IMG_SaveWEBP treats quality=100 as lossless

[po-87]

The docs for IMG_SaveWEBP says the following about the quality parameter:

between 0 and 100. For lossy, 0 gives the smallest size and 100 the largest. For lossless, this parameter is the amount of effort put into the compression: 0 is the fastest but gives larger files compared to the slowest, but best, 100.

But there is no way to specify whether the image should be saved as lossless or not. Instead SDL uses lossless if quality is 100, and lossy otherwise. I don't know if this is intentional but it should at least be reflected in the documentation.

[slouken]

That's a good point. Right now the behavior is that quality 100 is lossless with the highest compression and lower quality is lossy with the given quality. I can clarify the documentation, but is that behavior what you want?

[po-87]

I understand if it's too late to change now but I think the easiest to understand and explain would have been if there was an additional bool or enum parameter to specify whether you want lossy or lossless. This would also have been more consistent with other tools.

Either you care about the image being lossless, or you don't. It would feel more reassuring to be explicit about this than to rely on a specific floating-point value.

Note that lossy quality=100 does not imply lossless with the official webp tools (such as cwebp). It's still lossy and the size and time tradeoffs are not the same. The lossy image might be larger than the lossless version for some simple images and the lossless compression might take much more time than the lossy compression for photos and other non-trivial images.

That said, I suspect the current behaviour is probably able to handle most real use cases because most people that want lossy compression will not use quality=100 (and you could probably get close enough by using something like 99.999f).

I personally like the idea of being able to change the compression level even for lossless but not being able to change it is consistent with other IMG_Save* functions for the lossless formats. Not sure highest/slowest compression is the best tradeoff for general use though. cwebp uses 75 by default so maybe that's a better value to use if it cannot be changed.

[GPIforGit]

One Solution could be
0 to 100 = lossy
-100 to -1 = lossless