[Bf-cycles] Alpha pass independent from transparent option

[Brecht Van Lommel]

Hi,

On Sat, Dec 8, 2012 at 3:37 PM, Adriano Oliveira <adriano.ufrb at gmail.com> wrote:
> I don't think that having the alpha channel dependent on Transperent option
> in rendering a good aproach. I would like to have the alpha in composition
> even if the background is visible.
> I propose we have an aplha pass independent from the transperant option in
> the film slide.

This could be done, however you can't actually do correct alpha
operations if the background is part of the combined result. The
reason is that for semi-transparent materials or antialiasing the
colors of the background and foreground get mixed and there is no way
to separate afterwards. So if you want to get correct results you will
need to use the environment pass anyway.

Personally I'm inclined to keep this restriction, so that the alpha
pass always matches the combined result. The way you could choose
alpha in blender internal caused a lot of confusion and incorrectly
composited results, I think this behavior helps guide people to the
right solution.

> It would be also nice to have the option of saving the alpha channel in a
> image file wihout transperency composed (not premultiplied).

This will be added, but not in the way it's done for blender internal.
This should be a feature of the image saving and dependent on the
image file format.

Brecht.