Keeping only reflection of object on floor to be able to replace the floor in comp?


I have an animated car I want to render on a reflective floor, but I want to be able to replace the floor in After Effects. Therefor I basically need to retain the reflections and shadows from the car on the floor, but lose the actual floor.

I’m using VRay and Maya 2018. How do I do this?


You have to create a reflection alpha matte. These aren’t standard AOVs in most renderers.

You leave the floor item as is and car as is in a new render layer. There are no lights.
You apply an vray Light material at full white to the entire car as an override in that layer.
Make the car invisible to direct rays. But still visible to reflections/secondary rays…

No other lights. This makes it so that scene is generally fully black except where the car is reflected in the floor.
Then you have a normally lit pass of the ground with the usual lighting with the car still invisible to camera.

You then use this black and white reflection alpha matte pass to key in the reflection of the car in the normally lit ground pass in the overall comp.