Feature-monitored shape unifying for lossy SPM-JBIG2

Shape unifying is a very efficient preprocessing technique used in lossy SPM-JBIG2 systems. It permits isolated errors between the current bitmap and its reference to improve refinement coding efficiency. Compared to lossless coding, it can improve compression by about 32% while causing very little visual information loss. When bigger error clusters are permitted in shape unifying, further compression gain can be achieved but at the price of more noticeable visual information loss and even character substitution errors. We propose a feature monitored shape unifying procedure that can significantly lower the risk of substitution errors when permitting bigger errors. Experiments show that, compared to the unmonitored shape unifying, the feature monitored version can suppress more than 2/3 of all substitution errors while achieving additional compression improvements of 30-40%.