[uarrgh]

very useful could be also multiple different alpha channels for individual objects in a scene achieved during only one render pass.
the main image should rendered in this pass as well.

as I understood it is only possible when you toggle "hidden" for example for the background and render only the alpha channel, but then you have to launch the render over and over again if you want to get the channels for other objects too.

enlighten me if i am on a wrong path ....

[Mihai]

Maybe you're confusing the matte material (available through the material channel) with rendering alphas? If you activate the alpha channel, it renders the alpha of everything in the scene, not per object. You could also use the object ID channel if you want to separate objects in the scene, you get a file where every object in the scene has a different solid color allowing for easier selection in your image editing application.

[uarrgh]

Hola mihai.
I was thinking about this scenario:
Background stage, two glasses overlapping each other
When I am using matte I would get a solid color around the glass(es), but not the transparency information within the glass. Alpha would do this, but I would have to toggle objects invisible and render Alpha's separately.

Wouldn't it be cool to get alpha channels in different files per object in one shot?

Maybe you're confusing the matte material (available through the material channel) with rendering alphas? If you activate the alpha channel, it renders the alpha of everything in the scene, not per object. You could also use the object ID channel if you want to separate objects in the scene, you get a file where every object in the scene has a different solid color allowing for easier selection in your image editing application.