Re-compression of pictures that are already well compressed

[Johan-foreva]

Pitch

Yes, one can understand when a picture is huge (in pixels or megabytes) and it is resized and re-compressed in WebP. WebP is a good format. But why to compress lossy, if the original picture arrives already in WebP and weighs less than the result? The same applies to JPEG, but for some reason does not affect PNG, while it can always be compressed in WebP without loss, getting a benefit in size.

It would be good to add minimal input format checking and not to take unnecessary actions if they will knowingly degrade the image but not give a gain in size.

Motivation

This will make life easier for authors who understand and use compression correctly, as well as for subscribers.