1. A method, comprising: compressing an initial image signal into a compressed image signal, the initial image signal including a sequence of pixels, each pixel being allocated with plural digital color components, for each color component the compressing including: subdividing the sequence of pixels into several groups of several adjacent pixels, for each group of at least a portion of the groups, generating a sequenced set of pixels associated with the group, the generating including monotonically sequencing the pixels of the group as a function of respective amplitudes of the color component of the pixels, and generating approximations of the amplitudes of the color component of the pixels of the set using a monotone approximation function with one variable related to positions of the sequenced pixels in the set, wherein the compressed image signal includes, for each color component and for each group of pixels belonging to the at least one portion, an item of information including indications of position of the sequenced pixels in the group and characteristics of the approximation function associated with the group.