DEFAULT_QUALITY causes problems for webp.

[btimby]

If you leave the default quality set to -1

When saving a webp image, you will receive the following exception ValueError: invalid configuration. This error originates from the webp handling in Pillow here:

https://github.com/python-pillow/Pillow/blob/main/src/_webp.c#L673C10-L673C28

Which in turn is calling libwebp's WebPValidateConfig() function, which is here:

https://github.com/webmproject/libwebp/blob/6a22b6709ce9e8fea5a487b37417f695df89b286/src/enc/config_enc.c#L99

It expects a value between 0 and 100. Therefore quality=-1 is an invalid value for webp images.

I am working around this by defining a default quality of 75, however, I would expect the default to be usable "out of the box". If you have some guidance on how you would like this fixed I will open a PR. I have a couple ideas:

• Add a special case for webp format that frames the quality value between 0-100.
• Catch this exception and if working with a webp file raise a more useful exception.
• Disable support for webp.
• Use the quality value from the source image (if this is available) in the default case.
• Omit the quality parameter unless the user defined it.
• Change the default to 75 or another valid value (see below).

Pillow docs for Image.save() say:

"The available options are described in the image format documentation for each writer."

On the file formats page, only jpeg, tiff and webp seem to accept quality. Jpeg accepts 0-95 (75 default), tiff is 0-100 (75 default) and webp accepts 0-100 (80 default). This is how I arrived at my default of 75.