Here are two presets knocking out black OR white from a layer based on the luminance to set it as transparent/alpha.

In this podcast tutorial, Creative Cow leader Aharon Rabinowitz adds to the concepts he covered in his tutorial Straight Vs. Premultiplied.

This example expression instructs a layer to be at the same position as the next higher layer in the Timeline panel, but delayed by a specified amount of time (in this case, 0.5 seconds). You can set similar expressions for the other geometric properties.

Cast 3D shadows onto live action footage
Use shadow diffusion and special material options
Tutorials for Visual Effects & Motion Graphics
Free After Effects Tutorials & More Hosted By: Andrew Kramer

The Difference Matte effect creates a transparency a mask or a matte by comparing the pixels in a source layer with the pixel in a defined difference layer, and then keying out (eliminating) pixels in the source layer that match both the position and color in the difference layer.

Step through time with this innovative look at the evolution of energy. Use 3D animation, layering, colorization and more to create this incredibly unique opener.
length: 36:30