Premultiplied alpha interpreting weirdness

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  06 June 2010
Question Premultiplied alpha interpreting weirdness

Hey guys, I'm having a bit of trouble with interpreting alpha channels of renders coming out of Maya and 3ds Max. It seems that no matter how I render or interpret the alpha channel I'm always left with some kind of border around the render, whether it's white or black or more or less there's always something there. I've rendered out targas and tiffs in straight and premultiplied from my 3d apps (I presume they are premultiplied with black?) and I've imported the footage and interpreted each in all the available options: straight, premult with black and premult with white.

Here's what I'm getting in AE:
Straight Render -> Straight interpret in AE:

Straight Render -> Premultiplied with white in AE:

Straight Render -> Premultiplied Black in AE:

Premultiplied Render -> Straight in AE: (this is probably the best option for this particular case, but it varies from render to render depending on the background, and even here there is a slight grey border.)

Premultiplied Render -> Premultiplied White in AE:

Premultiplied Render -> Premultiplied Black in AE:

So I hope you can see those ok, and you'll see I get a border in each possible combination. Premultiplying with a grey of some sorts doesn't help matters much and even if I use a remove matte colour effect it still shows on most renders. I've also tried to use a separate alpha and use it as a track matter but that didn't seem to work either =\ What am I missing here? I think this problem may have been there before and maybe I just didn't notice it. I notice the border is still there with older renders and it does this for renders coming out of max and maya (Vray and mental ray). I read somewhere that some compositing apps don't support premultiplied alphas correctly and that AE does the maths for them wrong or something, is that possible? Does anybody have any advice on how to do alphas right for AE?
  06 June 2010
*bump* any compositing gurus wanna offer some advice?
Steven Olver - 3D and motion graphics artist in the UK
  07 July 2010
Hi Steven,
in my experience no matter how you render/ interpret the footage it will show you the halo effect around the matte. You should always use straight alpha since that is the cleanest option so far.

I've been getting best results when i import my passes as separate elements and also render or get a main beauty pass from the 3D animator that includes all of the elements combined as one.

The main beauty pass, i always put on the bottom to patch all of the passes above it so that it fills all of the gaps created by the alpha.

Hope this helps.
"Don't live that negative way, so make way for a positive day!!"
Bob Marley
  07 July 2010
Try dropping the "remove color matting" effect on it.

Explanations of how that works: 597-7b90.html

video tut by Aharon Rabinowitz
If it's not fun, what's the point?
  07 July 2010
hey guys thanks for the replies i've actually managed to sort out the problem now, the solution was to always render on a black background (by the just plugging a pure black blinn into the environment slot in the mia_physical_sky node) and then interpret as premult with black. there was definitely no halo with that one, i searched high and low for any signs of it but it's been perfect in every render so far! ah the things they don't teach you in college xD
Steven Olver - 3D and motion graphics artist in the UK
  07 July 2010
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