Dynamic range coding for images and video

1. A HDR video decoder comprising an image color processing apparatus arranged to transform an input color of a pixel of an input image, which input image has a first luminance dynamic range, into a red, green and blue color component output color of a pixel of an output image, which output image has a second luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times lower than the peak luminance of the second dynamic range or vice versa, comprising a coarse mapping unit arranged to apply a three-segment brightness re-grading curve which consists of a linear segment for a dark sub-range of the range of lumas of the input image colors comprising the darkest input luma values, which is determined by a slope variable, a second linear segment for the lightest input luma values in a bright sub-range, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments.

2. An HDR video decoder as claimed in claim 1 in which the image color processing apparatus is arranged to apply the three-segment curve to low dynamic range lumas.

3. An HDR video decoder as claimed in claim 1 in which the image color processing apparatus is arranged to yield as a result of applying the three-segment curve an output luma which is related to the luminance of the color being the output result of applying the three-segment curve to the input color by a power function with a power 1/N, wherein N is preferably 2.

4. An HDR video decoder as claimed in claim 1 in which the image color processing apparatus is arranged to determine the output color by comprising a multiplier arranged to apply a multiplication with as a multiplicative factor the output luma to a preferably non-linear color representation of the color of the currently processed pixel.

5. An HDR video decoder as claimed in claim 1 in which the image color processing apparatus is arranged to obtain the non-linear color representation which is technically defined from linear R, G, B color components by a non-linear function, which is close in shape to a square root function, and which is preferably a power function with a power 1/N with N an integer value.

6. An HDR video decoder as claimed in claim 1 comprising a reading unit arranged to read from received metadata the first and second slope variable, and arranged to supply it to the coarse mapping unit.

7. An HDR video decoder as claimed in claim 1 in which the reading unit is further arranged to read a width of a parabolic region between the two linear segments from received metadata, and arranged to supply it to the coarse mapping unit.

8. A method of video decoding to yield a decoder HDR video of consecutive images, comprising applying a three-segment brightness re-grading curve which consists of a linear segment for a dark sub-range of the range of lumas of the input image colors comprising the darkest input luma values, which is determined by a slope variable, a second linear segment for the lightest input luma values in a bright sub-range, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments, which yields an output luma to be supplied to a multiplication with a preferably non-linear RGB color representation of the input color being color transformed.

9. A HDR video encoder comprising an image color processing apparatus arranged to transform an input color of a pixel of an input image, which input image has a second luminance dynamic range, into a output color of a pixel of an output image, which output image has a first luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times lower than the peak luminance of the second dynamic range or vice versa, comprising a coarse mapping unit arranged to apply a three-segment brightness re-grading curve which consists of a linear segment for a dark sub-range of the range of lumas of the input image colors comprising the darkest input luma values, which is determined by a slope variable, a second linear segment for the lightest input luma values in a bright sub-range, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments.

10. A HDR video encoder as claimed in claim 9 in which the image color processing apparatus is arranged to apply the three-segment curve to a color representation of the input color which is in a non-linear domain, wherein its color components are defined by non-linear functions based on linear red, green and blue additive color components.

11. A HDR video encoder as claimed in claim 9 , in which the image color processing apparatus is arranged to apply the three-segment curve to a linear red, green and blue color representation of the input color.

12. A HDR video encoder as claimed in claim 9 in which the image color processing apparatus is arranged to determine the output color in a representation comprising a non-linear luma which is related to the luminance of the output color by a non-linear function.

13. A HDR video encoder as claimed in claim 9 in which the image color processing apparatus is arranged to determine the output color in a representation defined by a standard dynamic range video encoding, wherein the output luma color component is defined by a Rec. 709 Opto-electrical transfer function or a square root.

14. A method of HDR video encoding, yielding a HDR set of images encoded in low dynamic range images, comprising transforming an input color of a pixel of an input image, which input image has a second luminance dynamic range, into a output color of a pixel of an output image, which output image has a first luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times lower than the peak luminance of the second dynamic range or vice versa, comprising applying a three-segment brightness re-grading curve which consists of a linear segment for a dark sub-range of the range of lumas of the input image colors comprising the darkest input luma values, which is determined by a slope variable, a second linear segment for the lightest input luma values in a bright sub-range, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments.

15. A computer-readable memory comprising code which when run by a processor will apply all steps as defined in claim 8 .