Method and device for reconstructing a display adapted HDR image

1. A method for reconstructing image data (I 3 ) representative of original image data (I 1 ) from decoded image data (I 2 ) and parameters obtained from a bitstream, wherein the reconstructed image is adapted to a characteristic of a presentation display, the method comprising:

desaturating a luma component according to parameters for adjusting the desaturation;

inverse-mapping said desaturated luma component to obtain a reconstructed luma component;

correcting two chroma components to obtain two reconstructed chroma components according to said desaturated luma component and said reconstructed luma component;

characterized in that the chroma correction is responsive to the value of a single modulation factor mod representative of:

a luminance information data (D_PL) of the presentation display;

a luminance information data (SDR_PL) of a typical Standard-Dynamic-Range image; and

a luminance information data (C_PL) of the original image data (I 1 ) or the mastering display used to grade the original image data, and wherein the value of the modulation factor mod is computed according to following formula:

mod

=

D_PL

-

SDR_PL

C_PL

-

SDR_PL

2. The method of claim 1 wherein the value of SDR_PL is 100 nits.

3. The method of claim 1 wherein the desaturating operation further comprises obtaining ( 31 ) a desatured luma component y′ 1 as follows:

y′ 1 =y ′+Max(0, a ×mod× u′+b ×mod× v ′)

where a and b are two control parameters for adjusting the desaturation, y′ is the luma component and u′ v′ are the chroma components.

4. The method of claim 1 wherein the inverse-mapping is using a look-up table constructed using a gamma function modulated by the modulation factor according to following formula:

y= 2.0+0.4×(1−mod).

5. The method of claim 1 wherein the chroma correcting operation uses a look-up table lutCC defining the correction to apply to the chroma components for a given luma component, the look-up table lutCC being derivated based on a saturation gain function sgf( ) whose impact is modulated by the modulation factor as follows:

lutCC( Y )= f ( Y )·(1/ Y ) with f ( Y )=1/( R ·( sgf (1/ Y )·mod+(1−mod)/ R )

with R being a constant value equal to 2.

6. The method of claim 1 wherein the chroma correcting operation comprises:

computing a parameter T based on parameters k0, k1, and k2 obtained from a set of parameters SP, the parameter T being responsive to a value of the single modulation factor mod as follows:

T=k 0× U′×V′+k 1× U′×U′×U′+k 2× V′×V′

wherein when the parameter T is smaller than 1, computing the combined compression correction as follows:

S =√(1− T ), U=U ′ and V=V ′; and

wherein when the parameter T is greater than or equal to 1, computing the combined compression correction as follows:

S= 0, U=U′/√T, V=V′/√T.

7. A device for reconstructing image data (I 3 ) representative of original image data (I 1 ) from decoded image data (I 2 ) and parameters obtained from a bitstream, said parameters having been processed from said original image data (I 1 ), wherein the reconstructed image is adapted to a characteristic of a presentation display, the device comprising a processor configured to:

desaturate a luma component according to parameters for adjusting the desaturation;

perform an inverse-mapping said desaturated luma component to obtain a reconstructed luma component;

correct two chroma components to obtain two reconstructed chroma components according to said desaturated luma component and said reconstructed luma component;

characterized in that the chroma correction is responsive to the value of a single modulation factor mod representative of:

a luminance information data (D_PL) of the presentation display;

a luminance information data (SDR_PL) of a typical Standard-Dynamic-Range image; and

a luminance information data (C_PL) of the original image data (I 1 ) or the mastering display used to grade the original image data

wherein the value of the modulation factor is computed according to following formula:

mod

=

D_PL

-

SDR_PL

C_PL

-

SDR_PL

8. The device of claim 7 wherein the value of SDR_PL is 100 nits.

9. The device of claim 7 wherein the desaturating operation further comprises obtaining ( 31 ) a desatured luma component y′ 1 as follows:

y′ 1 =y ′+Max(0, a ×mod× u′+b ×mod× v ′)

where a and b are two control parameters for adjusting the desaturation, y′ is the luma component and u′ v′ are the chroma components.

10. The device of claim 7 wherein the inverse-mapping is using a look-up table constructed using a gamma function modulated by the modulation factor according to following formula:

y= 2.0+0.4×(1−mod).

11. The device of claim 7 wherein the chroma correcting operation uses a look-up table lutCC defining the correction to apply to the chroma components for a given luma component, the look-up table lutCC being derivated based on a saturation gain function sgf( ) whose impact is modulated by the modulation factor as follows:

lutCC( Y )= f ( Y )·(1/ Y ) with f ( Y )=1/( R ·( sgf (1/ Y )·mod+(1−mod) R )

with R being a constant value equal to 2.

