Best practice for rendering out render elements only

[MANUEL_MOUSIOL]

Sometimes I need additional render elements like mattes.
so I don't need the final image or any other computation. In CPU mode I used to turn off gi, lights and shadows etc to speed things up. (I think I had a problem with Antialiasing then though)
I think in GPU mode that's not possible anymore anyways..
What would be the best way to quickly get render elements rendered out?
Thanks for any input!

[svetlozar.draganov]

so I don't need the final image or any other computation. In CPU mode I used to turn off gi, lights and shadows etc to speed things up. (I think I had a problem with Antialiasing then though)

It's expected to have AA issues with such drastic changes, V-Ray samples the scene differently and this will affect also multimattes.
You could fix that by adjusting Image Sampler: Min Subdivs value, or by loading already completed render output in ExtraTex-render element screen-space.

It not very clear whether you would like to extract masks from CPU or GPU render engine! Does it matter actually where the masks comes from?
Even if it's a GPU render, does something prevents you from using the described technique to generate the desired masks via CPU rendering?

[MANUEL_MOUSIOL]

No, it I guess it doesn't matter what engine I use, as long as I get my masks as fast as possible after the first rendering. It would be nice if I didn't have switch engines though..
i didn't get the extra Tex method though. How does it work exactly?

[M.Max]

in addition to what's suggested above I will use this script to quickly create mattes if I needed them later .
http://tepavicharov.com/?p=25

[MANUEL_MOUSIOL]

Thanks! This looks as awesome as it is fast and simple.