Threshold matrix generation method, image data generation method, storage medium storing program, threshold matrix, and image data generation apparatus

A non-transitory storage medium storing a program to cause a computer to generate a threshold matrix to be compared with a multi-tone original image in performing halftoning of said original image, said program being executed by a computer to cause said computer to perform:

• a) preparing a matrix space which is a set of elements arranged in a row direction and a column direction, said matrix space being a storage area in said computer;
• b) setting a plurality of periodic areas disposed periodically and distributed uniformly in said matrix space, each of said plurality of periodic areas including a plurality of elements;
• c) setting elements existing every other one in said row direction and said column direction in at least each periodic area, as target elements;
• d) assigning threshold values ranging from one on a most highlight side to a predetermined switching threshold value sequentially to target elements included in said plurality of periodic areas;
• and
• e) assigning remaining threshold values sequentially to remaining elements to each of which no threshold value is assigned after said operation d),
• wherein assuming that elements to which threshold values are assigned are regarded as determined elements, when each threshold value is assigned in said operation d), under a condition that the number of determined elements is almost same in said plurality of periodic areas and each determined element is positioned adjacent to any one determined element in each periodic area, in consideration of repetitive application of said threshold matrix in performing halftoning of said original image, a target element whose distance to all determined elements is largest is specified within said plurality of periodic areas and said each threshold value is assigned to said target element, thereby to grow a cluster of determined elements in said each periodic area.

An image data generation apparatus for generating image data, comprising:

• a matrix storage part storing a threshold matrix;
• and
• an image data generator for generating halftone dot image data in which a multi-tone original image is halftoned by comparing said original image with said threshold matrix, wherein
• when a plurality of halftone tint images having respective tone values ranging from one on a most highlight side to a predetermined switching tone value are generated, in said plurality of halftone tint images, dots are formed only in target pixels existing every other one in a row direction and a column direction in a plurality of periodic areas disposed periodically and distributed uniformly, each of said plurality of periodic areas including a plurality of pixels, and
• a position to which a dot is added in a transition from a halftone tint image having one tone value to a halftone tint image having a next tone value among said plurality of halftone tint images is a target pixel whose distance to all already-existing dots is largest, under a condition that the number of dots is almost same in said plurality of periodic areas and each dot is positioned adjacent to any one dot in each periodic area in each halftone tint image, thereby to grow a cluster of dots in said each periodic area.