Beating PNG is not actually hard; zlib is not fast at compression, nor is it very good at compressing anything besides graphics. It doesn't even do RGB decorre

Beating PNG is not actually hard; zlib is not fast at compression, nor is it very good at compressing anything besides graphics. It doesn't even do RGB decorrelation, which can be a really big win (e.g. YCgCo). Ironically, on non-graphics material, most lossless video encoders beat PNG, too. FFV1 is particularly good at grainy images, and is probably quite difficult to beat without either a much fancier predictor (e.g. LPC instead of median) or much more costly entropy coding (context mixing).