Hardwired Paeth codec for portable network graphics (PNG)

This paper describes an execution unit capable of computing the Paeth predictor, as used in the portable network graphics (PNG) standard. PNG is a rather new, lossless compression method for real-world pictures. It features five prediction schemes, of which the modified Paeth predictor is the most computational intensive. This paper focuses on a hardware implementation of the Paeth predictor and a hardware Paeth codec capable of computing three different quantities: the Paeth predictor of three inputs, the difference of the current pixel and the Paeth predictor of the other inputs (coding), and the sum of the coded input and the Paeth predictor of the other three inputs (decoding). The proposed Paeth codec takes two cycles, where a cycle is comparable to an general purpose ALU cycle. Depending on the mode of operation, the proposed mechanism produces the predictor or the (de/en)-coded pixel value