Method and apparatus for deriving additional gray levels in a multi-gray

Abstract

A laser printer/copier performs a method which converts a gray pixel value of N possible levels, to a halftone image exhibiting n apparent pixel gray values (where N>n). Each pixel of the halftone image is rendered by a laser at one of n-1 non-white gray levels through use of the method. The method includes the steps of: sequentially, logically, tiling a dither matrix across the image and determining a relational value between a gray value of each image pixel and a logically superimposed dither matrix value. At each odd tile position, a determination is made for each such relational value, which of n-1 threshold values is exceeded and if an intermediate one of the n-1 threshold values is exceeded, the pixel is assigned a first gray modulation value. At each even tile position, a determination is made, for each such relational value, of which of the n-1 threshold values is exceeded and if the aforesaid intermediate threshold value is exceeded, the pixel is assigned a second, different one of the n-1 gray level modulation values. Thereafter, laser modulation values are examined for pixels that are correspondingly positioned at three sequential (odd/even/odd or even/odd/even) dither matrix tile positions. The gray modulation value assigned to the middle corresponding pixel is then adjusted in accordance with gray modulation values assigned to the adjoining corresponding pixels so as to provide a smoothing of gray value transitions. The smoothed gray level values are then applied to control a laser print engine.