Image processing apparatus which converts color images to monochrome with perceptible gradations in gray value, and image processing method and storage medium for same

An image processing method for printing a color image in monochrome, the image processing method comprising the steps of:

• converting a multidimensional color component value identifying a color included in the color image into a gray value;
• and
• modifying the gray value obtained in the converting, wherein
• in a case where a plurality of colors is included in the color image, in the modifying:
• a gray value difference indicating a difference between two adjacent gray values is calculated for a plurality of the gray values corresponding to the plurality of colors and arranged in order from the largest gray value;
• in a case where the calculated gray value difference is smaller than a threshold value, the corresponding gray value is changed so that the gray value difference becomes the same value as the threshold value;
• and
• in a case where the calculated gray value difference is larger than the threshold value, the corresponding gray value is changed so that the gray value difference becomes small in accordance with the magnitude of the gray value difference,
• wherein a total amount of changes relating to a gray value difference smaller than the threshold value is distributed in a case where a corresponding gray value is changed so that the gray value difference becomes the same value as the threshold value to a gray value difference larger than the threshold value in accordance with the magnitude of the gray value difference, and
• a corresponding gray value is changed so that a gray value difference larger than the threshold value becomes small by an amount corresponding to an amount of change distributed respectively.

A non-transitory computer readable storage medium storing a program for causing a computer to perform an image processing method for printing a color image in monochrome, the image processing method comprising the steps of:

• converting a multidimensional color component value identifying a color included in the color image into a gray value;
• and
• modifying the gray value obtained in the converting, wherein
• in a case where a plurality of colors is included in the color image, in the modifying:
• a gray value difference indicating a difference between two adjacent gray values is calculated for a plurality of the gray values corresponding to the plurality of colors and arranged in order from the largest gray value;
• in a case where the calculated gray value difference is smaller than a threshold value, the corresponding gray value is changed so that the gray value difference becomes the same value as the threshold value;
• and
• in a case where the calculated gray value difference is larger than the threshold value, the corresponding gray value is changed so that the gray value difference becomes small in accordance with the magnitude of the gray value difference,
• wherein a total amount of changes relating to a gray value difference smaller than the threshold value is distributed in a case where a corresponding gray value is changed so that the gray value difference becomes the same value as the threshold value to a gray value difference larger than the threshold value in accordance with the magnitude of the gray value difference, and
• a corresponding gray value is changed so that a gray value difference larger than the threshold value becomes small by an amount corresponding to an amount of change distributed respectively.