I have a comp that is setup like this:
http://vray.info/tutorials/basiccomping/
The difference being that I have rendered my FG and BG objects separately.
I rendered against a black background and am getting a black fringe where my FG objects are comped over the BG. Beauties look fine when comped together.
Seems to be right at the top of the stack:
Forground: (Diff*RawGI)+(Diff*Raw Light) comped over BG gives black fringe.
The solution was to unpremult exr before comping, then premult.
Yes that would solve the issue.
You can also try to avoid using Raw-elements generated from Vray and create custom Raw-elements by the following formula:
RawGI = Diffuse/GI
RayLight = Diffuse/Light
Thanks for the reply. Is there a problem with the raw passes?
Yes - this is the problem with the Raw passes, and it is better to create custom Raw passes like I explained you in the previous post.
Svetlozar bet me to it. I never use the RAW passes and create my own using the formula above. Works great in Nuke!
Would I be right in thinking that the difference between the RawReflection element and a RAWRefl=Reflection/ReflectionFilter is that the Raw element has a premultiplied result regardless of whether you unpremultiply it, where as the divided version doesn’t have any AA?
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Say you’re using a Multimatte to selectively adjust parts of the Reflection pass - should I render a non-AA version of the multimatte if I’m grading any unpremultiplied passes including the RawRefl=Refl/Refl Filter? I could use an integer ObjectID element in that case which can hold more objects.
Yep, the Raw passes have aa applied so when the passes are added back together, the borders don’t add properly.
I typically use the Multimatte straight from the render, but you can get some edge ringing depending on how drastic the changes. Others have mentioned that doing with a deep pixel approach resolves most of those issues. I have not figured out the deep pixel workflow yet between VRay and Nuke. It’s definitely on the to-do list.
I tried the deep approach today - it’s pretty neat. Render all your lovely teapots separately and bung them all together and it places one in front of the other without any user intervention - not tried with different elements yet.
So how did you go about exporting / saving data as deep exrs? I haven’t figured that part out. (Note: more like haven’t had the time to look into it too much yet.) 
Here’s a link to the 3.0 thread, I’ll add to that if any steps are missed, but better to get info in one place.
http://forums.chaosgroup.com/showthread.php?73945-Regarding-Vray-s-Deep-data-support