Shadows to Match Backplate Geometry

[dj_buckley]

I doing a photomontage at the minute, I need to make sure that my CGI shadows fall correctly on the existing building in the photograph.

I've modelled the existing building in 3D so that it receives the correct(ish) shadows.

Is there any way to have this geometry hidden in the render, but the shadows cast onto it visible in the shadow pass for overlaying in post?

Or is there a way to achieve what I'm trying to do

[TomG]

Should be able to just paint the object with the shadowcatcher (which works for more than just planes :) ). You can either have the original photo as the background in the render (with the same image in the Shadowcatcher as the background), or you can use the For Compositing mode in the Shadowcatcher to composite in post.

[dj_buckley]

Ok so let me get this correct, I don't want any backplates visible in the render, I want a black alpha, I do want the shadows from that geometry to overlay in post, but I don't want that geometry visible in the render.

If you're saying that's possible with the shadowcatcher, how to I set it up?

I've attached a screenshot showing the backplate on the left and the 3d file on the right.  The geometry in the 3d file is just to give me some shadows to overlay in post, the reason I don't want it visible in the render is that it overlaps the CG building slightly and doesn't 100% match the backplate.  Plus the backplate needs a lot of photoshop work to remove scaffolding before I can overlay the shadows.

Normally I'd just paint them in roughly but the shadows are being cast through perforated screens with a really intricate pattern

[romullus]

If i understood ithe task correctly, you should be able to achieve this with rayswitch material - simple Corona material to global illum slot, but leave other slots empty.