C4D Octane User Documentation

The Shadow Catcher material captures shadows on a plane object that can be composited into a background, as shown in the following image. The shadow catching object is visible only in areas that are receive shadows from objects in the scene. Essentially, the Shadow Catcher creates a mask for the shadow (joined with other scene objects) in the alpha channel. The Shadow Catcher is an option in both the Diffuse Material and the Universal Material, found in the Common tab of each material. 

Shadow Catcher Parameters

The Shadow Catcher node material is available via the Node Editor only, from the Materials section within the Node Editor. The Shadow Catcher is not available in the main Materials menu found in Live Viewer.

Enabled — The material is transparent unless there is some direct shadow cast onto the material, which makes it less transparent depending on the shadow strength.

Opacity — Controls the transparency of the shadows.

Custom AOV — Writes a mask to the specified custom AOV.

Custom AOV Channel — Determines whether the custom AOV is written to a specific color channel (R, G, or B) or to all the color channels.

Diffuse or Universal materials can act as shadow catchers, enabled by activating the Shadow Catcher option in the Common tab of the material. The material should be applied ONLY to surfaces intended to receive shadows. 

In the Render Kernel window, activate Alpha Channel and disable Keep Environment as shown in the illustration that follows. The resulting output is used in a compositing application to merge the object and the shadows into the final composition.