Essentially good compression needs prediction. Here the previous pixels are used but it seems only from the row. Given that this is a linear decompression (not

Essentially good compression needs prediction. Here the previous pixels are used but it seems only from the row. Given that this is a linear decompression (not a progressive) it would help a lot to instead of looking just at the row, also look at the above row where the other previously seen neighbor pixels are. With something like arithmetic coding and only the previously seen neighbor pixels as prediction contexts with some tweaking one can quickly get better compression than PNG offers. Doing this in a short C implementation is left to the reader ;)