They say it's technically compatible since older image decoders should recognize the PNG file is using a different compression algorithm than the default.
> Many programs already support the new PNG spec: Chrome, Safari, Firefox, iOS/macOS, Photoshop, DaVinci Resolve, Avid Media Composer...
This is intentionally ignoring the fact that there are countless PNG decoders out in the wild, many using libpng the standard decoder last updated 6 years ago; and they will not be able to read the new PNG v2 files.
They should have used a different file extension, PNG2, to distinguish this incompatible format. Otherwise, users will be confused why their newly saved PNG file cannot be read by certain existing programs.
Those are indeed the "magic" bytes of PNG. It's a very clever choice meant to ensure the transport layer didn't mess with it.
To start, there's a byte with the upper bit set which ensures an "8-bit clean" transport. If it's stripped, it becomes a harmless tab. Then the literal "PNG" text so you can see it in a text editor. Then a CR-LF pair to check for CR-LF to LF translations. Then, a CTRL-Z to stop display on DOS-like systems. And finally, another LF to check for LF to CR-LF translations.
It's a clever "magic" that basically ensures a binary transport layer. Things that mattered back in 1996.
It's clever but I'm not so sure it actually mattered - other formats have done just as well with simpler magic numbers. All it does in the end is that you get something that doesn't identify as a PNG file rather than a PNG file with bad data when a non-binary transport is used - both results are bad and immediately apparent.
Yes and no. It wasn't just about telling you a problem occurred, but failing early and being able to say exactly why. A "something in the chain is running in 7-bit mode" is more helpful than "CRC error in IDAT". Maybe the developers were being a bit too ambitious/hopeful, but an eight byte "magic" over a "simpler" four byte one isn't really worth crying over, even with 1996 download speeds.
They say it's technically compatible since older image decoders should recognize the PNG file is using a different compression algorithm than the default.
> Many programs already support the new PNG spec: Chrome, Safari, Firefox, iOS/macOS, Photoshop, DaVinci Resolve, Avid Media Composer...
This is intentionally ignoring the fact that there are countless PNG decoders out in the wild, many using libpng the standard decoder last updated 6 years ago; and they will not be able to read the new PNG v2 files.
They should have used a different file extension, PNG2, to distinguish this incompatible format. Otherwise, users will be confused why their newly saved PNG file cannot be read by certain existing programs.