Stack filters for 1-to-N bit image processing in electronic printers

A method for designing a stack filter, for enhancing a binary input image into a multiple-bit per pixel image, comprising the steps of:

• (a) obtaining a gray-scale image, wherein the image contains structures representative of structures appearing in documents to be enhanced from a binary to a multiple-bit per pixel gray-scale resolution;
• (b) thresholding the gray-scale image using a binary threshold level to produce a binary image;
• (c) thresholding the gray-scale image at second threshold, preferably not equal to the binary threshold, to produce a first level slice;
• (d) producing, as a function of the binary pixel levels in the binary image and the first level slice, a first difference image;
• and
• (e) generating, as a function of the first difference image and the binary image, a first level-slice filter having a plurality of templates which, upon receiving binary input signals in a pattern similar to that within a windowed subset of pixel locations within the binary image, will produce binary output signals equivalent to a corresponding pixel position in the first difference image.