render passes and premultiply

[dvmorris]

i'm using mentalray to render some passes, and I can't find a premultiply option in the render dialog. i always used it in maya / mentalray, but i can't find it here, and all my alpha channels are coming out matted on white.

 

any ideas? i'm rendering a bunch of trees, and i need to render them all in separate passes. also, a random question that doesn't belong here probably, is there a script for photoshop to composite tifs with their alpha channels really quickly. I always have to duplicate the alpha channel from one file to the final comp, and then move the layer, then control click the alpha channel and make it a mask. i know that's not the right way to do it, but i've never learned any other way.

 

thanks for the help,

dave

[DavidR]

For mental ray questions, try mental images' email-based forum -you'll get answers from film's top TDs. For Photoshop masks on same-sized images, control-click the alpha channel and shift-drag it into your comp using upper left tool (whatever it's called) then rename it in your comp. This is a one-at-a-time operation that I have to do for colour correction, and I really find it boring. If anyone else has a better way, I'd be very happy. I tried rendering as .rpf and colour correcting in Combustion, but I'm not very used to it, and the files are HUGE -more experimentation is need there.

[dvmorris]

awesome, i will try that. thanks for your help.

[William Alexander]

Did you check out premultiplied alpha in the reference guide?

 

It would appear all images are already pre-multiplied... though I would have to say not absolutely sure about that out of mental ray....

 

Targa Files give you the option to pre-mult and to split the alpha to a seperate image also. Actually kind of prefer .tga for comp over tiff. Final out put, more often than not out of comp, tiff is better