Mixed matrix image processing for rendering halftone images with variable dot sizes

Claim 2

A document copying apparatus of the type which generates a plurality of electrical image signals which are serial measures of imagewise information of successive picture elements of an original document to be reproduced, said apparatus comprising: means for comparing input image signals representing low optical density picture elements of the original document to a series of threshold values selected in sequential order from a first two-dimensional matrix halftone cell in which an output signal density pattern grows in a low spatial frequency manner; means for comparing input image signals representing high optical density picture elements of the original document to a series of threshold values selected in sequential order from a second two-dimensional matrix halftone cell in which an output signal density pattern grows in a high spatial frequency manner; and means for comparing input image signals representing mid-optical density picture elements of the original document to a series of threshold values selected in sequential order from said first or said second two-dimensional matrix halftone cells according to the optical density of the picture element such that at least one image signal, representing a first picture element having a higher optical density than a second picture element, is compared to a series of threshold values selected in sequential order from said first two-dimensional matrix halftone cell, and an image signal representing said second picture element is compared to a series of threshold values selected in sequential order from said second two-dimensional matrix halftone cell, whereby image signals representing mid-optical density picture elements compared to threshold values selected from said first cell are interspersed with mid-optical density picture elements compared to threshold values selected from said second cell.

Claim 3

A document copying apparatus as set forth in claim 1 wherein the printed sub-elements of the matrix halftone cell in which an output signal density pattern grows in a low spacial frequency manner grow around a core.

Claim 4

A document copying apparatus as set forth in claim 1 wherein the printed sub-elements of the matrix halftone cell in which an output signal density pattern grows in a low spacial frequency manner grow around a core central to the cell.

Claim 5

A document copying apparatus as set forth in claim 1 wherein low density is defined to be optical densities below about 60 on a 256 grey level scale.

Claim 6

A document copying apparatus as set forth in claim 1 wherein low density is defined to be optical densities below about 80 on a 256 grey level scale.

Claim 7

A document copying apparatus of the type which generates a plurality of electrical image signals which are serial measures of imagewise information of successive picture elements of an original document to be reproduced, said apparatus comprising: means for comparing input image signals representing picture elements in low optical density area of the original document to a series of threshold values selected in sequential order from a first two-dimensional matrix halftone cell in which an output signal density pattern grows in a low spatial frequency manner; means for comparing input image signals representing picture elements in high optical density areas of the original document to a series of threshold values selected in sequential order from a second two-dimensional matrix halftone cell in which an output signal density pattern grows in a high spatial frequency manner; and means for comparing input image signals representing picture elements in mid-optical density areas of the original document to a series of threshold values selected in sequential order from said first or said second two-dimensional matrix halftone cells according to the optical density of the area of the picture element such that at least one image signal, representing a first picture element having a higher optical density than a second picture element, is compared to a series of threshold values selected in sequential order from said first two-dimensional matrix halftone cell, and an image signal representing said second picture element is compared to a series of threshold values selected in sequential order from said second two-dimensional matrix halftone cell, whereby image signals representing mid-optical density picture elements compared to threshold values selected from said first cell are interspersed with mid-optical density picture elements compared to threshold values selected from said second cell.

Claim 8

A document copying apparatus of the type which generates a plurality of electrical image signals which are serial measures of imagewise information of successive picture elements of an original document to be reproduced, said apparatus comprising: means for comparing input image signals representing picture elements in low optical density area of the original document to a series of threshold values selected in sequential order from a first two-dimensional matrix halftone cell in which an output signal density pattern grows in a low spatial frequency manner; means for comparing input image signals representing picture elements in high optical density areas of the original document to a series of threshold values selected in sequential order from a second two-dimensional matrix halftone cell in which an output signal density pattern grows in a high spatial frequency manner; and means for comparing input image signals representing picture elements in mid-optical density areas of the original document to a series of threshold values selected in sequential order from said first or said second two-dimensional matrix halftone cells according to the optical density of the area of the picture element such that at least one image signal, representing a first picture element having a higher optical density than a second picture element, is compared to a series of threshold values selected in sequential order from said first two-dimensional matrix halftone cell, and an image signal representing said second picture element is compared to a series of threshold values selected in sequential order from said second two-dimensional matrix halftone cell, whereby image signals representing mid-optical density picture elements compared to threshold values selected from said first cell are interspersed with mid-optical density picture elements compared to threshold values selected from said second cell.

Claim 9

A document copying apparatus as set forth in claim 6 further comprising means for averaging the optical density of picture elements in said areas of the original document.

Claim 10

Apparatus for reproducing a gray level image in a bi-tonal medium, said apparatus comprising: means for comparing a picture element value with a threshold value to make a print or no-print decision characterized by a corresponding one of two print values; a first dot concentration type two-dimensional matrix halftone cell set; a second dot dispersion type two-dimensional matrix halftone cell set; and means for selecting said threshold value from said first set when the picture element is of low density level; for selecting said threshold value from said second set when the picture element is of high density level; and for selecting said threshold value from said first or said second sets when the picture element is of mid-density level such that at least one threshold value is selected from said first set for a first picture element having a higher optical density than a second picture element, and a threshold value for said second picture element is selected from said second set.

Claim 11

Apparatus for reproducing a gray level image in a bi-tonal medium, said apparatus comprising: means for comparing a picture element value with a threshold value to make a print or no-print decision characterized by a corresponding one of two print values; a first dot concentration type two-dimensional matrix halftone cell set; a second dot dispersion type two-dimensional matrix halftone cell set; and means for selecting said threshold value from said first set when the picture element is of low density level; for selecting said threshold value from said second set when the picture element is of high density level; and for selecting said threshold value from said first or said second sets when the picture element is of mid-density level such that at least one threshold value is selected from said first set for a first picture element having a higher optical density than a second picture element, and a threshold value for said second picture element is selected from said second set.