Methods, apparatus, and systems for extended high dynamic range (“HDR”) HDR to HDR tone mapping

1. A method for tone mapping an input image having a first dynamic range with a peak input luminance I max into an output image having a second lower dynamic range with a lower peak output luminance I o max , the method comprising

obtaining a luminance component of the input image;

determining a tone mapper curve from the first dynamic range to the second dynamic range;

determining said output image by applying the tone mapper curve to the luminance component of the input image;

wherein the tone mapper curve comprises a first linear part for dark and mid-tone levels and a second non-linear part for highlights, and

wherein the first linear part has a slope s that is a monotonically non-increasing function of a measure of uniformity of distribution of the luminance component in a bright region of a luminance histogram of the luminance component of the input image such that said slope s will be lower than or equal to 1 but higher than or equal to a ratio s l of the peak output luminance I o max and the peak input luminance I max .

2. The method of claim 1 , wherein the bright region of the luminance histogram is defined as being delineated between a lower bound ζ l and an upper bound ζ u positioned in a luminance range of the input image, wherein the lower bound ζ l proportional to the ratio s l of the peak output luminance I o max and the peak input luminance I max .

3. The method of claim 1 , wherein a mid-to-high threshold τ defined as separating the first linear part and the second non-linear part of the tone mapper curve is smaller than the peak output luminance I o max .

4. The method of claim 3 , wherein the mid-to-high threshold r is large enough to cover more than (100−n) % of the pixels of a frame or image, wherein n is superior or equal to 2.

5. The method of claim 1 , wherein the measure of uniformity of distribution of the luminance component in the bright region is determined as a ratio between a mean μ and a standard deviation υ of a number of pixels of said input image over bins of the bright region of said luminance histogram.

6. The method of claim 1 , wherein a relationship between the slope s and the measure of uniformity is non-linear.

7. The method of claim 6 , wherein the slope s is determined as follows

s

=

{

1

≤

ϱ

1

(

-

3

⁢

2

+

2

⁢

3

+

1

)

⁢

(

1

-

s

l

)

+

s

l

1

<

<

ϱ

2

s

l

≥

ϱ

1

where

=

µ

υ

-

ϱ

1

2

-

ϱ

1

,

and

where parameters 1 , 2 define transition points of the tone mapper curve representing the slope s as function of parameter .

8. The method of claim 1 , further comprising signaling information representative of the tone mapper curve.

9. The method of claim 8 , wherein the signaling is performed using at least one syntax element included in at least one of a Picture Parameter Set (PPS), a Sequence Parameter Set (SPS), a Supplemental Enhancement Information (SEI) message, a Video Usability Information (VUI), Consumer Electronics Association (CEA) message, and a header.

10. An apparatus for tone mapping an input image having a first dynamic range with a peak input luminance I max into an output image having a second lower dynamic range with a lower peak output luminance I o max , the apparatus comprising at least one processor configured for:

obtaining a luminance component of the input image;

determining a tone mapper curve from the first dynamic range to the second dynamic range;

determining said output image by applying the tone mapper curve to the luminance component of the input image;

wherein the tone mapper curve comprises a first linear part for dark and mid-tone levels and a second non-linear part for highlights, and

wherein the first linear part has a slope s that is a monotonically non-increasing function of a measure of uniformity of distribution of the luminance component in a bright region of a luminance histogram of the luminance component of the input image such that said slope s will be lower than or equal to 1 but higher than or equal to a ratio s l of the peak output luminance I o max and the peak input luminance I max .

11. The apparatus of claim 10 , wherein the bright region of the luminance histogram is defined as being delineated between a lower bound ζ l and an upper bound ζ u positioned in a luminance range of the input image, wherein the lower bound ζ l is proportional to the ratio s l of the peak output luminance I o max and the peak input luminance I max .

12. The apparatus of claim 10 , wherein a mid-to-high threshold τ defined as separating the first linear part and the second non-linear part of the tone mapper curve is smaller than the peak output luminance I o max .

13. The apparatus of claim 12 , wherein the mid-to-high threshold r is large enough to cover more than (100−n) % of the pixels of a frame or image, wherein n is superior or equal to 2.

14. The apparatus of claim 10 , wherein the measure of uniformity of distribution of the luminance component in the bright region is determined as a ratio between a mean μ and a standard deviation υ of a number of pixels of said input image over bins of the bright region of said luminance histogram.

15. The apparatus of claim 10 , wherein a relationship between the slope s and the measure of uniformity is non-linear.

16. The apparatus of claim 15 , wherein the slope s is determined as follows

s

=

{

1

≤

ϱ

1

(

-

3

⁢

2

+

2

⁢

⁢

3

+

1

)

⁢

(

1

-

s

l

)

+

s

l

ϱ

1

<

<

ϱ

2

s

l

≥

ϱ

1

where

=

µ

υ

-

ϱ

1

2

-

ϱ

1

,

and

where parameters 1 , 2 define transition points of the tone mapper curve representing the slope s as function of parameter .

17. The apparatus of claim 10 , wherein said at least one processor is further configured for comprising signaling information representative of the tone mapper curve.

18. The apparatus of claim 17 , wherein the signaling is performed using at least one syntax element included in at least one of a Picture Parameter Set (PPS), a Sequence Parameter Set (SPS), a Supplemental Enhancement Information (SEI) message, a Video Usability Information (VUI), Consumer Electronics Association (CEA) message, and a header.

19. An electronic device comprising the apparatus of claim 10 .

20. The electronic device according to claim 19 , selected from a group consisting of a mobile device, a communication device, a game device, a tablet, a laptop, a camera, an encoding chip, and a server.

21. A computer program product stored on a non-transitory computer readable medium comprising program code instructions to execute the steps of the method according to claim 1 when the program code instructions are executed on a computer.