A method compresses, into a compressed image signal, an initial image signal including a sequence of pixels having respective amplitudes of plural digital color components. The method includes, for each color component, subdividing the sequence into several groups of several adjacent pixels, and for each group of a portion of the groups, generating a sequenced set of pixels for the group by a monotone sequencing of the pixels of the group as a function of amplitudes of the color component, and approximating amplitudes of the color component of the pixels of the set by a monotone approximation function with one variable related to positions of the pixels in the set. The compressed image signal includes indications of position of the sequenced pixels in the group and characteristics of the approximation function associated with that group.