METHOD OF MAPPING SOURCE COLORS OF A SOURCE CONTENT

Abstract

Method of mapping source colors of a source content represented by source coordinates comprising: applying a reference display forward color transform characterizing a reference display device, applying a virtual display inverse color transform configured to model a virtual display device having approximately the same color primaries as a mastering display device used to master said source content.

Claims

1-14. (canceled)

15. A method of mapping source colors of a source content, wherein said source colors are represented by device-dependent source coordinates R,G,B in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform comprising: applying said reference display forward color transform to device-dependent source coordinates (R,G,B) representing said source colors, resulting in device-independent source coordinates (X,Y,Z) representing the same source colors in a device-independent linear color space, applying a virtual display inverse color transform (IT.sub.VD) to said resulting device-independent source coordinates X, Y, Z representing said source colors, resulting in device-dependent mapped coordinates R, G, B representing mapped colors in said reference device-dependent color space, wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content.

16. The method according to claim 15 wherein said virtual display device is further characterized by an EOTF corresponding to that of a mastering display device used to master said source content.

17. The method according to claim 15 wherein said virtual display device is further characterized by an EOTF corresponding to that of said reference display device.

18. The method according to claim 17 wherein said virtual display device is further characterized by a white point corresponding to that of said reference display device.

19. A method of mapping source colors of a source content, wherein said source colors are represented by first device-independent source coordinates X, Y, Z in a device-independent color space comprising: applying a virtual display inverse color transform (IT.sub.VD) to said device-independent source coordinates X,Y,Z representing said source colors, resulting in device-dependent source coordinates R,G,B representing mapped colors in the reference device-dependent color space of a reference display device, applying a reference display forward color transform characterizing said reference display device to said device-dependent source coordinates R,G,B representing said mapped colors, resulting in second device-independent source coordinates X,Y,Z representing the same mapped colors in the device-independent linear color space, wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized: by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content, by an EOTF corresponding to that of a mastering display device used to master said source content or to that of said reference display device.

20. The method according to claim 19 wherein said virtual display device is further characterized by an EOTF corresponding to that of a mastering display device used to master said source content.

21. The method according to claim 19 wherein said virtual display device is further characterized by an EOTF corresponding to that of said reference display device.

22. The method according to claim 21 wherein said virtual display device is further characterized by a white point corresponding to that of said reference display device.

23. The method for reproducing a source content on a target display device characterized by a target display inverse color transform, comprising: receiving device-dependent source coordinates R, G, B representing source colors of said source content in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform, receiving metadata representing color primaries of a mastering display device used to master said source content or extracting color primaries from a description of the color gamut of said source content, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by said color primaries, mapping said source colors into mapped colors according to the method of claim 15, resulting in device-dependent mapped coordinates R, G, B representing said mapped colors, applying said reference display forward color transform to said device-dependent mapped coordinates R, G, B, resulting into device-independent mapped coordinates X, Y, Z representing said mapped colors in device-independent color space, gamut mapping said device-independent mapped coordinates X, Y, Z from said reference color gamut towards said target color gamut and applying said target display inverse color transform to said gamut-mapped device-independent mapped coordinates X, Y, Z, resulting into device-dependent target coordinates R, G, B representing said mapped colors in the target device-dependent color space of said target display device, controlling said target display device by inputting said device-dependent target coordinates R, G, B, resulting in the reproduction of said source content.

24. The method for reproducing a source content on a target display device characterized by a target display inverse color transform, comprising: receiving first device-independent source coordinates X, Y, Z representing source colors of said source content, receiving metadata representing color primaries of a mastering display device used to master said source content or extracting color primaries from a description of the color gamut of said source content, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by said color primaries, mapping said source colors into mapped colors according to the method of claim 19, resulting in device-independent mapped coordinates X, Y, Z representing said mapped colors, applying said target display inverse color transform to said device-independent mapped coordinates X, Y, Z, resulting into device-dependent mapped coordinates R, G, B, controlling said target display device by inputting said device-dependent mapped coordinates R, G, B, resulting in the reproduction of said source content.

25. A color processing device for mapping source colors of a source content, wherein said source colors are represented by device-dependent source coordinates R, G, B in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform, comprising: a reference display forward color transform module configured for applying said reference display forward color transform to device-dependent source coordinates (R, G, B) representing said source colors, resulting in device-independent source coordinates (X, Y, Z) representing the same source colors in a device-independent linear color space, a virtual display inverse color transform module configured for applying a virtual display inverse color transform (IT.sub.VD) to the device-independent source coordinates X, Y, Z provided by said reference display forward color transform module, resulting in device-dependent mapped coordinates R,G,B representing mapped colors in said reference device-dependent color space, wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content.

