From optipng’s todo.txt: - Compression improvements: Use zlib's deflateTune(). Use 7zip's powerful deflation engine. (This is not possible with l

From optipng’s todo.txt: - Compression improvements: Use zlib's deflateTune(). Use 7zip's powerful deflation engine. (This is not possible with libpng, so a custom encoder is needed.) AdvanceCOMP uses the same deflate code as 7zip. PNGout uses a deflation engine that claims to be better than 7zip's.