A method of block-encoding a raster image by successive two-dimensional decompositions of blocks of the image in a base of functions using a combined application of a one-dimensional kernel of vertical decomposition of n pixels and of a one-dimensional kernel of horizontal decomposition of p pixels. In the method the horizontal dimension P of each block is determined as a multiple of p, P=k·p, and a decomposition at logp(P) level(s) of resolution is accomplished using the horizontal decomposition kernel, and the vertical dimension N of each block is determined as a multiple of n, N=l·n, and a decomposition at logp(N) level(s) of resolution is accomplished using the vertical decomposition kernel. For given values of n and p, the values of k and l are chosen such that the vertical dimension N is strictly less than the horizontal dimension P.