In for example the assignment of motion vectors, an array of pixel-to-pixel dissimilarity values is analyzed to identify a pixel which has a low pixel-to-pixel dissimilarity value and which has neighboring pixels which have a low pixel-to-pixel dissimilarity value. The pixel-to-pixel dissimilarity values are filtered with a filter aperture decomposed into two or more sectors with partial filters applied respectively to each sector. Outputs of the partial filters are combining by a non-linear operation, for example taking the minimum from diametrically opposed sectors.