METHODS AND DEVICES FOR ENCODING AND DECODING A COLOR PICTURE

1 . A method for encoding a High Dynamic Range (HDR) color picture and at least one first Standard Dynamic Range (SDR) color picture, said method comprising:

encoding a second Standard Dynamic Range (SDR) color picture obtained from the HDR color picture;

wherein the method further comprises:

determining a color remapping model composed of a first piece-wise linear function, a three by-three matrix and a second piece-wise linear function from the colors of said second Standard Dynamic Range (SDR) color picture and the colors of said at least one first Standard Dynamic Range (SDR) color picture.

2 . The method of claim 1 , wherein said at least one first Standard Dynamic Range (SDR) color picture is obtained from a color-graded version of said High Dynamic Range (HDR) color picture.

3 . The method of claim 1 , wherein encoding ( 101 ) a second SDR color picture obtained from said High Dynamic Range (HDR) color picture color picture comprises:

obtaining a luminance component (L) and two chrominance components (C 1 , C 2 ) from said second Standard Dynamic Range (SDR) color picture,

mapping the luminance (L) and chrominance (C 1 , C 2 ) components onto a final luminance component (L″) and two final chrominance components (C″ 1 , C″ 2 ) in order that the gamut of colors obtained from said final luminance (L″) and chrominance (C″ 1 , C″ 2 ) components maps onto the gamut of the colors of the high dynamic range color picture, the values of the final luminance component (L″) being always lower than the values of the luminance component (L).

4 . The method of claim 1 , wherein said method further comprises transmitting said color remapping model.

5 . The method of claim 3 , wherein said second Standard Dynamic Range (SDR) color picture, is delivered by said mapping.

6 . The method of claim 3 , wherein at least two distinct first Standard Dynamic Range (SDR) color pictures are respectively obtained from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, and

wherein, for each considered first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, a color remapping model is respectively determined from said second Standard Dynamic Range (SDR), delivered by said mapping to said considered first Standard Dynamic Range (SDR) color picture.

7 . The method of claim 3 , wherein at least two distinct first Standard Dynamic Range (SDR) color pictures are respectively obtained from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, and

wherein said second Standard Dynamic Range (SDR) is delivered by an invertible gamut mapping between said distinct color gamuts, said invertible gamut mapping, being performed after said mapping and before said encoding, and mapping one of said distinct color gamuts onto the other, and

wherein, for a considered first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, said corresponding color remapping model is determined from said second Standard Dynamic Range (SDR) to said other first Standard Dynamic Range (SDR) color picture, and

wherein for the other first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, said corresponding color remapping model is determined from a third Standard Dynamic Range (SDR), delivered by an inverse mapping operation performed after said invertible gamut mapping, to said other first Standard Dynamic Range (SDR) color picture.

8 . The method of claim 1 , wherein said color remapping model information is transmitted in a dedicated transmission channel distinct from a channel used for transmitting a bitstream comprising said second Standard Dynamic Range (SDR).

9 . A bitstream (B R ) obtained from a High Dynamic Range (HDR) color picture and at least one first Standard Dynamic Range (SDR) color picture, said bitstream comprising at least one encoded second Standard Dynamic Range (SDR) color picture, wherein said bitstream comprises also a color remapping model composed of a first piece-wise linear function, a three by-three matrix and a second piece-wise linear function from the colors of associated with said at least one encoded second Standard Dynamic Range color picture, said at least one piece of color remapping information being used to obtain an approximation of said at least one first Standard Dynamic Range color picture from said at least one encoded second Standard Dynamic Range color picture.

10 . A method for decoding a High Dynamic Range (HDR) color picture and at least one first Standard Dynamic Range (SDR) color picture, from a second Standard Dynamic Range (SDR) color picture of a received bitstream, said method comprising:

decoding said second Standard Dynamic Range (SDR) color picture;

wherein the method further comprises:

obtaining a color remapping model composed of a first piece-wise linear function, a three by-three matrix and a second piece-wise linear function, and associated with said second Standard Dynamic Range (SDR) color picture, and

applying said color remapping model to said second Standard Dynamic Range (SDR) color picture.

