clipka <ano### [at] anonymousorg> wrote:
> Am 05.07.2010 11:44, schrieb Mr:
>
> > sorry to get to blender again in the discussion, I am prepared for someone to
> > answer that it simply is buggy or wrong, but it behaves without any of its
> > million count userbase complaining. It doesn't seem to premultiply the
> > background as there is no trace of it when we use the show alpha button in the
> > image window. I made a little screenshot with the process to render and see what
> > I mean if you have a version of blender around:
>
> The alpha premultiplication issue only shows very openly with
> semi-transparent stuff.
>
> A simple test whether some piece of software does it right or not should
> be as follows:
>
> (1) Render a scene with anti-aliasing and/or semi-transparent items and
> some pitch black object behind it filling the whole view.
>
> (2) Remove that pitch black thing and render again with transparency
> output. View this image against a pitch black background.
>
> If the images don't look 100% the same, /something/ is wrong. If in (2)
> semi-transparent areas appear darker and anti-aliased edges seem to
> "shrink" a little bit, chances are the software handles the PNG alpha
> channel wrong.

If I understood your test correctly, blender passed it. I used the antialiasing
from this scene:
http://dl.free.fr/hj1KvmvaZ
-render with F12,
-then hit j to store in secondary buffer,
-back to main UI window, push 1 key on the non numpad keyboard to isolate the
layer without the black
object. (black object will disappear from 3D view and render)
-render again (F12)
-hit j key a few times to compare the two render buffers,
you can do so while zoomed with the mouse wheel.

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