26. The color processing device according to claim 25 wherein said virtual display device is further characterized by an EOTF corresponding to that of a mastering display device used to master said source content.

27. The color processing device according to claim 25 wherein said virtual display device is further characterized by an EOTF corresponding to that of said reference display device.

28. The color processing device according to claim 27 wherein said virtual display device is further characterized by a white point corresponding to that of said reference display device.

29. A color processing device for mapping source colors of a source content, wherein said source colors are represented by first device-independent source coordinates X, Y, Z in a device-independent color space, comprising: a virtual display inverse color transform module configured for applying a virtual display inverse color transform (IT.sub.VD) to said device-independent source coordinates X, Y, Z representing said source colors, resulting in device-dependent mapped coordinates R, G, B representing mapped colors in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform, a reference display forward color transform module configured for applying said reference display forward color transform to device-dependent mapped coordinates R, G, B provided by said virtual display inverse color transform module, resulting in device-independent mapped coordinates X, Y, Z representing the same mapped colors in the device-independent linear color space, wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content.

30. The color processing device according to claim 29 wherein said virtual display device is further characterized by an EOTF corresponding to that of a mastering display device used to master said source content.

31. The color processing device according to claim 29 wherein said virtual display device is further characterized by an EOTF corresponding to that of said reference display device.

32. The color processing device according to claim 31 wherein said virtual display device is further characterized by a white point corresponding to that of said reference display device.

33. A target display device characterized by a target display inverse color transform configured for reproducing a source content, comprising: a reception module configured for receiving device-dependent source coordinates R, G, B representing source colors of said source content in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform, a color primaries module configured to provide color primaries received as metadata representing color primaries of a mastering display device used to master said source content or extracted from a description of the color gamut of said source content, a color processing device according to claim 25, configured to map device-dependent source coordinates R, G, B provided by said reception module, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by color primaries provided by said color primaries module, resulting in device-dependent mapped coordinates R, G, B representing said mapped colors, a final color transform module configured to apply said reference display forward color transform and said target display inverse color transform to device-dependent mapped coordinates R, G, B provided by said color mapping device, resulting into device-independent mapped coordinates X, Y, Z representing said mapped colors in device-independent color space, configured to gamut map said device-independent mapped coordinates X, Y, Z from said reference color gamut towards said target color gamut and to apply said target display inverse color transform to said gamut-mapped device-independent mapped coordinates X, Y, Z, resulting into device-dependent target coordinates R, G, B representing said mapped colors in the target device-dependent color space of said target display device, a target display control module configured to control said target display device by inputting device-dependent mapped coordinates R, G, B provided by said final color transform module, resulting in the reproduction of said source content.

34. A target display device characterized by a target display inverse color transform, configured for reproducing a source content, comprising: a reception module configured for receiving device-independent source coordinates X, Y, Z representing source colors of said source content, a color primaries module configured to provide color primaries received as metadata representing color primaries of a mastering display device used to master said source content or extracted from a description of the color gamut of said source content, a color processing device according to claim 29, configured to map device-independent source coordinates X, Y, Z provided by said reception module, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by color primaries provided by said color primaries module, resulting in device-independent mapped coordinates X, Y, Z representing said mapped colors, a final color transform module configured to apply said target display inverse color transform to device-independent mapped coordinates X, Y, Z provided by said color mapping device, resulting into device-dependent mapped coordinates R, G, B, a target display control module configured to control said target display device by inputting device-dependent mapped coordinates R, G, B provided by said final color transform module, resulting in the reproduction of said source content.

35. A computer readable storage medium comprising stored instructions that when executed by at least one processor performs the method of claim 15.

36. A computer readable storage medium comprising stored instructions that when executed by at least one processor performs the method of claim 19.

37. A computer readable storage medium comprising stored instructions that when executed by at least one processor performs the method of claim 23.

38. A computable readable storage medium comprising stored instructions that when executed by at least one processor performs the method of claim 24.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0088] The invention will be more clearly understood on reading the description which follows, given by way of non-limiting example and with reference to the appended figures in which:

[0089] FIGS. 1 to 3, already mentioned, show different schemes of mapping methods according to the prior art;

[0090] FIG. 4 illustrates a general embodiment of the invention concerning a first situation where source colors to map are represented by device-dependent coordinates;

[0091] FIG. 5 illustrates the color gamut of a mastering display device in the CIE xy chromaticity space, and a position of a source color within this gamut;

[0092] FIG. 6 illustrates a technical effect of the invention;