11 . The method of claim 10 , wherein said at least one first Standard Dynamic Range (SDR) color picture has been obtained, during encoding, from a color-graded version of said High Dynamic Range (HDR) color picture.

12 . The method of claim 10 , wherein said decoding of said second Standard Dynamic Range (SDR) color picture further comprises:

obtaining a final luminance component (L) and two final chrominance components (C 1 , C 2 ) by applying an inverse mapping on the colors obtained from a luminance (L″) component and two chrominance components (C″ 1 , C″ 2 ) obtained from the bitstream; and

obtaining at least one color component (Ec) of said second Standard Dynamic Range (SDR) color picture from said final luminance (L) component and said two final chrominance (C 1 , C 2 ) components, the values of the final luminance component (L) being always higher than the values of the luminance component (L″).

13 . The method of claim 10 , wherein at least two distinct color remapping models associated with said second Standard Dynamic Range (SDR) color picture are obtained, and then applied to said second Standard Dynamic Range (SDR) delivering at least two distinct approximations of at least two distinct first Standard Dynamic Range (SDR) color pictures, obtained, during encoding, from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts.

14 . The method of claim 10 , wherein at least two distinct color remapping models associated with said second Standard Dynamic Range (SDR) color picture are obtained, and

wherein a first color remapping model of said at least two color remapping models, is applied to said second Standard Dynamic Range (SDR) delivering an approximation of one first Standard Dynamic Range (SDR) color picture, and

wherein a second remapping model of said at least two color remapping models, is applied to a third Standard Dynamic Range (SDR) delivered by an inverse operation of an invertible gamut mapping of said second Standard Dynamic Range (SDR), delivering an approximation of another first Standard Dynamic Range (SDR) color picture, said first Standard Dynamic Range (SDR) color pictures, being obtained, during encoding, from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, said invertible gamut mapping one of said distinct color gamuts onto the other.

15 . The method of claim 10 , wherein said color remapping model is obtained from a dedicated transmission channel distinct from a channel used for transmitting said bitstream comprising said second Standard Dynamic Range (SDR).

16 . A device for encoding a High Dynamic Range (HDR) color picture and at least one first Standard Dynamic Range (SDR) color picture, said device comprising a processor configured to:

encode a second Standard Dynamic Range (SDR) color picture obtained from the HDR color picture;

wherein the processor is further configured to:

determine a color remapping model composed of a first piece-wise linear function, a three by-three matrix and a second piece-wise linear function from the colors of said second Standard Dynamic Range (SDR) color picture and the colors of said at least one first Standard Dynamic Range (SDR) color picture.

17 . A device for decoding a High Dynamic Range (HDR) color picture and at least one first Standard Dynamic Range (SDR) color picture, from a second Standard Dynamic Range (SDR) color picture of a received bitstream, said device comprising a processor configured to:

decode said second Standard Dynamic Range (SDR) color picture;

wherein the processor is further configured to:

obtain a color remapping model composed of a first piece-wise linear function, a three by-three matrix and a second piece-wise linear function, and associated with said second Standard Dynamic Range (SDR) color picture, and

apply said color remapping model to said second Standard Dynamic Range (SDR) color picture.

18 . A computer program product comprising program code instructions to execute the steps of the encoding method according to claim 1 when this program is executed on a computer.

19 . A computer program product comprising program code instructions to execute the steps of the decoding method according to claim 10 when this program is executed on a computer.

20 . The device of claim 16 , wherein said at least one first Standard Dynamic Range (SDR) color picture is obtained from a color-graded version of said High Dynamic Range (HDR) color picture.

