Applying a Mask to Particles created in Adobe After Effects CS6
When using particles generator in Adobe After Effects, such as, but not limited to, Particle Playground, it is not possible to apply a Mask right away. In order to mask particles it is necessary to Pre-Compose the Layer and Move all the Attributes in the new Composition. Only at that point the Particles can be masked. Tutorial by Stefano Virgilli, Adobe Certified Instructor, Adobe Certified Expert.