PNG based on DEFLATE
pre-compression: filtering (prediction)
compression: DEFLATEPNG uses DEFLATE, a non-patented lossless data compression algorithm involving a combination of LZ77 and Huffman coding. Permissively licensed DEFLATE implementations, such as zlib, are widely available.
Compared to formats with lossy compression such as JPEG, choosing a compression setting higher than average delays processing, but often does not result in a significantly smaller file size.