21 . The device of claim 16 , wherein encoding a second SDR color picture obtained from said High Dynamic Range (HDR) color picture color picture comprises:

obtaining a luminance component and two chrominance components from said second Standard Dynamic Range (SDR) color picture,

mapping the luminance and chrominance components onto a final luminance component and two final chrominance components in order that the gamut of colors obtained from said final luminance and chrominance components maps onto the gamut of the colors of the High Dynamic Range (HDR) color picture, the values of the final luminance component being always lower than the values of the luminance component.

22 . The device of claim 16 , wherein said method further comprises transmitting said color remapping model.

23 . The device of claim 16 , wherein said second Standard Dynamic Range (SDR) color picture, is delivered by said mapping.

24 . The device of claim 16 , wherein at least two distinct first Standard Dynamic Range (SDR) color pictures are respectively obtained from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, and wherein, for each considered first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, a color remapping model is respectively determined from said second Standard Dynamic Range (SDR), delivered by said mapping to said considered first Standard Dynamic Range (SDR) color picture.

25 . The device of claim 16 , wherein at least two distinct first Standard Dynamic Range (SDR) color pictures are respectively obtained from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, and wherein said second Standard Dynamic Range (SDR) is delivered by an invertible gamut mapping between said distinct color gamuts, said invertible gamut mapping, being performed after said mapping and before said encoding, and mapping one of said distinct color gamuts onto the other, and wherein, for a considered first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, said corresponding color remapping model is determined from said second Standard Dynamic Range (SDR) to said other first Standard Dynamic Range (SDR) color picture, and wherein for the other first Standard Dynamic Range (SDR) color picture of said at least two distinct first Standard Dynamic Range (SDR) color pictures, said corresponding color remapping model is determined from a third Standard Dynamic Range (SDR), delivered by an inverse mapping operation performed after said invertible gamut mapping, to said other first Standard Dynamic Range (SDR) color picture.

26 . The device of claim 16 , wherein said a color remapping model is transmitted in a dedicated transmission channel distinct from a channel used for transmitting a bitstream comprising said second Standard Dynamic Range (SDR).

27 . The method of claim 17 , wherein said at least one first Standard Dynamic Range (SDR) color picture has been obtained, during encoding, from a color-graded version of said High Dynamic Range (HDR) color picture.

28 . The device of claim 17 , wherein said decoding of said second Standard Dynamic Range (SDR) color picture further comprises:

obtaining a final luminance component and two final chrominance components by applying an inverse mapping on the colors obtained from a luminance component and two chrominance components obtained from the bitstream; and

obtaining at least one color component of said second Standard Dynamic Range (SDR) color picture from said final luminance component and said two final chrominance components, the values of the final luminance component being always higher than the values of the luminance component.

29 . The device of claim 17 , wherein at least two distinct color remapping models associated with said second Standard Dynamic Range (SDR) color picture are obtained, and then applied to said second Standard Dynamic Range (SDR) delivering at least two distinct approximations of at least two distinct first Standard Dynamic Range (SDR) color pictures, obtained, during encoding, from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts.

30 . The method of claim 17 , wherein at least two distinct color remapping models associated with said second Standard Dynamic Range (SDR) color picture are obtained, and wherein a first color remapping model of said at least two color remapping models, is applied to said second Standard Dynamic Range (SDR) delivering an approximation of one first Standard Dynamic Range (SDR) color picture, and wherein a second color remapping model of said at least two color remapping models, is applied to a third Standard Dynamic Range (SDR) delivered by an inverse operation of an invertible gamut mapping of said second Standard Dynamic Range (SDR), delivering an approximation of another first Standard Dynamic Range (SDR) color picture, said first Standard Dynamic Range (SDR) color pictures, being obtained, during encoding, from at least two distinct color-graded versions of said High Dynamic Range (HDR) color picture by using respectively distinct color gamuts, said invertible gamut mapping one of said distinct color gamuts onto the other.

31 . The device of claim 17 , wherein said color remapping model is obtained from a dedicated transmission channel distinct from a channel used for transmitting said bitstream comprising said second Standard Dynamic Range (SDR).