How to render shadows for compositing CG vehicles onto photographic backplates.
A conundrum when compositing CG elements on backdrops is the question of shadows, since these are the link between the generated content and the real photograph, and therefore make or break the integration. In order to be perceived as natural the shadows should be cast by the subject, because approximating the shape by painting it manually seldom looks convincing, but since there’s no “set” around when rendering a little help is required in order to bring them into the compositor.
The solution is to use a “shadow catcher”. Apart from sounding new-agey, this is just what you would expect, a basic mesh whose sole purpose it is to capture a correct shadow in a way that will make it easy to sandwich between the backplate and the CG render. Fortunately for automotive compositing the shadows tend to be only on the ground, thereby simplifying the workflow since a simple flat plane will do.
Given the following scene:
Rendering a white ground plane with the car exerting influence but invisible to camera rays with an HDRI shot at the same time as the backplate results in a usable shadow when layered in multiply mode:
And the result composited in Photoshop looks like this: