1. A method for downscaling a digital coloured matrix image by selected ratios M 2 /M 1 and N 2 /N 1 , in which the matrix image includes N 1 rows, each row including M 1 pixels, so that the values of the pixels form the matrix and the pixels of different colours form the selected format, and in which scaling is used to form an output matrix, of a size M 2 ×N 2 , the pixels corresponding to sub-groups of the original matrix, in such a way that M 2 <=M1 and N 2 <=N 1 , and from the values Input(j,i) of which pixels (i,j) the value Output(l,k) of each output pixel (k,l) of the output matrix is calculated, characterized in that the coloured matrix image is read only once and the weighted sums of the values of the same-colour pixels (i,j) of the matrix image in the area of each output pixel (k,l) is formed parallel in line memories (Buffer[0], Buffer[1]) arranged for each colour, the weighting coefficient being the dimension share of the pixel (i,j) in the area of the output pixel (k,l) and each weighted sum is corrected by a scaling factor (f×M 2 /M 1 ×N 2 /N 1 ).