Threshold matrix, storage unit for storing threshold matrix as data, and raster image processor incorporating storage unit

1. A computer readable medium having stored therein a threshold matrix, said computer readable medium containing computer executable instructions for:

causing the threshold matrix to convert a continuous-tone image into a dot pattern representing a binary image,

wherein a minimum number of pixels making up dots of the dot pattern is 2×2, and

when a reference periphery length proportion per unit area of the dot pattern is defined as Ref_sur =(4×r×Q 1/2 )/R where r represents a pattern frequency, Q represents a blackened ratio, and R represents an output resolution, and a periphery length proportion determined per unit area for the dot pattern generated by the threshold matrix corresponding to the blackened ratio Q is represented by Mes_sur,

a threshold array of the threshold matrix is determined such that a dot pattern is generated where a dot pattern periphery length evaluation index defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackened ratios Q ranging from 0 to 1, and that a normalization threshold side size Nr which serves as an index indicative of how many times the pattern frequency r is repeated in one threshold matrix is defined as Nr=N×r/R where N represents a size of one side of the threshold matrix as a number of pixels, and the normalization threshold side size Nr is of an value greater than 65.

2. The program of claim 1 wherein the computer readable medium is a storage unit in a raster image processor.

3. A computer readable medium having stored therein a threshold matrix, said computer readable medium containing computer executable instructions for:

causing the threshold matrix to convert a continuous-tone image into a dot pattern representing a binary image,

wherein when a reference periphery length proportion per unit area of the dot pattern is defined as Ref_sur=(4×r×Q 1/2 )/R where r represents a pattern frequency, Q represents a blackened ratio, and R represents an output resolution, and a periphery length proportion determined per unit area for the dot pattern generated by the threshold matrix corresponding to the blackened ratio Q is represented by Mes_sur,

a threshold array of the threshold matrix is determined such that a dot pattern is generated where a dot pattern periphery length evaluation index defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackened ratios Q ranging from 0 to 1 is generated, and that a normalization threshold side size Nr which serves as an index indicative of how many times the pattern frequency r is repeated in one threshold matrix is defined as Nr=N r/R where N represents a size of one side of the threshold matrix as a number of pixels, and the normalization threshold side size Nr is of an value greater than 65, but not in excess of 75.

4. The program of claim 3 wherein the computer readable medium is a storage unit in a raster image processor.

5. A threshold matrix generating system comprising:

a threshold matrix generator; and

a threshold matrix storage unit for storing a threshold matrix which converts a continuous-tone image into a dot pattern representing a binary image,

wherein when a reference periphery length proportion per unit area of the dot pattern is defined as Ref_sur=(4×r×Q 1/2 )/R, where r represents a pattern frequency, Q represents a blackened ratio, and R represents an output resolution, and a periphery length proportion determined per unit area for the dot pattern generated by the threshold matrix corresponding to the blackened ratio Q is represented by Mes_sur,

a threshold array of the threshold matrix is determined such that a dot pattern is generated where a dot pattern periphery length evaluation index defined as Mes_sur/Ref_sur does not exceed 1.14 for all blackened ratios Q ranging from 0 to 1 is generated, and that a normalization threshold side size Nr which serves as an index indicative of a number of times the pattern frequency r is repeated in one threshold matrix is defined as Nr=N r/R, where N represents a size of one side of the threshold matrix as a number of pixels, and the normalization threshold side size Nr is of an value greater than 65, but not in excess of 75.

6. The threshold matrix generating system according to claim 5 , further comprising an output system which forms the dot pattern generated by the threshold matrix on printable media.