The Shadow CatcherThe Shadow Catcher can be used to create shadows cast by objects onto the surrounding background imagery. The shadows cast are not limited to simply a ground plane but can be cast onto other surfaces of varying shapes. option can create shadows cast by objects onto the surrounding geometry. You can cast shadows on the ground plane as well as other surfaces of varying shapes. This feature is enabled by activating the Shadow Catcher option on either the DiffuseAmount of diffusion, or the reflection of light photons at different angles from an uneven or granular surface. Used for dull, non-reflecting materials or mesh emitters. or Universal materials applied to the shadow-catching surfaces (figure 1).
Figure 1: A Model is integrated into an image using the Shadow Catching material
In the Render Kernel window, activate Alpha ChannelA greyscale image used to determine which areas of a texture map are opaque and which areas are transparent. and disable Keep Environment. When the image renders, the shadows appear over the transparent parts of the surface. This image can work in a compositing package to merge the object and the shadows into the composition.
Figure 3: Activating the Alpha and deselecting the Keep Environment checkboxes in the Render Kernel settings