[Bf-committers] Color Unpremultiply

[gespertino at gmail.com]

Hi. I have some questions regarding the new "Color Unpremultiply" option in
Blender.
I understand what it does, but I wonder if there isn't a more elegant or
accurate way to expose it.

First of all, the text in the tooltip is a little bit misleading. The
operation performed isn't done to avoid a fringe on light backgrounds. It's
done because colorspace conversions shouldn't be performed on premultiplied
data.
The resulting effect when doing it wrong is indeed a fringe, but what the
operation does is to deliver a properly converted image, not avoiding a
fringe.
This may sound silly, but I think that using a more accurate tooltip would
have an educational effect because people would know what it does or would
investigate more rather than "this is for reducing a fringe".
A fringe can come from a wrong compositing earlier, and believing that
color unpremultiply will fix it is wrong.

Apart from that, I wonder if it's really necessary to have that option
instead of just applying it whenever premultiplied alpha is selected and
the output file is gamma-corrected.

Actually, I wonder if Blender shouldn't use premultiplied or straight
(graying out sky) when output is RGBA and conversely offer just sky when
output is RGB.

I don't know if blender's internals allow this, but it would simplify the
UI a little and help to avoid some common mistakes caused by mere
distraction.
Apart from that it would have an educational value making easier to new
users to choose the right values.

Makes sense?