Coding and decoding method with color conversion and corresponding devices

1. A decoding method comprising:

decoding one standard dynamic range luminance component L′ and two standard dynamic range chrominance components U′ and V′ from a bitstream produced by an encoder;

color converting the standard dynamic range luminance component L′ into L and the two decoded standard dynamic range chrominance components U′ and V′ into Ur and Vr, respectively, as follows:

L=L ′+max(0, aU′+bV ′), Ur =β( L )* U′,Vr =β( L )* V′

where a and b are constants and where β(L) is a parameter that depends on L;

applying independently:

a dynamic expansion function on the color converted standard dynamic range luminance component L to obtain a high dynamic range luminance component Y, wherein the dynamic expansion function is an inverse of a dynamic reduction function applied to a high dynamic range luminance component on the encoder's side; and

a color transfer operation on the color converted standard dynamic range chrominance components Ur and Vr to obtain a first intermediate high dynamic range chrominance component and a second intermediate high dynamic range chrominance component in an intermediate color space; and

applying a color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain a high dynamic range picture in RGB color space.

2. The method according to claim 1 , wherein applying the dynamic expansion function on the color converted standard dynamic range luminance component L to obtain the high dynamic range luminance component Y comprises determining:

Y=g −1 ( Ba,L ))

where Ba is a modulation value and g −1 ( ) is the inverse of the dynamic reduction function applied to the high dynamic range luminance component on the encoder's side.

3. The method according to claim 1 , wherein applying a color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain the high dynamic range picture in RGB color space comprises:

calculating T as a linear combination of Ur 2 ,Vr 2 and Ur*Vr;

calculating S as √{square root over (1−T)} where T≤1 and setting S to 0 and dividing Ur and Vr by √{square root over (T)} otherwise;

calculating intermediate (R # ,G # ,B # ) components in RGB color space by applying a color conversion matrix on S, Ur, and Vr.

4. A decoder comprising a communication interface configured to access a bitstream produced by an encoder and at least one processor configured to:

decode one standard dynamic range luminance component L′ and two standard dynamic range chrominance components U′ and V′ from a bitstream;

color convert the standard dynamic range luminance component L′ into L and the two decoded standard dynamic range chrominance components U′ and V′ into Ur and Vr, respectively, as follows:

L=L ′+max(0 ,aU′+bV′ ), Ur=β ( L )* U′,Vr=β ( L )* V′

where a and b are constants and where β(L) is a parameter that depends on L;

apply independently:

a dynamic expansion function on the color converted standard dynamic range luminance component L to obtain a high dynamic range luminance component Y, wherein the dynamic expansion function is an inverse of a dynamic reduction function applied to a high dynamic range luminance component on the encoder's side; and

a color transfer operation on the color converted standard dynamic range chrominance components Ur and Vr to obtain a first intermediate high dynamic range chrominance component and a second intermediate high dynamic range chrominance component in an intermediate color space; and

apply a color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain a high dynamic range picture in RGB color space.

5. The decoder according to claim 4 , wherein to apply the dynamic expansion function on the color converted standard dynamic range luminance component L to obtain the high dynamic range luminance component Y comprises determining:

Y=g −1 ( Ba,L ))

where Ba is a modulation value and g −1 ( ) is the inverse of the dynamic reduction function applied to the high dynamic range luminance component on the encoder's side.

6. The decoder according to claim 4 , wherein to apply the color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain the high dynamic range picture in RGB color space comprises:

calculating T as a linear combination of Ur 2 ,Vr 2 and Ur*Vr;

calculating S as √{square root over (1−T)} where T≤1 and setting S to 0 and dividing Ur and Vr by √{square root over (T)} otherwise;

calculating intermediate (R # ,G # ,B # )components in RGB color space by applying the color conversion matrix on S, Ur, and Vr.

7. A non-transitory computer readable medium with instructions stored therein which, upon execution, instruct at least one processor to:

decode one standard dynamic range luminance component L′ and two standard dynamic range chrominance components U′ and V′ from a bitstream produced by an encoder;

color convert the standard dynamic range luminance component L′ into L and the two decoded standard dynamic range chrominance components U′ and V′ into Ur and Vr, respectively, as follows:

L=L ′+max(0 ,aU′+bV′ ) , Ur=β ( L ) *U′,Vr=β ( L ) *V′

where a and b are constants and where β(L) is a parameter that depends on L;

apply independently:

a dynamic expansion function on the color converted standard dynamic range luminance component L to obtain a high dynamic range luminance component Y, wherein the dynamic expansion function is an inverse of a dynamic reduction function applied to a high dynamic range luminance component on the encoder's side;

a color transfer operation on the color converted standard dynamic range chrominance components Ur and Vr to obtain a first intermediate high dynamic range chrominance component and a second intermediate high dynamic range chrominance component in an intermediate color space; and

apply a color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain a high dynamic range picture in RGB color space.

8. The non-transitory computer readable medium according to claim 7 , wherein to apply the dynamic expansion function on the color converted standard dynamic range luminance component L to obtain the high dynamic range luminance component Y comprises determining:

Y=g −1 ( Ba,L ))

where Ba is a modulation value and g −1 ( ) is the inverse of the dynamic reduction function applied to the high dynamic range luminance component on the encoder's side.

9. The non-transitory computer readable medium according to claim 7 , wherein to apply the color conversion matrix on the high dynamic range luminance component Y, the first intermediate high dynamic range chrominance component and the second intermediate high dynamic range chrominance component to obtain the high dynamic range picture in RGB color space comprises:

calculating T as a linear combination of Ur 2 ,Vr 2 and Ur*Vr;

calculating S as √{square root over (1−T)} where T≤1 and setting S to 0 and dividing Ur and Vr by √{square root over (T)} otherwise;

calculating intermediate (R # ,G # ,B # ) components in RGB color space by applying the color conversion matrix on S, Ur, and Vr.