Matte Shadow let’s you render Ambient Occlusion and Shadows onto an invisible plane so that you can render them on a background image. See Using Matte Shadow Plane for more info on how to setup.
MATTE SHADOW RENDER SETTINGS
Enable
Turns on/off Matte Shadow options.
Use the
Shadow Opacity,
Shadow Tint, and
Affect Lighting to dial in the look of how the shadow overlays onto the background.