Short Expressions
Chapter 24, Expressions

Expressions are mathematical formulas that let you control any parameter that can be animated, such as translation, rotation, scaling, material, or texture. You can create almost any association between parameters, from simple A=B relationships to very complex ones using predefined variables, standard math functions, random number generators, and more.

An expression is a string of characters that may include object and parameter names, math operators, and tokens representing functions or constants. For example, to constrain object A's Y rotation to object B's X translation, you might set an expression on A.Transform.Rotation.Y consisting of the string B.Transform.Position.X

What attributes can expressions refer to?

Expression strings refer to sockets by name. The sockets can be either input or output sockets. Expressions perform operations based on numerical values, therefore you do not access pixels in the context of expressions. However, you can still access numerical values of image attributes such as height and width.