[Gimp-developer] Enhanced smudge tool : Smudge with painting

[<b91502038 ntu edu tw>]

Hi everyone. I did an experiment that enhanced the smudge tool, which can not only smudge existing color on 
the canvas, but also blend brush color on it. This is useful in digital painting.

Details and test drawings
http://www.gimp-forum.net/Thread-Customized-smudge-tool-Smudge-with-painting

It was not difficult to implement. Only 6 files were modified and had no compatibility issues.

Current issues:

--Color from gradient and color brushes are not supported, only foreground color can be used.
I can try to fix it.

--Doesn't support layer modes (multiply, screen, add, etc.)
It's difficult to implement because smudge tool calculates brush-to-drawable blending by itself and calls 
gimp_brush_core_replace_canvas(), unlike other brushes.
MyPaint brush doesn't support this either.
I don't think this is a true problem. Normal mode is enough in practice. But I'm not sure other users' 
requirements.

--Have not modifird the relative script procedure
The "gimp-smudge" in Procedure Browser, I havn't added new parameters yet.

An optional idea:

--"No erasing effect" may be implemented as a layer property, like "Lock alpha channel", "Lock pixels" 
switches.
That is, for any operations effecting pixels' alpha
if(no_erasing_effect)
  now_alpha = MAX(updated_alpha, now_alpha);
else
  now_alpha = updated_alpha;
But I'm not sure the difficulty to implement and the usefulness.
As my experience, only smudge and MyPaint brush really need this.