Easing Lab

Easing Lab is a small Python easing library with a Pygame curve designer for seeing, shaping, and exporting motion.

The designer compares nine starting curves and lets you reshape eight of them.

pip install easing-lab

Here is a complete animation using only the standard library and Easing Lab:

import turtle
from easing_lab import interpolate, ping_pong
dot = turtle.Turtle("circle")
dot.penup()
def animate(frame=0):
    dot.setx(interpolate(-200, 200, ping_pong(frame / 60), "sine"))
    turtle.ontimer(lambda: animate(frame + 1), 16)
animate()
turtle.done()

Easing remaps time: instead of moving at one constant speed, an object can start gently, land with weight, overshoot, bounce, or settle like a spring.

Launch the designer

Install the optional Pygame app and run it:

pip install "easing-lab[app]"
easing-lab

python -m easing_lab opens the same designer. Use easing-lab --help for window size, startup file, screenshot, and version options.

The nine designer curves

The app keeps a focused 3×3 set of starting points. The library also provides separate In, Out, and In-Out variants for the Sine, Cubic, Quint, Back, Bounce, and Elastic families.

Curve	Feel
Linear	Constant speed
Sine / Cosine	Smooth circular projection; kept exact and locked
Smoothstep	Arrives and leaves at rest
Smootherstep	Softer endpoint acceleration
Cubic	A stronger S-curve
Quintic	Very soft endpoints
Back	Anticipation and overshoot
Bounce	Impact and landing
Elastic	Spring and settle

Controls

Action	Mouse / keyboard
Select a curve	Click its card
Move a control point	Drag it in the editor
Add a point	Click Add point, then click the graph
Remove an internal point	Right-click it, press Delete, or use Remove point
Reset the selected curve	Reset preset
Import / export JSON	Buttons or Ctrl+O / Ctrl+S
Pause / reset time	Space / R
Change speed	- / +, or 1 / 2 / 3 for 0.5× / 1× / 2×
Quit	Escape or close the window

Select an editable card before importing a custom curve. The import replaces only that card's working curve.

Use the library

The math library does not import Pygame or open a window. Inputs are normalized to 0..1, and evaluate and interpolate clamp that input range by default.

Evaluate and interpolate

from easing_lab import evaluate, interpolate

progress = evaluate("ease_out_cubic", 0.35)
x = interpolate(40.0, 600.0, 0.35, "back")

Back and Elastic curves can move outside the start/end range by design. Pass clamp_input=False when you also want to evaluate time outside 0..1.

The stable family-first keys are:

• linear, smoothstep, and smootherstep
• sine_in, sine_out, and sine_in_out
• cubic_in, cubic_out, and cubic_in_out
• quint_in, quint_out, and quint_in_out
• back_in, back_out, and back_in_out
• bounce_in, bounce_out, and bounce_in_out
• elastic_in, elastic_out, and elastic_in_out

The names used by easings.net are available in Python form too: ease_in_sine, ease_out_cubic, ease_in_out_back, and the matching names for every listed family. Short names such as sine, cubic, quintic, back, bounce, and elastic select the In-Out version. EASINGS exposes the full built-in registry.

Use vectors or your own value type

import pygame
from easing_lab import interpolate

start = pygame.Vector2(80, 240)
end = pygame.Vector2(720, 240)
position = interpolate(start, end, 0.4, "elastic_out")

interpolate works with any value type that supports subtraction, multiplication by a float, and addition. Pygame's Vector2 follows that small protocol, but Pygame is not required for floats or other compatible types.

Load a curve from the designer

from easing_lab import load_easing

ease = load_easing("my_jump.json")
height = ease(0.45)

See the versioned JSON format and the complete Pygame example.

Limitations

• The visual designer needs Pygame and a desktop supported by SDL; the math library does not.
• JSON format version 1 stores named built-ins or piecewise cubic Hermite curves. It is not a general animation timeline or keyframe format.
• CI covers Windows and Linux on Python 3.10 and 3.13. macOS is not part of the current automated matrix.

Development

git clone https://github.com/deviousname/easing-lab.git
cd easing-lab
python -m venv .venv
python -m pip install -e ".[dev]"
ruff format --check .
ruff check .
pytest
python -m build

The GIF can be reproduced with python tools/render_readme_gif.py. Read CONTRIBUTING.md before opening a pull request. Exact test conditions, artifact checks, dependency boundaries, and provenance are recorded in docs/verification.md.

Credits and license

The Back, Bounce, and Elastic families descend from Robert Penner's easing equations. The curve names and visual comparisons follow the conventions collected by easings.net.

Your exported curve data is unrestricted. JSON files created or exported with Easing Lab may be used, modified, redistributed, and sold in open- or closed-source projects, including commercial games, without attribution and without including Easing Lab's license. This permission applies to the exported curve data. If you copy or redistribute Easing Lab's application or library code, the MIT License still applies to that code.

Easing Lab's source is available under the MIT License. Pygame is an optional, separately licensed dependency used by the designer and Pygame example.