PERCEPTUALLY LOSSLESS IMAGE COMPRESSION

An image compression technique employs selecting a gray level threshold value for converting a gray level digital image input into a bi-level input which minimizes weak connectivity, wherein weak connectivity comprises a checkerboard pattern found in a 2×2 array or neighborhood of pixels. The threshold value is determined by traversing the array of pixels comprising the document in a single path, examining successive 2×2 neighborhoods and incrementing a plus register for the gray level value which a checkerboard pattern first appears and incrementing a minus register for the gray level value at which the checkerboard pattern no longer exists. The total number of weak connectivity checkerboards for each gray level threshold value is calculated based upon the difference between the values stored in the registers. An improved distance measure for determining a match between models comprises a modification of the Hausdorff measure by limiting such measure to a single quadrant, resulting in a lower mismatch rate.