3D Compositing Guide 1

“3D Compositing Guide – part1 ” by Simon Reeves
Whoops, I seem to have got a little ahead of myself there. Anyway this reflection pass, is on the plate, so instead of compositing it onto the last two passes, we can lay it straight onto the plate. Smashing.
? Reflection Matte Pass (RGB, Alpha channel not needed)

Now you can’t just slap the reflection onto the plate in this instance, because there are already reflections on the floor that would be occluded by the signs were actually there (they would block them). So I found a good way to create a matte for where the reflections will be is to make a pass where the signs materials to a constant white material, and have that as the only thing reflecting in the scene. N.B This needs to be inverted in post truthfully in the film I made the objects black, and rendered them on a white surface and a white environment – but now I realise that is way to overcomplicated as you can just invert it.
You can multiply this pass onto the plate before the main reflections to matte out those reflections you don’t want.

Another note about the reflections I’m afraid. If you look at the plate, you can see the differences between the soaked floor, and the standing water/puddles, the puddles are sharp reflections while where the water has soaked in, the reflections are blurred. So before those passes are rendered I painted a matte based on the plate – and used a camera projection to apply that texture onto the floor, to control the amount of blur for the reflection. Hopefully that made sense.
? Ambient Occlusion (RGB, Alpha channel not needed)

This pass effects the CG elements as well as the plate (although often you may want to render 2 separate AO Passes), so to layer it onto both, before this we need to layer the CG passes we have so far, over the plate (including the reflections). For this pass, we just want to keep the darker values, so to remove/hide the white, we will use ‘Multiply’.
? ID Pass (RGBA)

     

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