LUMINANCE CHANGING IMAGE PROCESSING WITH COLOR CONSTRAINTS

摘要

To be able to do good color mapping between color encodings for gamuts with considerably different luminance dynamic range while not introducing significant color errors, we describe an image color processing apparatus (201) arranged to transform an input color (L,x,y) of a pixel specified in a color representation corresponding to a first luminance dynamic range into an output color (L*,x,y) of a pixel specified in a color representation corresponding to a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor 1.5, comprising a tone mapping deformation unit (203) arranged to determine on the basis of an input tone mapping (301) and a quantity linearly related to the luminance (L) of the input color, an adjusted output luminance (L*, 309), wherein the determining is arranged so that the adjusted output luminance (L*, 309) obtained by applying the input tone mapping to the input luminance (L) of all possible input luminances in its extent of valid values [0,1] giving the highest output (L*,L_HDR), is not higher than a maximum luminance Lmax(x,y) which for the chromatic coordinates (x,y) of the input color is maximally achievable in the gamut corresponding to the second luminance dynamic range for those chromatic coordinates (x,y).

主权项

1. An image color processing apparatus arranged to transform an input color of a pixel specified in a color representation corresponding to a first luminance dynamic range into an output color of a pixel specified in a color representation corresponding to a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor of 1.5, comprising:
a tone mapping deformation unit arranged to determine an adjusted output luminance for the output color from an input luminance of the input color and on the basis of an input tone mapping defining output luminances as a function of input luminances, and a quantity linearly related to the input luminance, the quantity being indicative of a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in at least one of a gamut corresponding to the first luminance dynamic range and a gamut corresponding to the second luminance dynamic range for those chromatic coordinates; wherein the tone mapping deformation unit is arranged to generate the adjusted output luminance by applying an adjusted tone mapping to the input luminance; and the tone mapping deformation unit is arranged to determine the adjusted tone mapping by adapting the input tone mapping dependent on the quantity so that any adjusted output luminance obtained by applying the adjusted tone mapping to any input luminance of all possible input luminances in its range of valid values 0 to 1 is not higher than a maximum luminance which for the chromatic coordinates of the input color is maximally achievable in the gamut corresponding to the second luminance dynamic range for those chromatic coordinates.