grouping only some of the pixels in the first pixel array into sets, each of the sets including a triad of pixels of the first array, each triad including the three different primary colors of the first array, the pixels of the triad of pixels of each set being formed from columns xk and x(k+1) and rows yp and y(p+1), where k is an integer such that 1???k<M and p is an integer such that 1???p<N,