How do I keep the occlusion but lose the faces?

I’m attempting to comp a blender built shed into a photo of a garden as I’m trying to improve my photo realism skills, but I realized pretty early on that the image would never work without some natural occlusion. So I put a few planes around my shed to simulate the floor and fences. (See image)

I know how to achieve shadows on a Z transparent plane, but shadows are not an issue as the garden is fully in the shade, but I need some AO, the trouble is as soon as I make the planes transparent they no longer affect the AO. Is there a way to achieve AO with transparent objects or am I going to have to fake it with some soft shadow only lamps.

Any advice appreciated, Thanks


After playing with shadows to try and fake the occlusion I came up with another solution that seems to be working out okay. I UV unwrapped the planes and did an ambient occlusion bake for each one, having saved the bakes I’m applying them as procedurals affecting the negative alpha channel with a plain black non-specular material.

With a bit of tweaking to the contrast and brightness settings the results seem to be pretty good, much better than I was getting with shadow only lamps, I’ll post another image to show the results shortly.

Now I just have to finish building the shed!

A simpler way would be setting the walls and floor material to Shadows Only and Ambient to 1, you can find this in Shadow and Shader tabs of the material. Then rendering a separate AO pass and excluding it from combined allows even better control when compositing.

edit: but your baking solution should be faster to render now that I think…!
-this was for 2.5x-

Thanks for the advice Liero, I always forget about that ambient slider, however this seems to be working out for now, here’s my next test render with the occlusion applied to the alpha channel and a bit of a tweak to the wood colour.

Still a long way to go and I plan to re-bake and tweak the occlusion textures again once the scene is complete, this is just to test the principle.

Once I get further along I may try your method too and see which works out best.