Apple Motion 5.1.1 - Manual (Page 949)

Chapter 21 3D compositing 949
You can select both Receives Shadows and Shadows Only, which makes the object invisible
except for the regions where a shadow is cast upon it.
Shadows and complex 3D objects
When using shadows with objects such as particle systems, text objects, or replicators that
are set to 3D, shadows are cast from one part of the object onto another part. This is called
self-shadowing.
Modifying the object in a way that causes rasterization prevents shadows from displaying.
In some cases you may be able to find another way to perform the effect that does not require
rasterization. For example, in the following images, rather than modifying the opacity of the
particle emitter, which causes rasterization, you can modify the opacity of the particle cells and
maintain the shadows.
Emitter set to 100% Opacity Emitter set to 60% Opacity Particle cells set to 60% Opacity
67% resize factor