How to render multimatte behind alpha masks (or transparent objects)

[theedge]

Hi,
I'm working on a large interior where the ceilings are all made of perforated metal panels. Behind those are lighting fixtures. I have setup a multimatte pass thinking it would allow me to select the visible parts of the lights behind the perforations so I can apply some nice glow effects in photoshop. But what a deception when I found that multimatte actually renders the perforated panels as a plain object.

here's a simplified image of the situation :



I also tried putting the map in the refractino slot instead but it doesn't work neither. My guess is that logically, matte passes are performed according to the geometry. And the plane being... a plane, I cannot see the holes.
If I could model real panels with holes inside them my problem would be solved... but, of course, that would be way too heavy as there are lots of these everywhere, also I would loose the flexibility to change the scale of the holes anytime.
is there any way to achieve that ?

thanks.

[pjkw]

I don't think you'll manage to get multimatte to do this for you, however, you could make separate renders that would allow you to get selections for the lights. For example;

Turn off reflections/refractions, turn off GI, turn off lights.
Make all diffuse colors black. Make a vraylightmtl with all red color (255,0,0), and apply it to your objects you want selection for. You could ofcourse get selections for 3 different sets of objects by using both red, blue and green materials in the same render.

[theedge]

Well the reason I wanted a direct output of the selection as a pass is because that project is changing all the time (3/4 times a day !) so I wanted to setup a sort of quick pipeline to do the post.
But maybe I can speed up the process by using a scene state with the particular settings you describe.
Thanks for your help pjkw

[dans]

You could use psd-manager. It uses the transparency information from the G-Buffer to produce mattes.

Here is a video that shows how to do it (I have used a VRayMtl with a pattern applied as the opacity map.).
http://www.screencast.com/users/gizm...0-75ec87b5beb8

And here is the PSD file from the video:
http://rapidshare.com/files/10928343...pping.psd.html

[gfa2]

You can also set the Alpha to -1 on the ceiling material only and in the alpha channel you will have the mask that you want. I recently did this with some people planes that I wanted to be able to adjust in post.

[theedge]

Thanks a lot guys !
PSD manager looks very powerful and actually seems to be able to do much more than I need
I think for the moment I'll stick with the trick of putting alpha to -1 and extract the channel through the alpha mask

[joconnell]

Try the wirecolor element - I think that'll do opacity too.