Comping Renderman Passes

[user_deleted]

Hey all. I am currently working on a project compositing passes of a cg character that were rendered out of Maya utilizing Renderman for Maya. I have not had much experience compositing renderman passes, so I have consulted a couple of tutorials on this. I am experiencing a slight black outline around the character, now I know everyone is immediately going to jump on premultiplicaion errors. But let me finish .

The passes from RfM (diffuse, specularity, shadow, etc) do not contain thier own individual alpha channels. When layering them I use the alpha from the default RGBA beauty render that RfM produces with the passes. I have chatted with Pixar support via thier private forum, and they are convinced that it's a premultiplication error. They tell me "although the AOVs do not contain, or exhibit an alpha channel, they have already been multiplied by alpha, so any function that multiplies them again will result in that outline effect."

Any ideas on how to mDiv and mMult an image that has been premultiplied but doesnt actually contain an embedded alpha channel? I have tried it doesn't work.

Or if you have experience compositing Renderman passes, please clue me in to your technique. If you would like a small image of my shake tree let me know. I have been discussing this with Pixar for a couple of weeks now, and havent been able to figure this out.

Sorry this is such a long post. Thanks so much for your help everyone.

[user_deleted]

so you need to put the alpha of your beauty pass in the other passes for mdiv ?.. switchmatte or copy "a"
I am not aware of renderman specific multipass tuts, but the same apply with mentalray output buffers/passes http://www.highend3d.com/shake/tutorials/general/8.html
Actually when rendering passes using mr, each one has its own alpha though.... Using adds and mults should work anyway.

[user_deleted]

QUOTE(brandonshane @ 03/24/06, 03:17 AM) [snapback]232136[/snapback]
Any ideas on how to mDiv and mMult an image that has been premultiplied but doesnt actually contain an embedded alpha channel? I have tried it doesn't work.

Absolutely correct. These passes come without alpha. The way I work normally is to make a second branch on the beauty (that's the one with the alpha) and reorder it "aaaa". This is your "global divisor".
Now divide each and every pass by this "global divisor" using an IDiv.

Now you have all your passes unpremultiplied (still without alpha, but that doesn't matter for now) and can start adding, multing, colour correcting them. When you're happy with what you have, just switchmatte (and premult) the alpha of the beauty pass in and comp it.

FR

[user_deleted]

Thanks, that helped a great deal. Appreciate your help. I am quite new to Renderman and compositing it's passes. Learning so much so fast.