1. A method of encoding a high dynamic range image (M_HDR), comprising the steps of: converting the high dynamic range image to an image of lower luminance dynamic range (LDR_o) by applying: a) normalization of the high dynamic range image to a scale of the luma axis being [0,1] yielding a normalized high dynamic range image with normalized colors having normalized luminances (Yn_HDR), b) calculating a gamma function on the normalized luminances yielding gamma-converted luminances (xg), c) applying a first tone mapping yielding lumas (v) which is defined as v=log(1+(RHO−1)*xg)/log(RHO), with RHO having a predetermined value, and d) applying an arbitrary monotonically increasing tone mapping function mapping the lumas to output lumas (Yn_LDR) of the lower dynamic range image (LDR_o); and outputting in an image signal (S_im) a codification of the pixel colors of the lower luminance dynamic range image (LDR_o), and outputting in the image signal (S_im) values encoding the function shapes of the above color conversions b to d as metadata, or values for their inverse functions, which metadata allows a receiver to reconstruct a reconstructed high dynamic range image (Rec_HDR) from the lower luminance dynamic range image (LDR_o), wherein RHO or a value being a function of RHO is outputted in the metadata.