Lossy compression is not possible. PNG hasn't got one. However, some apps intentionally do various forms of preprocessing to make the data more compressible. This has nothing to do with PNG, per se.

There is one particular app called ImageAlpha that has this behavior as its only reason for existence - to make PNG files really small. It does this by blurring, quantizing colors, and using dithering against a custom color palette with 8-bit lookup. None of these techniques are part of the PNG 24 file format.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)