A high dynamic range video encoding circuit, configured to encode a high dynamic range image (IM_HDR) of a first maximum pixel luminance (PB_C1), together with a second image (Im_LWRDR) of lower dynamic range and corresponding lower second maximum pixel luminance (PB_C2), wherein the second image is functionally encoded as a luma mapping function for decoders to apply to pixel lumas (Y_PQ) of the high dynamic range image to obtain corresponding pixel lumas of the second image, the encoder comprising a data formatter configured to output to a video communication medium the high dynamic range image and metadata encoding the luma mapping function, the functional encoding of the second image being based also on a color lookup table (CL(Y_PQ)) which encodes a respective multiplier constant for each of all possible values of the pixel lumas of the high dynamic range image, which multiplier constants are for multiplication by chrominances (Cb, Cr) of the pixels to obtain resultant chrominances of the second image, and the formatter being configured to output this color lookup table in the metadata, characterized in that the high dynamic range video encoding circuit comprises: - a gain determination circuit which receives pixels of the high dynamic range image (IM_HDR), each pixel having a luma and two chrominances, configured to determine for each pixel a luma gain value (G_PQ) which equals a ratio of a first output luma for a luma equal to a normalized luminance of the luma of each pixel, divided by a second output luma for the luma of the pixel, wherein the first output luma is obtained by applying the luma mapping function to the normalized luminance, and the second output luma is obtained by applying the luma mapping function to the luma of the pixel; wherein the high dynamic range video encoding circuit comprises a color lookup table determination circuit configured to determine the color lookup table (CL(Y_PQ)) based on values of the luma gain value for various lumas of pixels present in the high dynamic range image, wherein the values of the multiplier constant for lumas from at least a sub-set of the possible values of the lumas (Y_PQ) correlate with the value of the determined luma gain (G_PQ) for those lumas. wherein the second image is functionally encoded as a luma mapping function for decoders to apply to pixel lumas (Y_PQ) of the high dynamic range image to obtain corresponding pixel lumas of the second image, the encoder comprising a data formatter configured to output to a video communication medium the high dynamic range image and metadata encoding the luma mapping function, the functional encoding of the second image being based also on a color lookup table (CL(Y_PQ)) which encodes a respective multiplier constant for each of all possible values of the pixel lumas of the high dynamic range image, which multiplier constants are for multiplication by chrominances (Cb, Cr) of the pixels to obtain resultant chrominances of the second image, and the formatter being configured to output this color lookup table in the metadata, characterized in that the high dynamic range video encoding circuit comprises: - a gain determination circuit which receives pixels of the high dynamic range image (IM_HDR), each pixel having a luma and two chrominances, configured to determine for each pixel a luma gain value (G_PQ) which equals a ratio of a first output luma for a luma equal to a normalized luminance of the luma of each pixel, divided by a second output luma for the luma of the pixel, wherein the first output luma is obtained by applying the luma mapping function to the normalized luminance, and the second output luma is obtained by applying the luma mapping function to the luma of the pixel; - a gain determination circuit which receives pixels of the high dynamic range image (IM_HDR), each pixel having a luma and two chrominances, configured to determine for each pixel a luma gain value (G_PQ) which equals a ratio of a first output luma for a luma equal to a normalized luminance of the luma of each pixel, divided by a second output luma for the luma of the pixel, wherein the first output luma is obtained by applying the luma mapping function to the normalized luminance, and the second output luma is obtained by applying the luma mapping function to the luma of the pixel; wherein the high dynamic range video encoding circuit comprises a color lookup table determination circuit configured to determine the color lookup table (CL(Y_PQ)) based on values of the luma gain value for various lumas of pixels present in the high dynamic range image, wherein the values of the multiplier constant for lumas from at least a sub-set of the possible values of the lumas (Y_PQ) correlate with the value of the determined luma gain (G_PQ) for those lumas.