[0093] FIG. 7 illustrates a first variation of the general embodiment shown on FIG. 4;

[0094] FIG. 8 illustrates a second variation of the general embodiment shown on FIG. 4;

[0095] FIG. 9 illustrates an application of the general embodiment shown on FIG. 4 for the reproduction of source colors on a target display device;

[0096] FIG. 10 illustrates a general embodiment of the invention concerning a second situation where source colors to map are represented by device-independent coordinates;

[0097] FIG. 11 illustrates a first variation of the general embodiment shown on FIG. 10;

[0098] FIG. 12 illustrates a second variation of the general embodiment shown on FIG. 10;

[0099] FIG. 13 illustrates an application of the general embodiment shown on FIG. 10 for the reproduction of source colors on a target display;

[0100] FIG. 14 illustrates an example of implementation of the general embodiment shown on FIG. 4;

[0101] FIG. 15 illustrates an implementation of the example of FIG. 14 on a whole image workflow;

[0102] FIGS. 16, 17, and 18 illustrates respectively a first, second, and a third example of implementation of the general embodiment shown on FIG. 10;

[0103] FIG. 19 illustrates an implementation of the general embodiment shown on FIG. 10 applied on a whole image workflow.

DESCRIPTION OF EMBODIMENTS

[0104] It will be appreciated by those skilled in the art that block diagrams and the like presented herein represent conceptual views of illustrative circuitry embodying the invention. They may be substantially represented in computer readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

[0105] The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software.

[0106] A source content is provided but is formatted to be reproduced by a reference display device, for instance as standardized according to ITU-R BT.2020, i.e. based on a wide color gamut. This source content has been mastered on a given mastering display device, notably characterized by given color primaries.

[0107] We will now describe how such source colors could be advantageously mapped into mapped colors adapted to be reproduced by a target display device: in a first situation, the mapping of source colors is performed in the reference device-dependent color space of the reference display devices; in a second situation, the mapping of source colors is performed in device-independent color space.

[0108] The general embodiment of a mapping in the first situation is illustrated in FIG. 4, already explained. Source colors have been produced using a given mastering display device. Most of the source colors to be mapped are hereby within the color gamut of this mastering display device (which should then be able to reproduce most of these source colors). In order to be transmitted to a target display device for reproduction, these source colors are represented, i.e. encoded, by trichromatic color coordinates R,G,B in the color space of a reference display device. The reference display device is generally different from the mastering display device, but may be equal. Usually, the color gamut of the reference display device is larger than that of the mastering display device. For example, the mastering display device can be a cinema projector with P3 color gamut while the reference display device can be a ITU-R BT.2020 compliant display device having a larger color gamut than P3. When the color gamut of the reference display device is larger than that of the mastering display device, all source colors are located in the color gamut of the reference display device (and could then be reproduced by the reference display device without any mapping).

[0109] In this general embodiment of a mapping in the first situation illustrated on FIG. 4, the method of color mapping according to the invention first transforms R,G,B color coordinates representing source colors of the source content in the color space of the reference display device into device independent color coordinates X,Y,Z representing the same colors in the CIE XYZ device independent color space, by using a forward transform RGB->XYZ modeling the reference display device. By definition, this forward transform of the reference display device is able to transform R,G,B color coordinates of any color located in the color gamut of the reference display device into X,Y,Z color coordinates defining the color that the reference display device would actually reproduce when controlled by those R,G,B color coordinates.

[0110] In a second step of this method, the method applies the inverse transform IT.sub.VD of a virtual display device to the X,Y,Z color coordinates obtained from the first step above. Through this inverse transform, R,G,B device dependent color coordinates are obtained that represent mapped source colors, still in the color space of the reference display device. This virtual display device is characterized through a RP177 model (see above) by an EOTF, namely EOTF.sub.VD, and a matrix M.sub.VD. According to the invention, EOTF.sub.VD is defined as the EOTF of the reference display device and the matrix M.sub.VD is defined in reference notably to the primaries of the mastering display device according to the following equations:

