Different implementations of DEFLATE possibly... https://en.wikipedia.org/wiki/DEFLATE
Or different compression algorithms used...
Or both?

For a sanity check: open each file (original, GIMP png, and Krita png) in gimp and save as PPM (human readable) then diff the files... there should be no differences between them all.

If there are differences, then something has gone wrong somewhere since both gimp png and krita png should be exactly like the original. If there are no differences then nothing has gone wrong and the two programs must use different compression implementations so pick which ever is best (I guess Krita since its smaller?).

You can diff the bits (hex editor) ofcourse, to get a better idea of what is different between the compressed versions, but working back from that... erm... not me.

Both are open source so futher digging in the relvant code areas would yeild the correct reason for this. Personally have no idea, never been bothered to look at the source for gimp (Although I'm pretty sure it uses zlib)... not used krita at all.

run pngcrush on it and see how small it gets!

I just tried pngcrush and it doesn't really do as good a job as Krita, the result is closer to GIMP.

spiroyster wrote:

For a sanity check: open each file (original, GIMP png, and Krita png) in gimp and save as PPM (human readable) then diff the files... there should be no differences between them all.

Thanks, I'll try that.

I use Ken Silverman's PNGOUT to optimize the size of PNG files.

Thanks for the tip, PNGOUT gives a slightly better result than Krita.

UPD: However I noticed that PNGOUTWin reduces the ppi resolution of compressed images while Krita keeps the same ppi as the source file.

MrFlibble wrote:

However I noticed that PNGOUTWin reduces the ppi resolution of compressed images while Krita keeps the same ppi as the source file.

I don't see this with irfanview's implementation.

MrFlibble wrote:

However I noticed that PNGOUTWin reduces the ppi resolution of compressed images while Krita keeps the same ppi as the source file.

It's probably stripping the pHYs chunk so the PPI you see is some default value of the viewer (typically 72, or maybe that's part of the PNG spec?). Tell it to keep that chunk.