Image processing apparatus that sets a spatial frequency of a chromatic foreground image of a watermark to be lower than a spatial frequency of an achromatic foreground image of a comparable watermark, associated image forming apparatus, image processing method and recording medium

An image processing method comprising steps of:

• generating watermark data which denote a chromatic watermark and an achromatic watermark for copy control, which appear as a result of making a copy;
• generating composite image data which denote an image composed by joining an original image with a watermark, on a basis of the generated watermark data and original image data obtained by reading an original document;
• generating a chromatic image pattern to form a chromatic watermark;
• generating an achromatic image pattern to form an achromatic watermark;
• and
• generating watermark data on a basis of the generated chromatic image pattern or the generated achromatic image pattern,
• wherein the chromatic image pattern includes a first chromatic image pattern, which appears as a result of making a copy,
• wherein the achromatic image pattern includes a first achromatic image pattern, which appears as a result of making a copy,
• wherein a spatial frequency of the first chromatic image pattern is set in low screen ruling and a spatial frequency of the first achromatic image pattern is set in high screen ruling.

A non-transitory computer-readable medium, in which a computer program is recorded,

• the computer program comprising steps of:
• causing a computer to generate watermark data which denote a chromatic watermark and an achromatic watermark for copy control, which appear as a result of making a copy;
• causing a computer to generate composite image data which denote an image composed by joining an original image with a watermark, on a basis of the generated watermark data and original image data obtained by reading an original document;
• causing a computer to generate a chromatic image pattern to form a chromatic watermark;
• causing a computer to generate an achromatic image pattern to form an achromatic watermark;
• and
• causing a computer to generate watermark data on a basis of the generated chromatic image pattern or the generated achromatic image pattern,
• wherein the chromatic image pattern includes a first chromatic image pattern, which appears as a result of making a copy,
• wherein the achromatic image pattern includes a first achromatic image pattern, which appears as a result of making a copy,
• wherein a spatial frequency of the first chromatic image pattern is set in low screen ruling and a spatial frequency of the first achromatic image pattern is set in high screen ruling.