12. The device of claim 7 wherein the chroma correcting operation comprises:

computing a parameter T based on parameters k0, k1, and k2 obtained from a set of parameters SP, the parameter T being responsive to a value of the single modulation factor mod as follows:

T=k 0× U′×V′+k 1× U′×U′+k 2× V′×V′

wherein when the parameter T is smaller than 1, computing the combined compression correction as follows:

S =√(1− T ), U=U ′ and V=V ′; and

wherein when the parameter T is greater than or equal to 1, computing the combined compression correction as follows:

S= 0, U=U′/√T, V=V′/√T.

13. The method of claim 5 wherein the desaturating operation further comprises obtaining ( 31 ) a desatured luma component y′ 1 as follows:

y′ 1 =y ′+Max(0, a ×mod× u′+b ×mod× v ′)

where a and b are two control parameters for adjusting the desaturation, y′ is the luma component and u′ v′ are the chroma components.

14. The method of claim 2 wherein the inverse-mapping is using a look-up table constructed using a gamma function modulated by the modulation factor according to following formula:

□=2,0+0,4×(1−mod).

15. The method of claim 2 wherein the chroma correcting operation uses a look-up table lutCC defining the correction to apply to the chroma components for a given luma component, the look-up table lutCC being derivated based on a saturation gain function sgf( )whose impact is modulated by the modulation factor as follows:

lutCC( Y )= f ( Y )·(1/ Y ) with f ( Y )=1/( R ·( sgf (1/ Y )·mod+(1−mod)/ R )

with R being a constant value equal to 2.

16. The method of claim 2 wherein the chroma correcting operation comprises:

computing a parameter T based on parameters k0, k1, and k2 obtained from a set of parameters SP, the parameter T being responsive to a value of the single modulation factor mod as follows:

T=k 0 ×U′×V′+k 1 ×U′×U′+k 2 ×V′×V′

wherein when the parameter T is smaller than 1, computing the combined compression correction as follows:

S =√(1− T ), U=U ′ and V=V ′; and

wherein when the parameter T is greater than or equal to 1, computing the combined compression correction as follows:

S= 0, U=U′/√T, V=V′/√T.

17. The device of claim 8 wherein the desaturating operation further comprises obtaining ( 31 ) a desatured luma component y′ 1 as follows:

y′ 1 =y ′+Max(0, a ×mod× u′+b ×mod× v ′)

where a and b are two control parameters for adjusting the desaturation, y′ is the luma component and u′ v′ are the chroma components.

18. The device of claim 8 wherein the inverse-mapping is using a look-up table constructed using a gamma function modulated by the modulation factor according to following formula:

□=2,0+0,4×(1−mod).

19. The device of claim 8 wherein the chroma correcting operation uses a look-up table lutCC defining the correction to apply to the chroma components for a given luma component, the look-up table lutCC being derivated based on a saturation gain function sgf( )whose impact is modulated by the modulation factor as follows:

lutCC( Y )= f ( Y )·(1/ Y ) with f ( Y )=1/( R ·( sgf (1/ Y )·mod+(1−mod)/ R )

with R being a constant value equal to 2.

20. The device of claim 8 wherein the chroma correcting operation comprises:

computing a parameter T based on parameters k0, k1, and k2 obtained from a set of parameters SP, the parameter T being responsive to a value of the single modulation factor mod as follows:

T=k 0 ×U′×V′+k 1 ×U′×U′+k 2 ×V′×V′

wherein when the parameter T is smaller than 1, computing the combined compression correction as follows:

S =√(1− T ), U=U ′ and V=V ′; and

wherein when the parameter T is greater than or equal to 1, computing the combined compression correction as follows:

S= 0, U=U′/√T, V=V′/√T.

21. A non-transitory processor-readable medium storing program code instructions to execute the steps of the method according to any of claim 2 , 13 , 14 , 15 when the program code instructions are executed on a computer.

22. A non-transitory processor-readable medium storing program code instructions to execute the steps of the method according to claim 2 when the program code instructions are executed on a computer.

23. A non-transitory processor-readable medium storing program code instructions to execute the steps of the method according to claim 13 when the program code instructions are executed on a computer.

24. A non-transitory processor-readable medium storing program code instructions to execute the steps of the method according to claim 14 when the program code instructions are executed on a computer.

25. A non-transitory processor-readable medium storing program code instructions to execute the steps of the method according to claim 15 when the program code instructions are executed on a computer.