Yusuf's Thoughts

Expressions and Parameters

3 min read

Every parameter in TouchDesigner has four modes, switchable by clicking the small mode indicator to the left of the parameter field:

ModeColourDescription
ConstantGreyA plain, unchanging value you type in.
ExpressionBlueA Python expression evaluated every cook.
ExportGreenA CHOP export drives this value externally. Think green for CHOPs.
BindPurpleTwo-way sync with another parameter.

Right-click any parameter → Set to Expression (or press = in the field) to enter expression mode.

Time References

These are the most commonly used time variables in expressions:

ExpressionReturns
absTime.secondsSeconds elapsed since the application started
absTime.frameAbsolute frame count since the application started
me.time.secondsSeconds within the local component’s timeline
me.time.frameFrame within the local timeline
me.time.rateThe cook rate (FPS) of the local timeline
iop.Geometry1.par.txThe tx parameter on a sibling node Geometry1

Example: Make a node’s X-translate oscillate at 0.5 Hz:

math.sin(absTime.seconds * math.pi) * 5

Referencing Other Operators

# Get a CHOP channel value
op('lfo1')['chan1']
 
# Get a CHOP channel by index
op('noise1')[0]
 
# Get a DAT cell
op('table1')[1, 2]        # row 1, column 2
op('table1')['Name', 1]  # row with label 'Name', column 1
 
# Get another node's parameter value
op('geo1').par.tx.val
 
# Use 'me' to reference siblings relative to the current node
op(me.path + '/../sibling')[0]

The me Object

me always refers to the operator the script is running inside. It’s more reliable than hardcoded paths because it works wherever you copy the node.

me.parent()          # The parent COMP
me.parent().par.w    # A parameter of the parent
me.inputs[0]         # First connected input
me.outputs           # List of all connected outputs
me.digits            # The trailing number in a node's name (e.g. 3 in 'lfo3')
me.name              # Node name as a string
me.path              # Full path e.g. /project1/base1/lfo1

The parent() Shortcut

Instead of writing op(me.path + '/../'), use:

parent()         # One level up
parent(2)        # Two levels up
parent().par.Inputtex  # A custom parameter on the parent

Useful Expression Tricks

Map a range (e.g. LFO -1→1 to 0→1):

(op('lfo1')['chan1'] + 1) / 2

Conditional expression:

1 if op('button1')['chan1'] > 0.5 else 0

Smoothstep (using numpy, available in TD):

x = op('lfo1')['chan1']
x * x * (3 - 2 * x)

Common Gotchas

  • Green background on a parameter field means expression mode is active — it’s not an error.
  • Division by zero in an expression silently evaluates to 0 rather than crashing.
  • absTime.seconds keeps counting even when a component’s local timeline is paused. It only stops if you pause the root timeline or hit the power button. Use me.time.seconds for timeline-relative time.
  • Using op('path') with an absolute path is fragile. Prefer relative references or me / parent().

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