Negative alpha out of renderlayers #32598
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%%%Due to AA filters renderlayers can return negative Alpha and RGB pixels on the borders of objects.
This negative pixels are the root of all kinds of problems that get solved by clamping them in the compositor. But first a question, is there any circumstance where we would need/like those negative values out of the renderlayer? What is a negative alpha? more transparent than transparent?
See the white border around the sphere (press F12)
https://dl.dropbox.com/u/255376/Blender-Compositing/Strand-Render-error.blend
And this is how it gets fixed, a ton of nodes i you ask me just to clamp the values
https://dl.dropbox.com/u/255376/Blender-Compositing/Strand-Render-Fix-proper.blend
There are a few incorrect solutions to this problem that I and many people have used, the first is recurring to FSA. FSA does the filtering after the composition and that seems to do the trick, however if you save your result to EXR files for later re-composition this EXR files will still have the negative values baked in and will result in the same trouble again, forcing us to clamp
The worst solution is to use dilate/erode nodes to choke the alpha, and yet people do this because it's hard to realize that the renderlayer is outputting this garbage pixels
Back to the begging, is there a reason why we would not want to clamp this nastiness right out of the renderer?%%%
Changed status to: 'Open'
%%%Committed a clamping fix now, note that in this .blend file you still need to use full sample to fix the white border.%%%
Changed status from 'Open' to: 'Resolved'
%%%Thank you very much!%%%
%%%@brecht:
Thank you! This gave me tons of headaches.
Regarding the problem with the strand renderer, in this example where emitter and hair are in different render layers it's not a bug, since the rendered strands' alpha that is both correlated and uncorrelated to the emitter's alpha (some strands are in front of the emitter and some of them behind), and consequently the alpha-over operation fails for correlated portion of the alpha channel.
As you pointed out, FSA fixes that, but there's another way to get a correct compositing of strands: adding both alphas, then setting the resulting alpha to the predivided alpha-over result. That's what was done in the second example file linked, but in that case the alphas had to be clamped to give proper results. Now with your fix those ramps can be taken away, making simpler composites and more importantly, getting more predictable results when dealing with alphas.
Thanks for that! :-)%%%