Expression

Expression is a Drivers effect.

What it does

Type an expression - time * 2, sin(time) * 50, [comp_width / 2, comp_height / 2], [1, 0.5, 0] - and the box hands out its answer on a wire, so a parameter can follow a line of arithmetic instead of its keyframes. It is the same language a property’s own expression is written in, with the same names in scope: time, the comp’s size and frame rate, layer("…") and comp("…").

The box always has four sockets - Value, Colour, Point x and Point y - and what the expression returns decides which of them carry anything on a given frame. A number fills Value. A pair, [x, y], fills Point x and Point y. Three or four numbers, [r, g, b] or [r, g, b, a], fill Colour. The other sockets carry nothing, and a parameter wired to one of them reads its own keyframes for that frame.

An expression that cannot run - a typing mistake, a name that is not in scope, a division by nought, a result that is none of the three shapes - carries nothing at all, so whatever it drives keeps its keyframes rather than jumping to a number that only looks like an answer. The editor shows the sentence about why.

The answer depends on the text and on the frame - the comp, the layer and the time - and on nothing else, so the preview and the export agree exactly.

Parameters

Parameter	Control	Range	Default	Unit
Edit expression…	Button	-	-	-

What each control does

• Edit expression…: opens the editor for the text the box runs. The text itself is not a parameter - it has no keyframes and nothing to wire into - which is why the box has one button and no sliders.

Related

• Math - the arithmetic you can see, for one operation on two numbers.
• Split - a colour taken apart into four numbers, the same four-socket shape.
• Expressions - the language, and the names it knows.
• The node graph - where a driver is added and wired.
• Drivers
• Effects