Hdr out of gamut adjusted tone mapping

Abstract

To get easy correction of typical out of gamut mapping issues when liberally applying desired luma mapping functions to luma/chrominance color representations, the inventor proposes an apparatus (), or method, for transforming an input pixel color (C_Hb) of a high dynamic range input image into an output color (C_Sb) of an output image, wherein the input color is represented as a gamma-log-defined triplet comprising an initial first color component, an initial second color component and an initial third color component according to an opto-electronic transfer function which has a gamma-log shape, wherein the apparatus comprises a color transformation circuit () arranged to transform the initial first, second and third color component into an intermediate red color component (R″_im), an intermediate green color component (G″_im) and an intermediate blue color component (B″_im) by mapping the initial color components with a function which is the same for each color component, wherein the function has a slope larger than 1 for a sub-range comprising the darkest color component values of the initial color components, characterized in that the apparatus comprises: —a first maximization circuit () arranged to calculate a first value (Mx) which is for each input color the largest one of the three intermediate color components; —a first subtractor () arranged to subtract from the value a second value (V) which is equal to a value obtained from applying the opto-electronic transfer function to a tertiary value, which tertiary value is equal to a target display maximum luminance for the output image (ML_To) divided by a maximum luminance associated with the opto-electronic transfer function (ML_PQ), which subtraction yields a fourth value (V); —a second maximization circuit () arranged to calculate a fifth value which is the highest one of the fourth value and zero; —a second subtractor (), a third subtractor () and a fourth subtractor () arranged to calculate three output color components (R″_out, G″_out, B″_out) by subtracting the fifth value from the corresponding intermediate color component, and arranged to set the output color component equal to zero if the subtraction yields a negative value.