Image processing method, image processing apparatus, and computer-readable storage medium for computer program

An image processing apparatus for extracting an edge from a color image comprising first pixels each of which expressed by color component values of three R, G, and B colors, comprising:

• a memory that stores therein a lightness value of each of the pixels calculated based on the color component values of three colors corresponding to said each of the pixels;
• a determination portion that determines which one of low, medium, and high lightness regions the lightness value thus stored in the memory belongs to, the low, medium, and high lightness regions being defined by dividing a predetermined lightness range into three;
• an image generation portion that generates an extraction image comprising pixels corresponding to said each of the first pixels of the color image,
• the image generation portion determining, for a pixel of which the lightness value belongs to the medium lightness region, a pixel value based on the color component values of three colors determining, for a pixel of which the lightness value belongs to the low lightness region, a pixel value based on the color component values of said given first pixel excluding at least one of the color component values of three colors, and determining, for a pixel of which the lightness value belongs to the high lightness region, a pixel value based on the color component values of two colors of the color component values of three colors, or, based on the lightness value and the color component value of B;
• and
• an extraction portion that extracts an edge based on the pixel value of the extraction image thus generated by the image generation portion.

A non-transitory computer-readable storage medium storing thereon a computer program executed by a computer provided in an image processing apparatus that extracts an edge from a color image comprising first pixels each of which expressed by color component values of three R, G, and B colors, the computer program, when read and executed by the computer, causing the image processing apparatus to implement:

• a calculation portion that calculates a lightness value of each of the first pixels based on the color component values of three colors corresponding to said each of the first pixels;
• a determination portion that determines which one of low, medium, and high lightness regions defined by dividing a predetermined lightness range into three the lightness value thus calculated belongs to;
• an image generation portion that generates an extraction image comprising pixels corresponding to said each of the first pixels of the color image,
• the image generation portion determining, for a pixel of which the lightness value belongs to the low lightness region, a pixel value based on the color component values excluding at least one of the color component values of three colors, determining, for a pixel of which the lightness value belongs to the medium lightness region, a pixel value based on the color component values of three colors, and determining, for a pixel of which the lightness value belongs to the high lightness region, a pixel value based on the color component values of two colors of the color component values of three colors or, based on the lightness value and the color component value of B;
• and
• an extraction portion that extracts an edge based on the pixel value of the extraction image generated by the image generation portion.