METHOD AND APPARATUS OF ENCODING AND DECODING A COLOR PICTURE

1 . A method for encoding a color picture having color components (Ec), comprising:

obtaining a luminance component (L) comprising:

obtaining a modulation value (Ba) from the luminance (Y) of the color picture;

obtaining a scaled luminance by dividing the luminance (Y) of the color picture by said modulation value (Ba);

obtaining the luminance component (L) by applying a non-linear function on said scaled luminance in order that the dynamic of said luminance component (L) is reduced compared to the dynamic of said scaled luminance;

obtaining two chrominance components (C1, C2) comprising:

obtaining a factor (r(L(i)) that depends on the value of the pixel (i) of said luminance component (L(i)) and the luminance value (Y(i)) of the co-located pixel (i) in the color picture;

obtaining at least one intermediate color component (E′c) by multiplying each color component (Ec) by said factor (r(L(i)); and

obtaining said two chrominance components (C1, C2) from said at least one intermediate color components (E′c); and

encoding said luminance (L) and two chrominance components (C1, C2).

2 . The method of claim 1 , wherein the non-linear function is either a gamma curve or a S log curve according to the pixel values of said scaled luminance (Y).

3 . The method of the claim 2 , wherein the method further comprises generating an information data (Inf) that indicates whether either the non-linear function is said gamma correction or said S log correction.

4 . The method of claim 1 , wherein the method further comprises storing in a local or remote memory and/or adding to a bitstream at least one of the following values:

the modulation value (Ba);

parameters of the non-linear function;

the information data (Inf).

5 . The method of claim 1 , wherein the factor (r(L(i)) is a ratio of the value (L(i)) of the pixel (i) of said luminance component over the luminance value (Y(i)) of the co-located pixel (i) in the color picture.

6 . The method of claim 1 , wherein the factor r(L(i)) is given by:

r



(

L



(

i

)

)

=

max





{

5

,

L



(

i

)

}

2048





max





{

0.01

,

Y



(

i

)

}

where L(i) is the value of the pixel of said luminance component and Y(i) the luminance value of the co-located pixel in the color picture.

7 . The method of claim 1 , wherein obtaining said two chrominance components (C1, C2) from said at least one intermediate color components (E′c) comprises:

obtaining three intermediate components (Dc) by applying an OETF on each intermediate color component (E′c); and

linearly combining together the three intermediate components (Dc).

8 . The method of the claim 7 , wherein the OETF is a square root.

9 . The method of the claim 7 , wherein the OETF is a cubic root.

10 . A method for decoding a color picture from a bitstream, comprising:

obtaining a first component (Y) comprising:

obtaining a luminance component (L) from the bitstream;

obtaining a resulting component by applying a non-linear function on said luminance component (L) in order that the dynamic of said resulting component is increased compared to the dynamic of the luminance component (L);

obtaining a modulation value (Ba) from the luminance of the color picture to be decoded;

obtaining the first component (Y) by multiplying said resulting component by said modulation value;

obtaining two chrominance components (C1, C2) from the bitstream;

obtaining a factor (r(L(i))) that depends on the value (L(i)) of the pixel (i) of said luminance component (L);

obtaining at least one color component (Ec) from said first component (Y), said two chrominance component (C1, C2) and said factor (r(L(i))); and

forming a decoded picture by combining together said at least one color component (Ec).

11 . The method of claim 10 , wherein obtaining at least one color component (Ec) comprises:

obtaining three intermediate color components (E′c) from said first component (Y) and the two chrominance components (C1, C2); and

obtaining said at least color component (Ec) by dividing each intermediate color component (E′c) by said factor (r(L(i))).

12 . The method of claim 10 , wherein obtaining at least one color component (Ec) comprises:

obtaining two intermediate chrominance component (C′1, C′2) by dividing each chrominance component (C1, C2) according to the factor (r(L(i))); and

obtaining said at least one color components (Ec) from said first component (Y) and said two intermediate chrominance components (C′1, C′2).

13 . The method of claim 12 , wherein obtaining at least one color component (Ec) comprises:

obtaining two intermediate chrominance component (C′1, C′2) by dividing each chrominance component (C1, C2) by a value equal to the square root of the factor (r(L(i))); and

obtaining said at least one color components (Ec) comprises:

obtaining a second component (S) by combining together the two intermediate chrominance components (C′1, C′2) and the first component (Y);

obtaining at least one intermediate color components (Dc) by linearly combining together the intermediate chrominance component (C′1, C′2) and said second component (S); and

obtaining the three color components (Ec) by taking the square of each intermediate color components (Dc).

14 . The method of claim 10 , wherein the non-linear function is the inverse of either a gamma curve or a S log curve according to the pixel values of said scaled luminance (Y).

15 . The method of claim 10 , wherein the method further comprises obtaining at least one of the following values from either a local or remote memory and/or from a bitstream:

the modulation value (Ba);

parameters of the non-linear function;

an information data (Inf) that indicates whether either the non-linear function is a gamma correction or a S log correction.

16 . The method of claim 10 , wherein the factor (r(L(i)) is a ratio of the value (L(i)) of the pixel (i) of said luminance component over the luminance value (Y(i)) of the co-located pixel (i) in said first component (Y).

17 . The method of claim 10 , wherein the factor (r(I(i))) is obtained from either a local or remote memory or from a bitstream.

18 . A device for encoding a color picture having color components (Ec), comprising a processor configured to:

obtain a modulation value (Ba) from the luminance (Y) of the color picture;

obtain a scaled luminance (Y) by dividing the luminance (Y) of the color picture by said modulation value (Ba);

obtain the luminance component (L) by applying a non-linear function on said scaled luminance (Y), in order that the dynamic of said luminance component (L) is reduced compared to the dynamic of said scaled luminance (Y);

obtain two chrominance components (C1, C2) comprising:

obtaining a factor (r(L(i)) that depends on the value of the pixel (i) of said luminance component (L(i)) and the luminance value (Y(i)) of the co-located pixel (i) in the color picture;

obtaining at least one intermediate color component (E′c) by multiplying each color component (Ec) by said factor (r(L)); and

obtaining said two chrominance components (C1, C2) from said at least one intermediate color components (E′c); and

encode said luminance (L) and two chrominance components (C1, C2).

19 . A device for decoding a color picture from a bitstream, comprising a processor configured to:

obtain a first component (Y) comprising:

obtaining a luminance component (L) from the bitstream;

obtaining a resulting component by applying a non-linear function on said luminance component (L) in order that the dynamic of said resulting component is increased compared to the dynamic of the luminance component (L);

obtaining a modulation value (Ba) from the luminance (Y) of the color picture to be decoded;

obtaining the first component (Y) by multiplying said resulting component by said modulation value;

obtain two chrominance components (C1, C2) from the bitstream;

obtain a factor (r(L(i))) that depends on the value (L(i)) of the pixel (i) of said luminance component (L);

obtain at least one color component (Ec) from said first component (Y), said two chrominance component (C1, C2) and said factor;

the decoded picture being formed by combining together said at least one color component (Ec).

20 . 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.

21 . 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.

22 . A processor readable medium having stored therein instructions for causing a processor to perform at least the steps of the encoding method according to claim 1 .

23 . A processor readable medium having stored therein instructions for causing a processor to perform at least the steps of the decoding method according to claim 10 .

24 . Non-transitory storage medium carrying instructions of program code for executing steps of the method according to claim 1 , when said program is executed on a computing device.