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If we compare the original GI rendering to the composited image, they look more or less identical. Apart from one thing: The interior of the car looks flat in the composite. This is due to a shortcoming of the ambient occlusion shader as it doesn't support transparency. We would have to tradeoff our rendering time to transparency if it was supported anyway :) One workaround for this situation is by rendering a separate occlusion pass for the interior and compositing it over with a mask image for the windows

For creating the mask image we apply a Matte/shadow Material to the glass portions of the car and render it to obtain a mask from the alpha channel. You can do this by rendering the image to a TGA file and checking theĀ  Alpha Split and Pre-Multiplied Aplha from the Targa file settings. The mask will render in 52 seconds

We'll use this mask as the layer mask for the ambient occlusion pass. With the Ambient Occlusion layer selected choose Add Later Mask>Reval All from the Layer pulldown menu. A white layer mask will be added next to the layer. Next we'll have to copy the mask image we rendered and paste it to this layer mask. This can be done via the Channels palette. We enable the transparency channel of the ambient occlusion layer(pressing the eye icon) and paste the mask image into it. We now created a mask that will enable us to see through the interior of the car.

As the last step, I will render out an occlusion pass just for the interior. Same method as described before, but this time I will be applying the Ambient Occlusion shader just to the interior parts of the car and the Matte/Shadow to the remaining objects to help me with the compositing. The pass will render in less than a minute

I then apply this layer with "Darken" blend mode which can vary depending on the image and desired look



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