Rig an Expression with Controls

Turning a working expression into something an animator can tune — sliders, a colour, a checkbox — without them opening the code.

The task

1. Decide which values in the expression should be tunable.
2. Create a control for each one, on a layer you choose.
3. Rewrite the expression to read the controls instead of the literals.
4. Keep the two in sync from then on.

How it goes in plain After Effects

1. Pick a host layer. Usually a null: Layer → New → Null Object.

2. Add one control per value: Effect → Expression Controls → Slider Control (or Angle, Colour, Checkbox, Point, 3D Point, Layer, Dropdown Menu).

3. Rename each one in Effect Controls — select it, press Return, type the name.

4. Point the expression at each control, either by dragging AE's pickwhip onto the control's value or by typing the reference:

   thisComp.layer("Ctrl").effect("Radius")("Slider")
   

5. Repeat for every value. Each control you add is its own undo step, and each reference is typed or picked separately.

What changes with eXLib

The values become variables, and the controls get created for you.

• Detect reads the expression and proposes which literals are worth a variable, each with its reason. You accept the ones you want — a row at a time, or Tokenize all.
• Each variable is wired to a target: a New control, an existing property via Link property…, or Ask on apply.
• The control's kind is chosen on the control's own row, and the menu marks the kind the variable actually needs. A kind that contradicts the need still applies — it warns, it doesn't block.
• The destination layer is yours to pick. The picker is titled Choose the destination layer, defaults to the layer you're applying to, and offers Control null (new layer) if you'd rather the controls landed on a fresh null.
• Apply creates every control, substitutes each one's real, collision-safe name into the code, and sets the expression — as one undo group.

Or deliver them as one pseudo effect instead. The delivery selector at the top of the Variables panel switches from Native controls to Pseudo effect: the same controls arrive as parameters of a single custom effect rather than as a stack of separate ones. Pseudo Effect Authoring is where that rig is arranged.

Step by step

Step	In plain After Effects	With eXLib
1. Decide what's tunable	Read the code and pick the numbers out by eye	Detect proposes them, each with its reason — accept one, or Tokenize all
2. Name them	Not a step — the literal has no name	Rename a variable inline; every {{name}} in the code is rewritten with it, as one undo
3. Choose where controls live	Create/choose a layer, then add effects to it manually	Pick it once in Choose the destination layer — the target layer by default, any other layer, or Control null (new layer)
4. Create the controls	Effect → Expression Controls → …, once per control	Nothing to do — Apply creates them
5. Name the controls	Select each in Effect Controls, Return, type	Nothing to do — the variable's name is the control's name
6. Pick each control's type	Choose the right Expression Control from the menu yourself	The kind menu marks the kind the variable needs; every kind stays selectable, a contradicting one warns
7. Wire the code to the controls	Pickwhip or type one reference per control	Nothing to do — Apply substitutes each control's real name into the code
8. Commit	Several separate steps, several undo entries	One Apply, one undo group
9. Avoid a 6-effect stack	Not addressed — the controls are separate effects	Switch delivery to Pseudo effect: one custom effect carrying every parameter
10. Change the rig later	AE lets you rename only the effect instance; its internal controls are not editable there	Pull the rig into Pseudo Effect Authoring, edit it, then place it back — keyframes and expressions migrate

Related

• Creating Variables
• Wiring the Bay
• Delivering Controls
• Pseudo Effect Authoring
• Pull, Edit, Place Back