Optimized decoded high dynamic range image saturation
To enable better color and in particular color saturation control for HDR image handling systems which need to do luminance dynamic range conversion, e.g. from a SDR image to an image optimized for rendering on a display of higher display peak brightness and dynamic range, the inventors invented an apparatus ( 400 ) for processing a color saturation (C′bL, C′rL) of an input color (Y′L, C′bL, C′rL) of an input image (Im RLDR) to yield an output color (Y′M, Cb′M, Cr′M) of an output image (Im3000nit) corresponding to the input image, which output image is a re-grading of the input image characterized by the fact that its pixel colors have a different normalized luminance position (Y2) compared to the normalized luminance positions of the input colors (Y1), the normalized luminances being defined as the luminance of a pixel divided by the respective maximal codeable luminance of the image's luminance representation, whereby the ratio of the maximum codeable luminance of the input image and the maximum codeable luminance of the output image is at least 4 or larger, or ¼ th or smaller, the apparatus comprising a luminance processor ( 401 ) arranged to apply a display tuned luminance mapping function (F_L_da) to determine an output luminance (Y′M) from the input luminance (Y′L) of the input color, the apparatus further comprising a saturation factor determination unit ( 402 ) arranged to calculate a final saturation processing strategy (b; Bcorr) based on an initial saturation processing strategy (F_sat) and based on a secondary luminance value (Y′_H) which is derivable from the output luminance (Y′M) by applying a luminance mapping function (F M2H) which is based on the luminance mapping function (F_L_s2h).
1. An apparatus for processing a color saturation of an input color of an input image comprising:
a receiver circuit,
wherein the receiver circuit is arranged to receive a luminance mapping function and an initial saturation processing function,
wherein the luminance mapping function defines a mapping between a luminance of the input color of the input image and a reference luminance,
wherein the initial saturation processing function defines saturation multipliers for different values of the luminance of the input color;
a display tuning circuit, wherein the display tuning circuit is arranged to calculate a display tuned luminance mapping function based on the luminance mapping function and at least one of a display peak brightness and a minimum discernable black;
a luminance processor circuit, wherein the luminance processor circuit is arranged to apply the display tuned luminance mapping function so as to determine an output luminance from the luminance of the input color;
a saturation processing circuit, wherein the saturation processing circuit is, arranged to map an input color saturation to a color saturation of an output color on the basis of a saturation processing function which specifies saturation multipliers for normalized luminance values; and
a saturation multiplier determination circuit,
wherein the saturation multiplier determination circuit is arranged to calculate a final saturation processing function based on an initial saturation processing function and based on a secondary luminance value,
wherein the secondary luminance value is derivable from the output luminance by applying a luminance mapping function based on the luminance mapping function,
wherein the saturation processing circuit is arranged to calculate the color saturation of the output color by applying the final saturation processing function,
wherein the output image corresponds to the input image,
wherein the output image is a re-grading of the input image characterized by the fact that its pixel colors have a different normalized luminance position compared to normalized luminance positions of the input colors,
wherein the normalized luminances are defined as the luminance of a pixel divided by a respective maximal codeable luminance of the image's luminance representation.
2. The apparatus as claimed in claim 1 , wherein the secondary luminance value is used as an input coordinate of the initial saturation processing function to get a multiplier which is the value of the initial saturation processing function for that secondary luminance value.
3. The apparatus as claimed in claim 2 ,
wherein the input luminance and the secondary luminance value is a luminance calculated with a first luminance mapping function,
wherein the first luminance mapping function optimizes for a first rendering capability of the darkest colors of the input image of a receiving side display,
wherein the secondary luminance value is a luminance calculated with a second luminance mapping function which optimizes for a second rendering capability of the darkest colors of the input image of an actual receiving side display.
4. The apparatus as claimed in claim 1 , wherein the secondary luminance value is used to calculate a new output coordinate of the final saturation processing function for each possible normalized luminance of the input color.
5. The apparatus as claimed in claim 4 ,
wherein the input luminance and the secondary luminance value is a luminance calculated with a first luminance mapping function,
wherein the first luminance mapping function optimizes for a first rendering capability of the darkest colors of the input image of a receiving side display,
wherein the secondary luminance value is a luminance calculated with a second luminance mapping function which optimizes for a second rendering capability of the darkest colors of the input image of an actual receiving side display.
6. The apparatus as claimed in claim 1 ,
wherein one of the input luminance and the secondary luminance value is a luminance calculated with a first luminance mapping function,
wherein the first luminance mapping function optimizes for a first rendering capability of the darkest colors of the input image of a receiving side display, and
wherein the other one of the input luminance and the secondary luminance value is a luminance calculated with a second luminance mapping function,
wherein the first luminance mapping function optimizes for a second rendering capability of the darkest colors of the input image of an actual receiving side display.
7. The apparatus as claimed in claim 1 , wherein the apparatus is arranged to output an image with a peak luminance of at least 500 nit.
8. A method of processing a color saturation of an input color of an input image comprising:
receiving a luminance mapping function and an initial saturation processing function,
wherein the luminance mapping function defines a mapping between a luminance of the input color and a reference luminance,
wherein an initial saturation processing function defines saturation multipliers for different values of the luminance of the input color;
calculating a display tuned luminance mapping function based on the luminance mapping function and at least one of a display peak brightness and a minimum discernable black;
applying the display tuned luminance mapping function so as to determine an output luminance of an output image from the input luminance of the input color;
calculating a final saturation processing function based on an initial saturation processing function and based on a secondary luminance value,
wherein the secondary luminance value is derivable from the output luminance,
mapping an input color saturation to obtain a color saturation of an output color on the basis of a saturation processing function which specifies saturation multipliers for normalized luminance values; and
wherein the color saturation of the output color is calculated by applying the final saturation processing function,
wherein the output image is a re-grading of the imput image characterized by the fact that its pixel colors have a different normalized luminance position compared to normalized luminance positions of the input colors,
wherein the normalized luminances are defined as the luminance of a pixel divided by a respective maximal codeable luminance of the image's luminance representation.