Easing (Data Type)

A String naming an interpolation curve: how a value travels from its start to its end over time, instead of jumping straight there.

Names ignore case and separators, so ease_in_out, easeInOut and EASE-IN-OUT are the same curve.

Special curves#

Value	Description
linear	Constant rate. The default everywhere an easing is optional.
step	No interpolation. Holds the starting value and snaps to the end value at the very end. Use this for on/off properties like visible.
catmullrom	Smooth spline through the surrounding keyframes rather than a straight line between two of them. Only meaningful between keyframes; behaves as linear for a fade. Alias: smooth.
smoothstep	Classic 3t² − 2t³ S-curve. Gentle ease in and out.
smootherstep	Perlin’s 6t⁵ − 15t⁴ + 10t³. Like smoothstep but flatter at both ends.

Standard curves#

Each family comes in three forms — ease_in_* starts slow, ease_out_* ends slow, ease_in_out_* does both. Ordered from gentlest to sharpest:

Family	Values	Feel
Sine	ease_in_sine, ease_out_sine, ease_in_out_sine	Barely there. Good for subtle idle motion.
Quadratic	ease_in_quad, ease_out_quad, ease_in_out_quad	The safe default for poses.
Cubic	ease_in_cubic, ease_out_cubic, ease_in_out_cubic	Noticeably snappier.
Quartic	ease_in_quart, ease_out_quart, ease_in_out_quart	Sharp.
Quintic	ease_in_quint, ease_out_quint, ease_in_out_quint	Very sharp.
Exponential	ease_in_expo, ease_out_expo, ease_in_out_expo	Near-instant at one end.
Circular	ease_in_circ, ease_out_circ, ease_in_out_circ	Flat then abrupt, like a quarter circle.
Back	ease_in_back, ease_out_back, ease_in_out_back	Overshoots past the target and settles back. Anticipation.
Elastic	ease_in_elastic, ease_out_elastic, ease_in_out_elastic	Springs past the target and oscillates.
Bounce	ease_in_bounce, ease_out_bounce, ease_in_out_bounce	Hits the target and bounces off it.

back and elastic deliberately leave the 0–1 range mid-curve, so the animated value overshoots its keyframe. That is the point of them, but it means a visible or hidden transformation should not use them.

Aliases#

Alias	Resolves to
ease_in	ease_in_quad
ease_out	ease_out_quad
ease_in_out	ease_in_out_quad
smooth	catmullrom
hold	step
constant	step
none	linear

Example#

{
  "model_part": "right_arm",
  "type": "pitch",
  "value": -1.5708,
  "override_animation": true,
  "easing": "ease_out_back",
  "duration": 6
}

The arm swings up over 6 ticks, overshoots slightly, and settles.