Ink jet and xerographic binary graphic printers are disclosed which make reproductions by marking the pixels within a raster pattern on a copy sheet with a high or low density (e.g. black or white) mark in response to electrical printing signals. The printing signals are generated by comparing the magnitude of video signals representative of a continuous tone raster image with electrical screen signals organized into halftone cells according to a pattern. A shift circuit processes received video signals and identifies the center of the black (or white) video signals within the confines of a screen halftone cell. The pattern of screen signals within a halftone cell is shifted to the center of the black video signals. The cell shifting suppresses beat frequencies in reproductions of originals containing screens or image detail with frequencies near that of the electrical screen.
While a general large layer dither volume provides great flexibility in dither cell design, by decoupling different intensity levels, they are inefficient to access since each color component requires the retrieval of a different bit from the volume. Therefore in a multi threshold dither volume for digital halftoning a contone color image, in the form of an array of contone color pixel values to bi-level dots, there is allocated for each dither cell location a fix set of n thresholds defining n+1 intensity intervals within which said dither cell location is defined to be alternatively not set and set.
Method and apparatus for compressing and enhancing gray-level, halftone, image data in copiers and printers. The input data is digitally halftoned to provide input halftone cells containing a plurality of individual pixels. The pixel density distribution is examined to determine the relative position within the cell of the densest pixels. The gray-level density value for the entire cell is also ascertained. The distribution determines the particular threshold matrix which is used to convert the density value of the input cell into pixel density values which form an output halftone cell. The various matrices have distribution patterns which place the lower threshold values at the position in the output cell corresponding to the position of the densest pixels in the input cell. The cell density value and the matrix selection designator are stored in memory at different bit levels.
A method of determining image input contrast to dynamically adjust screen amplitude. The screen amplitude is controlled on a dot by dot basis to selectively enhance the original image. There is a higher amplitude when the input contrast is low and a lower amplitude when the input contrast is high. Thus, the partial dots more closely follow image detail at high contrast, while at low contrasts, noise is not enhanced. In a specific embodiment, depending upon the input contrast, a predetermined screen amplitude is used.
A method of forming a plurality of printing screens, each having a selected frequency and angle, from a single masterdot of mxm elements where each element specifies a weight accorded a printing signal.
Processing used to represent a gradation by changing the number of black pixels and the number of white pixels in a matrix of 8.times.8 pixels. First, edge information sampled from the input data is allocated to corresponding pixel positions of the output data with priority. A residual gradation value is determined by subtracting a value corresponding to the edge information already allocated from the average gradation of the input data, and black pixels and white pixels are alternately allocated to the remaining pixel positions of the output data except for the edge region, in accordance with the ratio of the number of those remaining pixels to the residual gradation value. When the region except for the edge region is divided into a plurality of regions, an average gradation for each of the divided regions is determined and a residual gradation value is distributed to each of the divided regions in accordance with the ratio between the average gradations, followed by allocating black pixels and white pixels to each of the divided regions in accordance with the ration of the distributed residual gradation value to the number of pixels in that region.
