Old PNGs will work just fine. And forward compatibility is much less important.

The main use case for PNG is web browsers and all of them seem to be on board. Using old web browsers is a bad idea. You do get these relics showing up using some old version of internet explorer. But some images not rendering is the least of their problems. The main challenge is actually going to be updating graphics tools to export the new files. And teaching people that sRGB maybe isn't good enough any more. That's going to be hard since most people have no clue about color spaces.

Anyway, that gives everybody plenty of time to upgrade. By the time this stuff is widely used, it will be widely supported. So, you kind of get forward compatibility that way. Your browser already supports the new format. Your image editor probably doesn't.

Better compression can also mean a new set of filter methods or a new interlacing algorithm. But yeah, any of them would cause an instant incompatibility. As noted in the relevant issue [1], we will need a new media type at the very least.

[1] https://github.com/w3c/png/issues/39#issuecomment-2674690324

It has fields to say what compression is used. Adding another compression form should be handled by existing software as recognizing it as a valid PNG that they can't decompress.

The PNG format is specifically designed to allow software to read the parts they can understand and to leave the parts they cannot. Having an extensible format and electing never to extend it seems pointless.

I don't think that will super be an issue. How often has "progressive jpeg" ever caused problems? That's the same thing.

They could, for example, use lossy compression for the compatibility layer and then fill it in the rest of the way to lossless using incompatible new compression objects. Legacy uses will see some fidelity degradation, but they are already being stuck with sRGB downmixes, so that’s fine — and those who are bothered by it can just emit a lossless-pixels (but lossy-color and lossy-range) compatibility layer and reserve the compression benefits for the color and dynamic range.

I’m not saying this is what will happen — but if I was able to construct a plausible approach to compression in ten minutes, then perhaps it’s a bit early to predict the doom of compatibility.

Can't you improve a compression algorithm and still produce a still valid decompression input? PNG is based on zip, there's certainly ways to improve zip without breaking backwards compatibility.

That being said, they also can do dumb things however, right at the end of the sentence you quote they say:

> we want to make sure we do it right.

So there's hope.

What backward compatibility are we talking about here? Backwards compatibility of images will be fine, backwards compatibility of decoders might be impacted, but the article says the major image viewers (browsers) and image editors already support the 3rd version. Better compression is only planned for the 5th version of the spec.

Also if you forbid evolving existing formats, the only alternative to improve is to introduce a new format, and I argue that it would be causing even more fragmentation and be more difficult to adopt to. Look at all the drama surrounding JPEG XL.

Does usb-c spec break backward compatibility ?, a 2018 macbook work perfectly fine with 2025 usb c charger

One could imagine a PNG file which contains a low-resolution version of the image with a traditional compression algorithm, and encodes additional higher-resolution detail using a new compression algorithm.

+1 why not name it png4 or something. It's better if compatibility is obvious upfront