Abstract: Cumulative histograms generated based on threshold matrixes with respect to halftone-dot area percentage data of the four plates C, M, Y, K are referred to for thereby determining the existential probability of halftone dots. Then, average colorimetric data are calculated from predetermined colorimetric data with respect to every 2.sup.n colors using the area percentage as a weighting coefficient. An anti-aliasing filtering process is effected on the average colorimetric data to produce device-independent image data for a color printer. The cumulative histograms are emphasized and the average colorimetric data are emphasized to accurately reproduce a peculiar pattern such as a rosette image which will appear on a printed document on a proof which is outputted from a low-resolution color printer.