[00003]$M_{\mathrm{VD}}={\mathrm{IM}}_{\mathrm{VD}}.Math.W_{\mathrm{VD}}.Math..Math.\mathrm{and}.Math..Math.{\mathrm{IM}}_{\mathrm{VD}}=\left[\begin{array}{ccc}{X}_{\mathrm{MD}-R}& X_{\mathrm{MD}-G}& X_{\mathrm{MD}-B}\\ Y_{\mathrm{MD}-R}& Y_{\mathrm{MD}-G}& Y_{\mathrm{MD}-B}\\ Z_{\mathrm{MD}-R}& Z_{\mathrm{MD}-G}& Z_{\mathrm{MD}-B}\end{array}\right]$$\mathrm{and}.Math..Math.W_{\mathrm{VD}}=\left[\begin{array}{ccc}{w}_{\mathrm{VD}-R}& 0& 0\\ 0& w_{\mathrm{VD}-G}& 0\\ 0& 0& w_{\mathrm{VD}-B}\end{array}\right]$$\mathrm{and}.Math.\left[\begin{array}{c}{w}_{\mathrm{VD}-R}\\ w_{\mathrm{VD}-G}\\ w_{\mathrm{VD}-B}\end{array}\right]={\mathrm{IM}}_D^{-1}\left[\begin{array}{c}{x}_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\\ 1\\ z_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\end{array}\right].$

where X.sub.MD-RY.sub.MD-RZ.sub.MD-R, X.sub.MD-GY.sub.MD-GZ.sub.MD-G and X.sub.MD-BY.sub.MD-BZ.sub.MD-B are the X,Y,Z color coordinates of, respectively, the Red, Green and Blue primaries of the mastering display device and x.sub.VD-W, y.sub.VD-W, z.sub.VD-W are the chromaticity coordinates of the white point of the reference display device in the XYZ color space.

[0111] We have then IT.sub.VD(XYZ)=EOTF.sup.1.sub.VD (M.sup.1.sub.VD[XYZ]). This inverse transform IT.sub.VD of the virtual display device is capable to transform XYZ coordinates of any color located within the color gamut of this virtual display devicei.e. of the mastering display deviceinto RGB color coordinates representing the same color in the color space of this virtual display device. The color gamut of this virtual display device consists of all colors defined by color coordinates X,Y,Z where these color coordinates X,Y,Z can be transformed by IT.sub.VD into valid R,G,B color coordinates. When X,Y,Z color coordinates are valid in the range [0,1], usually valid RGB color coordinates are in the range [0,1], too.

[0112] The data defining the Red, Green and Blue primary colors of the mastering display device could be sent as metadata together with the content to be reproduced, for instance by the content creator. Such metadata can be advantageously compliant with a standard, as for instance the MPEG proposal entitled Indication of SMPTE 2084 and 2085 and carriage of 2086 metadata in HEVC from January 2014, which proposes color primaries as SEI metadata, defined as follows: This SEI message provides metadata for specifying the color volume (the color primaries, white point, and luminance range) of the display that was used in mastering video content.

[0113] If no data are available concerning the mastering display device, Red, Green and Blue primaries of the mastering display device are replaced by Red, Green and Blue primaries extracted from a gamut boundary description describing the color gamut of the source content to calculate the matrix IM.sub.VD above.

[0114] As the second step above applies the inverse of the EOTF of the reference display device, this step is equivalent to a gamut mapping from the color gamut of the mastering display device to the color gamut of the reference display device.

[0115] As already explained above, source colors of the content to be reproduced by the target display device is generally within the color gamut of the mastering display device because this content is precisely generated by this mastering display device. Therefore, colors represented by X,Y,Z color coordinates obtained through the first step above are within the color gamut of the virtual display device, because this virtual display device is characterized by the same primary colors as those of the mastering display device. If these Primary colors are represented in the CIE xy chromaticity space by coordinates xr,yr for the red primary, xg,yg for the green primary, and xb,yb for the blue primary, these three primaries xr,yr and xg,yg and xb,yb form a chromaticity gamut triangle within the CIE xy chromaticity space. This gamut triangle corresponds to the color gamut of the mastering display device. As shown on FIG. 5, since any source color is generally within the color gamut of the mastering display device, its chromaticities xs,ys represented in the CIE xy are within this color gamut triangle. Since mastering and virtual displays have the same primary colors, the chromaticities xs,ys of a source color are within the gamut triangle of the virtual display, too. In general, primary colors of a display device are the main characteristics defining the color gamut of this display device. Since the virtual display device has the same primaries as the mastering display device, their gamuts are thus very close. Since the source colors are generally within the color gamut of the mastering display device, they are also within the color gamut of the virtual display device, too.

[0116] An important element of the method of color mapping of this general embodiment based on the first situation in which source colors are represented by R,G,B color coordinates is that the output RGB of the virtual display inverse transform are reference display dependent color coordinates, i.e. is that the obtained mapped colors are represented in the color space of the reference display device.

