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Comment by debazel | Hacker News Reader

You can’t really do that with PNG today either, because of all the PNG optimizers that quantize images before encoding them to PNG for better compression.

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Independent preprocessing doesn't make PNG itself lossy.

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

Yes, then that pipeline is lossy, not the lossless encoder that's part of it.

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

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.

Arguably the same is true of JPEG. The format itself can record all the frequencies in full detail (with some tiny rounding errors I'm ignoring here), but the optimizer creates a different image by cutting elements that are less important and wasteful of bits. And then that different image is encoded losslessly.

By that definition, all formats are lossy, because any preprocessing could be lossy, and the word loses all meaning … which is why we don't use that definition.

The word doesn't lose definition, it has different definitions for codecs and images.

If you reencode a jpeg to a png, you used a lossless format but your image is very lossy.

By what definition? I didn’t say anything was lossy; I said you couldn’t be sure that the image was lossless just from the extension. Of course, it might still be, and you might have other reasons to know it to be.

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