Interactively control camera focus distance.
The camera will focus wherever your null or solid happens to sit. This expression is golden when you're doing a lot of Depth of Field work.

A simple, elegant, telecine-style color-correction tool for coloring or grading your movie.

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.

From Adobe's LiveDocs:
You can create an expression without using properties from other layers. For example, you can make a layer revolve in a perfect circle.

From Adobe's LiveDocs:

This expression maintains the objects apparent size as it moves away from camera.
It's helpful for taking a flat layout and making a "multiplane" layout from it for animation.

Apply this expression to your 'Scale' attribute and it'll automatically 'flip' the layer in 3d space when the camera goes behind it. This way, your text will never be seen from behind, for instance.

Apply this expression to the opacity attribute and it'll disappear whenever it's flipped away from the camera. You need to have an active camera in your composition for this one to work.

Often very useful to wiggle only horizontal or vertical

More experimentation with sampleImage().
from http://www.creative-workflow-hacks.com