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Black Halo After Precomping 3D Render Passes
Hello! I think I don’t understand the way Ae renders what I’m doing.
So I have some render passes from 3ds max with antialiased edge. When I’m stacking them onto a background picture, everything looks fine. But if I precomp those render passes, a black sharp line appears around the precomp.
As I understand it, since an antialiased edge of every render pass contains semi-transparent pixels, when I combine these passes using “add” blending mode, two overlaid semi-transparent pixels make a completely opaque one and a chain of such pixels create that dark outline. But why then if I put a background picture on the bottom of the comp it doesn’t happen like this and I have a clean edge?
I’m planning a lot of 3d compositing onto photos and videos and this technical moment doesn’t allow me to move on.
So could you tell me what are the calculations behind all of this?
Thanks! -
13 Replies
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Maybe this will clarify the issue:
https://tv.adobe.com/watch/creative-cow-after-effects-tutorials/straight-vs-premultiplied/
Tudor “Ted” Jelescu
Senior VFX Artist -
Unfortunately, no matter whether I interpret passes as straight or premultiplied with black, after precomping they get that nasty fringe.
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How are you rendering the alpha from your 3d app?
Tudor “Ted” Jelescu
Senior VFX Artist -
Well, I’m rendering on a black background into 32bit *.exr format. In the output settings, I choose zip compression and RGBA. There’s no premultiply checkbox or something like that.
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My guess is that this is the reason you get the dark outline- premultiplied alpha on black.
Here’s a helpful link:
https://docs.autodesk.com/3DSMAX/15/ENU/3ds-Max-Help/index.html?url=files/GUID-E49782BE-6486-4B9D-929A-7A02535F1829.htm,topicNumber=d30e609223Tudor “Ted” Jelescu
Senior VFX Artist -
[Ben Waflik] “Well, I’m rendering on a black background into 32bit *.exr format. In the output settings, I choose zip compression and RGBA. There’s no premultiply checkbox or something like that.”
If you’re using the EXtractoR effect to extract your passes from the EXR then the premultiplied/straight interpret footage options in AE have no effect.
Instead you’ll want to use the ‘Unmult’ check box in the EXtractoR effect to interpret the footage as premultiplied, otherwise the assumption made by the effect is that the EXR has a straight alpha.
Also, if possible I’d recommend rendering to half float 16-bit EXRs as the file size is half as large and the quality is generally not much different.
Darby Edelen
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“My guess is that this is the reason you get the dark outline- premultiplied alpha on black.
Here’s a helpful link:
https://docs.autodesk.com/3DSMAX/15/ENU/3ds-Max-Help/index.html?url=files/GU...”Tudor “Ted” Jelescu, thanks, really helpfull info. But I don’t know how to render out “straight” in 3ds max, I’ll have to find it out.
“Instead you’ll want to use the ‘Unmult’ check box in the EXtractoR effect to interpret the footage as premultiplied, otherwise the assumption made by the effect is that the EXR has a straight alpha.”
Darby Edelen, As you suggested I’ve tried EXtractoR and checked “Unmult”. What it changed is that now, in the render passes precomp, I have clean edges even on a transparent background! Great! But when I switch to the main comp, I see that render passes precomp still has that black fringe.
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Try applying Effects/Channel/Remove Color Matting on the precomp and see if that works.
Tudor “Ted” Jelescu
Senior VFX Artist -
My last ditch effort would be to solo the offending pre-comp and the background, if the problem still persists turn off any effects on the pre-comp, if it still persists make sure there are no layer styles that got mysteriously applied.
If you could post screen shots of the problem layers both inside and outside the pre-comp that might help deduce the issue as well.
Darby Edelen
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Sure, I will upload all the images asap, thank you for helping me!
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