calculator.
  • the invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or combinations thereof.
  • the invention may be notably implemented as a combination of hardware and software.
  • the software may be implemented as an application program tangibly embodied on a program storage unit.
  • the application program may be uploaded to, and executed by, a machine comprising any suitable architecture.
  • the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPU"), a random access memory (“RAM”), and input/output (“I/O") interfaces.
  • the computer platform may also include an operating system and microinstruction code.
  • the various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU.
  • various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Image Processing (AREA)
  • Color Image Communication Systems (AREA)

Abstract

Selon l'invention, un opérateur de mise en correspondance de couleur est sélectionné en fonction de l'application d'une règle sur la base d'informations d'image qui se rapportent à un aspect de couleur de cette image.
PCT/EP2014/067431 2013-10-23 2014-08-14 Procédé et appareil pour la mise en correspondance des couleurs d'une image selon le contenu de cette image WO2015058877A1 (fr)

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EP13306459 2013-10-23
EP13306459.2 2013-10-23

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070291179A1 (en) * 2004-11-01 2007-12-20 Sterling Michael A Method and System for Mastering and Distributing Enhanced Color Space Content
US7903178B1 (en) * 2005-12-07 2011-03-08 Marvell International Ltd. Color management unit
US8154560B2 (en) * 2008-09-29 2012-04-10 Renesas Electronics Corporation Display drive circuit

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070291179A1 (en) * 2004-11-01 2007-12-20 Sterling Michael A Method and System for Mastering and Distributing Enhanced Color Space Content
US7903178B1 (en) * 2005-12-07 2011-03-08 Marvell International Ltd. Color management unit
US8154560B2 (en) * 2008-09-29 2012-04-10 Renesas Electronics Corporation Display drive circuit

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
GREEN PHIL: "Gamut mapping for the Perceptual Reference Medium Gamut", 2013 COLOUR AND VISUAL COMPUTING SYMPOSIUM (CVCS), IEEE, 5 September 2013 (2013-09-05), pages 1 - 6, XP032501360, DOI: 10.1109/CVCS.2013.6626285 *

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