Created attachment 103278 [details] Levels No Channel by control The Filter Levels doenst have levels by channel, like blue, red or green modifications, just have a simple RGB Levels, this options it's need to control normal maps and texture decals with different img on each channel. Gimp and Photoshop they have this important option.
Marked as wish, since it's a feature request, not a bug.
I'm thinking about working on this as a first issue. I might be able to confirm this bug and figure out the path to relevant the source code.