1. A method of generating an image signal encoding a representation of a captured physical scene, the image signal comprising a first pixelised picture encoding and a second pixelised picture encoding, pixelised picture encodings having a luminance component and a colour component being sets of code words of a number N of bits, corresponding to a two-dimensional array of pixel positions; the method comprising: encoding a first image of the captured physical scene with luminance and color code words in the first pixelised picture encoding; and encoding, instead of colour values for corresponding pixel positions, in any colour component of the second pixelised picture encoding, dynamic range extension data in the colour component of the second pixelised picture encoding, the dynamic range extension data corresponding to at least one region of pixels and comprising data representing a dynamic range extension transformation in a luminance direction to obtain an increased dynamic range image on the basis of the first pixelised picture, wherein the dynamic range extension data either codifies a transformation, or a modification of a transformation.