2D and 3D MasksHere is a review of the principal masks, each one has it's own options.
The demonstrations of the 3D masks is made by masking both a 3D Transform and a diffuse component of the sphere Surface. As you may see on the Placement Mask image, a Perspective has been used.
Color Mask (and chroma key)
Depth Mask (using depth buffer of 3D layers)
Age Mask over a 3D transform with a rotation in a particle system
Layer Mask (use Mesh UV)
Random Mask (per vertex)
UV mask ( Horizontal / Vertical / Angles / Radius )
Velocity Mask (only for particles)