[userbinator]

I wonder how it compares with simply LZMA'ing (i.e. 7zip) a BMP. In my experience that has always been significantly smaller than PNG (which is itself a low bar --- deflate/zlib is a simple LZ+Huffman variant which is nowhere near the top of general-purpose lossless compression algorithms.)

Along the same lines, I suspect BMP+LZMA would likely be beaten by BMP+PPM or BMP+PAQ, the current extreme in general-purpose compression.

[PetahNZ]

Do you have any examples of zipped bmp's compared to png. Seem strange if that was indeed better.

[Dylan16807]

It shouldn't be that surprising. PNG has a maximum window size of 32KB. That means you could use a small set of identical tiles to make an image, and PNG would have to store a new copy every row, because the previous row is out of range.

[clarry]

Well I just took a screenshot and tried it. BMP: 6MB. lzma-compressed bmp: 75K. PNG through optipng and pngcrush: 178k.

Not very surprising imho. PNG was never a strong compression format to begin with.