[0117] We will now describe in reference to FIG. 14 an example of implementation of the general embodiment of the first situation in which source colors to reproduced are represented by R,G,B color coordinates in the color space of the reference display device. in this example, the mapping method maps source colors of a source content from a source content color gamut in a reference color gamut. The source content color gamut may correspond to the color gamut of a mastering color display device used to master the source content, or is simply the color gamut of the source content itself. The source content color gamut is described by a gamut boundary description. The source colors are represented by device-dependent reference color coordinates R,G,B in the reference device dependent color space. The mapping described below maps the source colors into the reference color gamut. This reference color gamut is defined as the color gamut of a reference display device. This reference display device is notably characterized by a white point and a single electro-optical transfer function (EOTF).sub.RD. As already explained in detail above, this reference display device can be then modelled by a reference display device forward model FT.sub.RD capable of transforming R,G,B device-dependent color coordinates into X,Y,Z reference device-independent color coordinates and/or by a reference display device inverse model IT.sub.RD capable of transforming XYZ device-independent color coordinates into R,G,B reference device dependent color coordinates.

[0118] The mapping method comprises the following steps: [0119] 1. Obtainingin a manner known per seof primary colors from said gamut boundary description describing the source content color gamut. [0120] 2. From the obtained X.sub.S-RY.sub.S-RZ.sub.S-R, X.sub.S-GY.sub.S-GZ.sub.S-G and X.sub.S-BY.sub.S-BZ.sub.S-B coordinates of, respectively, the Red, Green and Blue primaries of the those primary colors in the CIE XYZ color space and from the chromaticities of the white point of the virtual display device x.sub.VD-W=x.sub.RD-W, y.sub.VD-W=y.sub.RD-W, that are set to the chromaticities x.sub.RD-W, y.sub.RD-W, of the white point of the reference display device, a source matrix M.sub.S is computed according to:

[00004]$M_S={\mathrm{IM}}_{\mathrm{VD}}.Math.W_{\mathrm{VD}}.Math..Math.\mathrm{and}.Math..Math.{\mathrm{IM}}_{\mathrm{VD}}=\left[\begin{array}{ccc}{X}_{S-R}& X_{S-G}& X_{S-B}\\ Y_{S-R}& Y_{S-G}& Y_{S-B}\\ Z_{S-R}& Z_{S-G}& Z_{S-B}\end{array}\right]$$\mathrm{and}.Math..Math.W_{\mathrm{VD}}=\left[\begin{array}{ccc}{w}_{\mathrm{VD}-R}& 0& 0\\ 0& w_{\mathrm{VD}-G}& 0\\ 0& 0& w_{\mathrm{VD}-B}\end{array}\right]$$\mathrm{and}.Math.\left[\begin{array}{c}{w}_{\mathrm{VD}-R}\\ w_{\mathrm{VD}-G}\\ w_{\mathrm{VD}-B}\end{array}\right]={\mathrm{IM}}_D^{-1}\left[\begin{array}{c}{x}_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\\ 1\\ z_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\end{array}\right].$ [0121] 3. Applying the reference device forward model as defined above to the RGB reference device dependent source color coordinates representing the source colors to map, resulting in XYZ device independent, linear source color coordinates representing the source colors in the XYZ CIE device-independent, linear color space. [0122] 4. Applying the inverse of the computed source matrix M.sub.S to these XYZ device independent, linear source color coordinates, resulting in R.sub.lG.sub.lB.sub.l device dependent, linear reference color coordinates representing the source colors in a linearized reference display color space; [0123] 5. Applying the inverse of the electro-optical transfer function EOTF.sub.RD of the reference display device to the R.sub.lG.sub.lB.sub.l device dependent, linear reference color coordinates resulting in RGB final device dependent, non-linear reference color coordinates representing mapped source colors in color space of the reference display device.

[0124] As a whole, the combination of the application of the source matrix M.sub.S and of the application of the inverse of the EOTF.sub.RD of the reference display device is equivalent to the application of the inverse model IT.sub.VD of a virtual display device such that IT.sub.VD (XYZ)=EOTF.sup.1.sub.RD (M.sup.1.sub.S[XYZ]).

[0125] An implementation of the above example on a whole image workflow is shown in FIG. 15, from the mastering of the content through the formatting according to BT.2020 up to the final rendering of the content on a target display device, namely a consumer display such as a LCD or a tablet.

[0126] In order to ensure valid device-dependent RGB color coordinates, the coordinates are clipped after application of inverse EOTF, such as shown in FIG. 1. The workflow of FIG. 15 starts with:

[0127] mastering of the source content resulting in RGB color coordinates representing source colors in the color space of the mastering display,

[0128] application of a forward model of the mastering display then of an inverse model of the BT.2020 reference display, resulting in RGB color coordinates representing source colors in the color space of the ITU-R BT.2020 reference display,

[0129] application of the mapping method as described in the example above, resulting in RGB color coordinates representing mapped source colors in the color space of the BT.2020 reference display,

[0130] application of the forward model of the BT.2020 reference display then of an inverse model of the consumer displayi.e. target display device, resulting in R,G,B color coordinates representing mapped source colors in the color space of this consumer display, that are adapted to control this consumer display for the rendering of the source content.

