Multi-level to bi-level raster shape converter

Abstract

A method and apparatus for converting multi-level raster shapes into bi-level raster shapes while preserving as much of the visual character of the shape is disclosed. The method and apparatus solve two main problems arising in the task: that of broken continuity (drop-outs) and that of large changes in the width of horizontal and vertical lines on the bi-level raster shape as a result of minute changes on the multi-level shape (stem width aliasing). Drop-outs are handled by identifying certain boundaries between pixels as drop-out warnings. Pixels adjacent to dropout warnings have their coverage values converted from multi-level to bi-level by a process that guarantees that at least one of them will be rounded up. Dropout warnings are found from the outline of the ideal shape used to generate the multi-level shape and represent an approximation to said outline. In a preferred embodiment, the set of drop-out warnings for an outline is the set of pixel boundaries containing the nearest pixel boundary to each intersection of the outline with mid-pixel lines. Stem width aliasing occurs because of rounding errors clustering along horizontal and vertical lines. The present invention minimizes stem width aliasing by a variable threshold technique in which the threshold value used to turn the multi-level coverage value of a pixel into a bi-level value depends on the pixel position. The multiple threshold values are used to scatter the rounding errors over neighboring pixels so that they do not cluster along horizontal or vertical lines and are therefore less apparent to a viewer. In a preferred embodiment, two different threshold values are used in a checkerboard pattern, i.e., the threshold value for every other pixel in the horizontal or vertical direction alternates between the two values.