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Imagine a scene with a camera facing a background plane, with suzanne in between partially obscuring some of the background plane. I'd like to do a 'project from view' from the perspective of the camera and see a suzanne shaped hole in whatever image was projected onto the background. Both meshes are a single object.

As I have my scene set up at the moment, the image being projected hits both the monkey and background.

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If I understand correctly, what you are looking for is to use an object as a mask. There are many ways to do it. Here are some:

In cycles, you can use holdout as a material on the monkey to get you what you want.

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Rendered image

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Also if you use a material index on the monkey's material you can substract the object from the background using the compositor.

The plane and the monkey have different materials.

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On the monkey's material change the pass index to 1.

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Enable material index on your render layers.

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In the compositor use the IndexMA as alpha for your image.

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To get this:

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In Blender internal you can set the material of the monkey to be used as mask.

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  • $\begingroup$ Thankyou for your answer. What I was hoping for is some combination of material nodes, possibly involving the light path or texture coordinate node. Whilst your method would work for this simple example, in my actual scene another camera is acting as a projector of this image with the UV project modifier on the plane/monkey object. Making masks/holdouts/clever compositing tricks unusable. $\endgroup$
    – Joe
    Commented Dec 8, 2014 at 22:32
  • $\begingroup$ @Joe could you upload your file? I just made a test with the UV project modifier and one object (plane and monkey joined in a single object, the monkey with holdout material and an empty as UV projector) and it works just fine. i.sstatic.net/sdiqp.png $\endgroup$
    – user1853
    Commented Dec 8, 2014 at 23:50
  • $\begingroup$ Perhaps I'm not explaining too well. Check out the .blend in my question. Thanks. $\endgroup$
    – Joe
    Commented Dec 9, 2014 at 2:05

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