[0131] A general embodiment of a mapping in the second situation in which source colors are represented in a device-independent color space by trichromatic color coordinates X,Y,Z will be now described in reference to FIG. 10, already explained. In this embodiment, the virtual display transform IT.sub.VD defined above is applied first resulting in reference display dependent color coordinates R,G,B representing mapped source colors in the color space of the reference display device. Then, device independent X,Y,Z color coordinates representing the same mapped colors in the CIE XYZ color space are obtained by applying the reference display forward transform as defined above.

[0132] We will now describe a first example of implementation of this general embodiment in reference to FIG. 16. The source content color gamut is described by a gamut boundary description. The source colors are represented by device-independent, linear source color coordinates X,Y,Z, in the CIE XYZ color space. As already explained, the mapping of source colors maps colors towards the reference color gamut. The reference color gamut is the color gamut of the reference display device having a white point and an electro-optical transfer function (EOTF) that are used to compute the inverse transform IT.sub.VD of the virtual display device.

[0133] The method comprises the following steps: [0134] 1. Extraction of primary colors from the gamut boundary description describing the source content color gamut. [0135] 2. From the extracted X.sub.S-RY.sub.S-RZ.sub.S-R, X.sub.S-GY.sub.S-GZ.sub.S-G and X.sub.S-BY.sub.S-BZ.sub.S-B coordinates of, respectively, the Red, Green and Blue primaries of those primary colors in the CIE XYZ color space and from the chromaticities of the white point of the virtual display device x.sub.VD-W=x.sub.RD-W, y.sub.VD-W=y.sub.RD-W that are set to the chromaticities x.sub.RD-W, y.sub.RD-W of the white point of the reference display device, a source matrix M.sub.S is calculated as follows:

[00005]$M_S={\mathrm{IM}}_{\mathrm{VD}}.Math.W_{\mathrm{VD}}.Math..Math.\mathrm{and}.Math..Math.{\mathrm{IM}}_{\mathrm{VD}}=\left[\begin{array}{ccc}{X}_{S-R}& X_{S-G}& X_{S-B}\\ Y_{S-R}& Y_{S-G}& Y_{S-B}\\ Z_{S-R}& Z_{S-G}& Z_{S-B}\end{array}\right]$$\mathrm{and}.Math..Math.W_{\mathrm{VD}}=\left[\begin{array}{ccc}{w}_{\mathrm{VD}-R}& 0& 0\\ 0& w_{\mathrm{VD}-G}& 0\\ 0& 0& w_{\mathrm{VD}-B}\end{array}\right]$$\mathrm{and}.Math.\left[\begin{array}{c}{w}_{\mathrm{VD}-R}\\ w_{\mathrm{VD}-G}\\ w_{\mathrm{VD}-B}\end{array}\right]={\mathrm{IM}}_D^{-1}\left[\begin{array}{c}{x}_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\\ 1\\ z_{\mathrm{VD}-W}/y_{\mathrm{VD}-W}\end{array}\right].$ [0136] 3. Applying the inverse of source matrix M.sub.S to the X,Y,Z device independent, linear source color coordinates representing the source colors to be mapped, resulting into R.sub.lG.sub.lB.sub.l device dependent, linear reference color coordinates. [0137] 4. Applying the inverse of the electro-optical transfer function EOTF.sub.RD of the reference display device to the R.sub.lG.sub.lB.sub.l device dependent, linear reference color coordinates resulting in RGB final device dependent, non-linear reference color coordinates representing mapped source colors in color space of the reference display device. [0138] 5. Applying the reference device forward model to the RGB final device dependent, non linear reference color coordinates resulting in XYZ device independent, linear source color coordinates representing the mapped source colors in device-independent, linear color space.

[0139] As a whole, the combination of the application of the source matrix M.sub.S and of the application of the inverse of the EOTF.sub.RD of the reference display device is equivalent to the application of the inverse model IT.sub.VD of a virtual display device such that IT.sub.VD (XYZ)=EOTF.sup.1.sub.RD (M.sup.1.sub.S[XYZ]).

[0140] We will now describe a second example of implementation of the general embodiment above in reference to FIG. 17. In this second example, the mapping method is amended by an additional step called merging of primary colors controlled by a color reproduction parameter which allows advantageously to control the mapping method according to a tradeoff between color hue fidelity and color chroma fidelity. In this second example, the primary colors are replaced by merged primary colors, the merged primary colors being a weighted average between primary colors that are extracted as shown above and primary colors of the reference display device, the weight being a color reproduction parameter computed such that the minimum value of this parameter results in that merged primary colors are identical to the extracted primary colors and such that the maximum value of this parameter results in that merged primary colors are identical to the primary colors of the reference display device.

[0141] This second example can be further simplified into a third example if the reference display device and the virtual display device are characterized by the same triple of EOTFs. In this case, neither an EOTF nor an inverse EOTF needs to be applied to color coordinates. This third example is shown on FIG. 18.

[0142] An implementation of the general embodiment above applied on a whole image workflow is shown on FIG. 19, from the mastering of the content through the formatting according to BT.2020 up to the final rendering of the content on a target display device, namely a consumer display such as a LCD or a tablet. This implementation considers source content that has been produced using a source display, also called mastering display. We further consider in this implementation a UHDTV reference display compliant to ITU-R BT.2020.

[0143] This implementation is then based on the following steps: [0144] Using a mastering display, artistic creation of an image the colors of which are represented by RGB color coordinates. [0145] Transforming the created mastering display device-dependent color coordinates R,G,B into X,Y,Z device-independent color coordinates using a forward model of the mastering display. [0146] Describing the source color gamut of the mastering display using X,Y,Z color coordinates of at least the red, green and blue as primary colors. These colors are measured using a colorimeter as output of the display controlled by at least three input signals. In case of a mastering display having 8 bit encoded R,G,B inputs, the input signals are (255,0,0), (0,255,0), (0,0,255), (255,0,255), respectively. [0147] Using SMPTE RP177 modeling of a display device, calculating from these primary colors and from the white point of the reference display a linear matrix M.sub.MD transforming device independent color coordinates into linear, pseudo device-dependent color coordinates:

[00006]$M_{\mathrm{MD}}={\mathrm{IM}}_{\mathrm{VD}}.Math.W_{\mathrm{VD}}.Math..Math.\mathrm{with}.Math..Math.{\mathrm{IM}}_{\mathrm{MD}}=\left[\begin{array}{ccc}{X}_{\mathrm{MD}-R}& X_{\mathrm{MD}-G}& X_{\mathrm{MD}-B}\\ Y_{\mathrm{MD}-R}& Y_{\mathrm{MD}-G}& Y_{\mathrm{MD}-B}\\ Z_{\mathrm{MD}-R}& Z_{\mathrm{MD}-G}& Z_{\mathrm{MD}-B}\end{array}\right]$$\mathrm{and}.Math..Math.W_{\mathrm{VD}}=\left[\begin{array}{ccc}{w}_{\mathrm{VD}-R}& 0& 0\\ 0& w_{\mathrm{VD}-G}& 0\\ 0& 0& w_{\mathrm{VD}-B}\end{array}\right]$$\mathrm{and}.Math.\left[\begin{array}{c}{w}_{\mathrm{VD}-R}\\ w_{\mathrm{VD}-G}\\ w_{\mathrm{VD}-B}\end{array}\right]={\mathrm{IM}}_D^{-1}\left[\begin{array}{c}{x}_{\mathrm{RD}-W}/y_{\mathrm{RD}-W}\\ 1\\ z_{\mathrm{RD}-W}/y_{\mathrm{RD}-W}\end{array}\right].$ [0148] where X.sub.MD-RY.sub.MD-RZ.sub.MD-R, X.sub.MD-GY.sub.MD-GZ.sub.MD-G and X.sub.MD-BY.sub.MD-BZ.sub.MD-B are the XYZ color coordinates of, respectively, the Red, Green and Blue primaries of the mastering display device and x.sub.RD-W, y.sub.RD-W, y.sub.zRD-W are the chromaticity coordinates of the white point of the reference display device in the xy chromaticity space. [0149] Applying this linear matrix to the XYZ device-independent, linear source color coordinates, resulting into R.sub.lG.sub.lB.sub.l device dependent, linear reference color coordinates. [0150] Applying the inverse of the (usually non-linear) electro-optical transfer function (EOTF) of the reference display device resulting into non-linear, pseudo device dependent color coordinates. [0151] Assuming as reference display an ITU-R BT.2020 compliant display, applying the inverse EOTF of this ITU-R BT.2020 compliant display to the non-linear, pseudo device-dependent color coordinates, and applying then a second linear matrix calculated from the primary colors and the white color of the ITU BT.2020 compliant display, resulting into device independent color coordinates XYZ representing mapped colors. [0152] Applying the inverse transform characterizing the consumer display used to reproduced the source colors, resulting in RGB color coordinates adapted to control this consumer display.

