A color graphics pipeline has a separate encoder with a dither table for processing each color value in pixel data received from a host processor or rasterizer. The encoders modify color values and dither noise values and also combine both positive and negative dither noise values with the color values, for independent optimization of each color and for better edge detection in a color recovery filter within a decoder of the pipeline. Each encoder comprises the following elements. A comparator mechanism compares a color value with a predetermined cutoff value. A dither table provides dither noise values. An offset mechanism modifies the dither noise values to derive a modified dither noise value when the color value is greater than or equal to the predetermined cutoff value. An aliasing mechanism aliases the color value with other color values below the predetermined cutoff value to derive an aliased color value when the color value is less than the predetermined cutoff value. An adder mechanism adds the unmodified dither noise or modified dither noise to the aliased color value or nonaliased color value, respectively, to derive the dithered color value. A clamping mechanism ensures that the dithered color value resides within a predetermined color range. Finally, a quantizing mechanism quantizes the dithered color value to derive a quantized/dithered color value for storage in memory, such as in a frame buffer.

An electronic halftoning image reproduction system wherein a halftone screen function is combined with pictorial information, typically by addition, to provide a sum function. In a second channel the pictorial signal is averaged over the area corresponding to one period of the halftone screen function and the average is utilized to determine the percent of the area to be turned white within that halftone dot. This is accomplished by thresholding the sum function with a dynamically adjusted threshold for each period of the halftone dot such that the percentage of white matches the average pictorial signal. In another embodiment wherein the desired gray scale of the reproduction is different from that of the original image, the average pictorial signal may be adjusted in some predetermined manner and the percentage of white is matched to the adjusted signal. In a digital implementation the result is obtained by starting at the sample location with the largest value of the sum function and setting subdots white until the correct area is white. All steps are repeated over each period of the halftone.

A tape printer is presented to improve the print quality of a border portion between print areas. Dots, which are smaller in area than dots printed in a previous print area, are printed after being shifted by .alpha. (1.0 dot) in the X direction from dots printed in an overlap area. Accordingly, even if the dots are printed slightly out of position, high-density overlap portions, where dots overlap with each other, and/or paper surface-exposed portions, where dots are away from each other, are not produced greatly or distributed unevenly.

A system for processing halftone image data in digital format. The image data is electronically screened to provide a binary output suitable for storage or printing, the image data being processed in discrete blocks or cells. The error between actual and printed average gray values of each image cell processed is determined and the electronic screen thresholds are modified in accordance with such error (if any) prior to processing the next image cell. At the beginning of the aforesaid process and at the beginning of the processing each line, an error is randomly chosen.

An ink jet printer in which refined half-tones or shades of gray are produced while retaining high contrast capability using a dot matrix in which the dots are of the same density for the same shading plus or minus one drop of ink at selected dot locations within the matrix but with each dot being printed in response to an individual shading density or darkness input signal.