zlib compression level from 1-9 (higher means smaller file size). Setting to 0 uses Huffman compression which often results in even smaller file size.

Data encoding filtering (can reduce file size) - a number between 1 and 5. 1 is sub, 2 is up, 3 is average, 4 is Paeth, and 5 is adaptive.

Mostly useful for images of natural landscapes (images without sequences of solid colors).

Define the number of colors. Useful for small/thumbnail images to reduce file size.