Independent preprocessing doesn't make PNG itself lossy.

The point is that you can’t tell whether it’s a lossless image by looking at the extension.

	▲	mnw21cam 39 minutes ago \| parent [-]
		It's not the PNG encoding that causes the loss. The optimisers are literally creating a different image and then losslessly encoding it as PNG. But it's also a valid point.

arthur-st 2 hours ago | parent | prev [-]

Yes, but a lossless encode of a lossy copy of an image results in a lossy image functionally.