Time Expressions in Houdini

A comprehensive guide to working with time-based expressions and animation controls in Houdini

Basic Time Variables

Houdini provides several built-in variables for working with time:

$F - Current frame number
$T - Current time in seconds
$TSTART - Start time of the timeline
$TEND - End time of the timeline

Boolean Operations

For boolean operations (switch, activation), expressions return either 0 (false) or 1 (true).

Common frame-based conditions:

// Activate after frame 30
$F > 30

// Activate before frame 60
$F < 60

// Activate between frames 30 and 60
$F > 30 && $F < 60

Time Shift Operations

You can use max/min functions to create time shifts and clamps:

// Will start at frame x
max($F, x)

// Will stop at frame x
min($F, x)

// Clamp between two frames
min(max($F, start_frame), end_frame)

Animation Controls

Create normalized animation values (0-1) based on frame ranges:

// Normalize frames 0-30 to range 0-1
clamp(fit(@Frame, 0, 30, 0, 1), 0, 1)

Custom Animation Interpolation

Use attribute wrangle for more control over animation interpolation:

// In an Attribute Wrangle node
float t = clamp(fit(@Frame, ch("frame_start"), ch("frame_end"), 0, 1), 0, 1);
float animation_value = chramp("../CONTROLS/animation_ramp", t);

setdetailattrib(0, "animation_value", animation_value, "set");

// Reference the value in other nodes using:
detail("../CONTROLS", "animation_value", 0)

This setup allows you to:

Control animation timing with custom parameters
Use ramp functions for non-linear interpolation
Share animation values across multiple nodes