Patent ID: 7796303

Claim:
An image processing system for performing certain image processing on image data and based on the results of said image processing, outputting dot data for forming dots on a recording medium, the image processing system comprising: encoding preparation unit that prepares in advance multiple classes of encoding correspondence relationships for the purpose of acquiring code values associated with the results of multilevel halftoning for the pixels making up said pixel groups, on the basis of representative tone values which are tone values representing pixel groups composed of multiple pixels; representative tone value determining unit that divides into multiple regions, each of which comprises multiple pixels, said pixel groups which have been extracted sequentially from said image, and determines for each region a representative tone value representing the tone value of said region; encoding unit that, using said determined representative tone value for each region, derives a code value for each said region by unit of referring to said encoding correspondence relationship assigned to said pixel group to which said region belongs, said correspondence relationship being any one of said multiple classes; decoding preparation unit that prepares in advance multiple classes of decoding correspondence relationships for the purpose of acquiring, from code values assuming a certain range of values, an output dot arrangement representing the dot on-off state for each pixel within said pixel group; decoding unit that, for each pixel group, receives said code value acquired for each region belonging to said pixel group, and that on the basis of said received code value, refers to said decoding correspondence relationship corresponding to said encoding correspondence relationship assigned to said pixel group to which said region belongs, and acquires said output dot arrangement; output unit for outputting said acquired output dot arrangement as dot data; dot forming unit that forms dots on a medium on which said image can be formed; and image forming unit that receives said output dot data and actuates said dot forming unit in accordance with said dot data, to form said image on said medium, wherein said encoding correspondence relationship is defined as a threshold value group consisting of threshold values in a number corresponding to the number of pixels included in said pixel group, wherein said plurality of encoding correspondence relationships has been prepared by assembling said threshold value group from a plurality of threshold values selected from a dither matrix wherein threshold values of different values are arranged according to a predetermined characteristic, said selection being made while sequentially changing the extraction location, and wherein said dither matrix is a global dither matrix containing threshold values in a greater number than the tone number of said image.