Invisible Objects that block Visible Objects

Is it possible in blender to for example have a large visible block and a small invisible block in front of the large visible block in which the invisible block blocks the camera from seeing the large block so there’s just alpha there?

Here’s a picture of what I mean:
http://cl.ly/2A1j3V2E010E0b3u0732/Blender_Idea.png

The reason I’d like this is so I can video record for example my desk, make a 3D mesh of the top, make the top invisible, and make flames come off the mesh and the flames are invisible when behind the invisible mesh so I can just render it and put the render on top of the video so the final product will look like my desk is on fire realistically. Cause the flames will look like they’re on the objects on my desk as well as behind and around them.

Can anyone help?

The only way i found was with green screen or chroma key . make the alpha cube green. I did not want to do it that way but I posted the same question with no replay.

Look up render layers and masking in the wiki

Thanks for the suggestions. I’d hate to have to use green screens but… well I’ll look up render layers and masking.

I hope it works. I’ll post if I find anything out.

How are you planning to to the fire? Is that going to be with particles?

for creating ALPHA for the “blocking-object” (how you call it.)
set in its material-options
under “options” the
Sky-checkbox.
Thats the window where the options are about Traceble, Full Oversampling, use Mist, Clay …

Then a render will use the Sky-background at this object-parts
and that is ALPHA - check in the final render-window the button to view it.
(draw image with RGB-colors and alpha-Transparency)

added a short screenshot …

Attachments


How are you planning to to the fire? Is that going to be with particles?

Yes.

for creating ALPHA for the “blocking-object” (how you call it.)
set in its material-options
under “options” the
Sky-checkbox.
Thats the window where the options are about Traceble, Full Oversampling, use Mist, Clay …

Then a render will use the Sky-background at this object-parts
and that is ALPHA - check in the final render-window the button to view it.
(draw image with RGB-colors and alpha-Transparency)

added a short screenshot …

Thanks test-dr! Works perfectly! :smiley: