Long time user, first time poster.
I switch between blender and Maya for cg illustration,
and recently wanted to Composite something I’d rendered out into separate layers from Maya:-
AO, diffuse, specular, ID, etc
In the past I have used nuke for compositing, but Nuke is for people with 2 grand and I prefer blenders’ system.
However, Nuke has a very good premult/unpremult node.
Blenders node system is amazing, but I’ve had a lot of problems with premultiplication and un-premultiplication.
The Alpha Convert node wasn’t giving me the results I was expecting,
and Blenders documentation on premultiplication is a little weak.
Generally if I was going to make a colour correction to an image with an alpha channel I would follow this rule:
Un-premultiply -> colour correct -> premultiply
Otherwise you can get incorrect results and black borders around images.
See for details:- http://compositing.wordpress.com/2010/09/13/premultiply-unpremultiply/
I wondered if anyone had a similar experience?
The easiest way round this I could think of was to make my own group node for premults/unpremults,
by separating R,G,B and A channels (I like having this level of control over the channels anyway!)
These groups are saved into my default .blend file as I will probably be using them a lot when I composite something I’ve rendered in maya mental ray and theres currently no system for saving groups.
Here are some images of the setup in case anybody wanted to do the same thing: