Thanks,your answer.
Simply load your footage,
then go to Effect>3D Channel.
There you'll see Depth of Field, Fog and others. >Careful, the settings are sensitive - for Maya RLA files ->1 to 1 should do the trick.
But,It doesn't solve my question.I knew what you said.
That is how to use "effect" using Z depth.
I just want to composite layer using Z information.
Digital Fusion can do that.
Whatever,I answer your question.
I have two way to solve your question.
1:If you have Maya.Go the RenderGrobals menu.
Choice any format like tga or tiff except "iff".
and Check "Zchannel" box.and Go rendering!
Maya will make 2files.One has RGB and Alpha,
and Other has only Z channel.This is you want image
that is Grayscale picture.
But this file isn's made "image" folder,I'm not sure
It may be "render data">"depth"folder.
2:Import footage file that has Zchannel like "RLA' on AE5
Then go to Effect>3D Channel.
You'll see Z-buffer Extract.So you can extract only
Z-buffer.
Then rendering as tga or tiff or something.
(But I haven't tryed this action.So I'm not sure to
work well....)