How to render just the alpha channel
 

Hi, I've got a man riding a bike and I want to render the two elements (man and the bike) seperatly. It renders fine, but there is a problem , when I composite the two layers in AE, they over lap.
for example when I put the two rendered layers together I can see both legs of the guy and the bike behind the guy or if I place the bike layer as the first layer, then it looks like the guy is detached from the bike! So I thought If I can render the the bike (with diffuse off, just the alpha channel) and the guy (with both diffuse and the alpha channel on) in one layer, then render the bike in another layer, then I would have the guy to look like he is sitting on the bike!!! Can anyone suggest how I can do that. Or if there's a better way of doing this? Thanks

you could put a black hole shader (background shader) on the bike when you render the man, then the bike would not show in the render but it will obscure the parts of the guy where the bike is and more importantly give you the correct Alpha.

Thanks man, it worked. I did it before and obviously the bike was coming out black as I had the block hole on, but i thought it must come out as white in the alpha channel!! Thanks dude, sorted.