[0153] It is to be understood that the mapping method according to 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. Moreover, 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. Preferably, 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. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit.

[0154] Therefore, further subjects of the invention are summarized below.

[0155] A subject of the invention is notably a color mapping device for mapping source colors of a source content, wherein said source colors are represented by device-dependent source coordinates R,G,B in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform comprising:

[0156] a reference display forward color transform module configured for applying said reference display forward color transform to device-dependent source coordinates (R,G,B) representing said source colors, resulting in device-independent source coordinates (X,Y,Z) representing the same source colors in a device-independent linear color space,

[0157] a virtual display inverse color transform module configured for applying a virtual display inverse color transform (IT.sub.VD) to the device-independent source coordinates X,Y,Z provided by said reference display forward color transform module, resulting in device-dependent mapped coordinates R,G,B representing mapped colors in said reference device-dependent color space, wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content.

[0158] A subject of the invention is also a color mapping device for mapping source colors of a source content, wherein said source colors are represented by first device-independent source coordinates X,Y,Z in a device-independent color space comprising:

[0159] a virtual display inverse color transform module configured for applying a virtual display inverse color transform (IT.sub.VD) to said device-independent source coordinates X,Y,Z representing said source colors, resulting in device-dependent mapped coordinates R,G,B representing mapped colors in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform,

[0160] a reference display forward color transform module configured for applying said reference display forward color transform to device-dependent mapped coordinates R,G,B provided by said virtual display inverse color transform module, resulting in device-independent mapped coordinates X,Y,Z representing the same mapped colors in the device-independent linear color space,

wherein said virtual display inverse color transform (IT.sub.VD) models a virtual display device characterized by the same primaries as the primaries of a mastering display device used to master said source content or by primaries extracted from a description of the color gamut of said source content.

[0161] A subject of the invention is also a target display device characterized by a target display inverse color transform characterized by a target display inverse color transform, configured for reproducing a source content, comprising

[0162] a reception module configured for receiving device-dependent source coordinates R,G,B representing source colors of said source content in the reference device-dependent color space of a reference display device characterized by a reference display forward color transform,

[0163] a color primaries module configured to provide color primaries received as metadata representing color primaries of a mastering display device used to master said source content or extracted from a description of the color gamut of said source content,

[0164] a color mapping device as summarized above that is configured to map device-dependent source coordinates R,G,B provided by said reception module, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by color primaries provided by said color primaries module, resulting in device-dependent mapped coordinates R,G,B representing said mapped colors,

[0165] a final color transform module configured to apply said reference display forward color transform and said target display inverse color transform to device-dependent mapped coordinates R,G,B provided by said color mapping device, resulting into device-independent mapped coordinates X,Y,Z representing said mapped colors in device-independent color space, configured to gamut map said device-independent mapped coordinates X,Y,Z from said reference color gamut towards said target color gamut and to apply said target display inverse color transform to said gamut-mapped device-independent mapped coordinates X,Y,Z, resulting into device-dependent target coordinates R,G,B representing said mapped colors in the target device-dependent color space of said target display device,

[0166] a target display control module configured to control said target display device by inputting device-dependent mapped coordinates R,G,B provided by said final color transform module, resulting in the reproduction of said source content.

[0167] A subject of the invention is also a target display device characterized by a target display inverse color transform characterized by a target display inverse color transform, configured for reproducing a source content, comprising:

[0168] a reception module configured for receiving device-independent source coordinates X,Y,Z representing source colors of said source content,

[0169] a color primaries module configured to provide color primaries received as metadata representing color primaries of a mastering display device used to master said source content or extracted from a description of the color gamut of said source content,

[0170] a color mapping device as summarized above that is configured to map device-independent source coordinates X,Y,Z provided by said reception module, using a virtual display inverse color transform (IT.sub.VD) modelling a virtual display device characterized by color primaries provided by said color primaries module, resulting in device-independent mapped coordinates X,Y,Z representing said mapped colors,

[0171] a final color transform module configured to apply said target display inverse color transform to device-independent mapped coordinates X,Y,Z provided by said color mapping device, resulting into device-dependent mapped coordinates R,G,B,

[0172] a target display control module configured to control said target display device by inputting device-dependent mapped coordinates R,G,B provided by said final color transform module, resulting in the reproduction of said source content.

[0173] Although the illustrative embodiments of the invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one of ordinary skill in the pertinent art without departing from the invention. All such changes and modifications are intended to be included within the scope of the present invention as set forth in the appended claims.