Sorry for this very basic question, the thing is I wanted to use MR and now I'm kinda stuck with this very simple step.
I know I've to read about render pass and probably store in channel options, or just find a good tutorial. I tried about the tutorial - they all require to go through quite long introductions and I'm quite in a rush now - and also read the documentation but I'm still very confused about this.

But the question is: how can I export the alpha matte for a particular object in my scene?

Quick setup, I've used in really short deadlines - select your beauty pass in explorer, ctrl+D to make copy, put your object in a new partition, apply white constant to partition, apply black constant to background partition, delete all others partitions (if there are any), render. Even if you have some complex lighting setup, it will render quickly, as constant material doesn't catch shading.

Probably last option would be MR object labels stuff - colors are random (on every next scene load), you have to add pixel coverage, so on.

Thank you so much Mathaeus, I'll try that.

Thanks Mathaeus, it works really fine.
In the meantime I kept looking for a good introduction to passes and partitions and I found this: http://vimeo.com/groups/66775/videos/16118812
It seems it's meant for XSI 6.5 but I guess it still works fine for SI 2011/12. Any other good resources about this topic?

Coming from C4D and Blender at first glance working with objects ID seemed a bit complicated in SI (well, C4D seems to be the easiest of the three). But I guess once one can fully understand how passes work in SI then it should looks like a very powerful tool. Or is it just actually too complicated?

Newcomer alert: please use the URL tags - HB

I really think you shouldn't shy away from reading the User's Guide once in a while. For the most part it's wel written, highly informative (like any User's Guide should be, obviously), relatively easy to comprehend and it can even be considered a "good read".

That said good tutorials on basic non-ICE related subjects are getting sparse. Luckily old tutorials for the most part are still valid (only the ones talking about the old (pre-ICE) particle system can be safely ignored (as this "legacy" system was removed from SI recently).
Some of the older stuff on Passes can be found in this thread - VAST - XSI Learning Movies on YouTube, which still is IMHO an unsurpassed training resource (but, like a said before: ignore the particles).
This thread - Softimage XSI 4 Principles DVD on YouTube - has some videos about passes also.
And while you're at it, make sure to check out the rest of our Tutorials forum also.

Hirazi Blue,

you're absolutely right. The User's Guide looks really good and well organized. For instance, it's been really easy to find out how to export the depth pass in a few simple steps. I was struggling with camera settings, shaders, but for my needs and a quick depth render the depth pass preset worked well.
I think I just got a bit scared by the concept of partitions and how SI handle all this stuff
It seems there's a lot to dig about it.

Thanks for the links.