Comment by AlienRobot
2 days ago
Honestly, I don't like how webp and now jpegxl support both a lossless and lossy mode.
Let's say you want to store images lossless. This means you won't tolerate loss of data. Which means you don't want to risk it by using a codec that will compress the image lossy if you forget to enable a setting.
With PNG there is no way to accidentally make it lossy, which feels a lot safer for cases you want lossless compression.
PNG can and is often used in a lossy way. Reducing the number of colors so PNG8 can be used instead of PNG24/PNG32 is the most common way to do that. Tools like pngquant exist, and for example Photoshop when exporting to PNG also has an option to reduce the colors, to flatten the image (remove alpha), or to change the colorspace.
16-bit PNG files can easily accidentally be reduced to 8-bit, which is of course a lossy operation. Animated PNG files can easily get converted into a still image (keeping only the first frame). CMYK images will have to be converted to RGB when saving them as PNG, which is also a lossy operation. It can happen that an image gets created as or converted to JPEG and then gets saved as PNG - which of course is a bad and lossy workflow, but it does happen.
So I don't agree that with PNG there is no way to accidentally make it lossy.
In any case: lossless or lossy is not a property of a format, but of a workflow. For keeping track of provenance information and workflow history, I would recommend looking into JPEG Trust / C2PA, which is a way to embed as metadata what happened to an image since it was captured/generated. Relying on the choice of image format for this is fragile and doesn't allow expressing the nuances, since reality is more complicated than just a binary "lossless or lossy".
You can use too few bits of color depth to get lossyness in PNG. More generally, I can't find myself very sympathetic to "I don't want a format that can do X and Y, because I might accidentally select X when I want Y in my software". You might accidentally choose JPG when you want PNG too. Or accidentally resample the image. Or delete your files.
If you want a robust lossless workflow, PNG isn't the answer. Automating the fiddly parts and validating that the automation does what you want is the answer.
Taking a look at the reference codec package at https://gitlab.com/wg1/jpeg-xl, they note:
> Specifically for JPEG files, the default cjxl behavior is to apply lossless recompression and the default djxl behavior is to reconstruct the original JPEG file (when the extension of the output file is .jpg).
You're right, however, that you do need to be careful and use the reference codec package for this, as tools like ImageMagick create loss during the decoding of the JPEG into pixels (https://github.com/ImageMagick/ImageMagick/discussions/6046) and ImageMagick sets quality to 92 by default. But perhaps that's something we can change.