IMAGE FORMING APPARATUS, CONTROL METHOD, AND PROGARM FOR COPY-FORGERY-INHIBITED DOT PATTERN CALIBRATION

A test pattern containing plurality of patches recorded using small dot patterns and large dot patterns is printed. The dot sizes arranged in the patches are different between patches. Then the test patterns are read. The detection rate and average density of additional information embedded in each large dot pattern are obtained. A large dot pattern whose detection rate and average density fall within a reference range and are closest to ideal values is determined. The average density of each small dot pattern is obtained. A small dot pattern whose average density falls within a reference range and is closest to the average density of the determined large dot pattern is determined. A copy-forgery-inhibited pattern image is generated using these dot patterns and combined with a print target image.