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ease.h

A single-header C library of easing functions.

Curves

Features

• Single header, no build step. Drop ease.h into your project.
• 10 easing families, each with in, out, and in_out variants. 30 functions total.
• Float and double precision, selectable independently via EASE_FLOAT / EASE_DOUBLE.
• Works as a normal header (extern declarations) or fully inlined into one translation unit via EASE_STATIC.
• Optional EASE_NO_PREFIX to drop the ease_ prefix if you've namespaced the include some other way.
• Plain C99, compiles clean under -Wall -Wextra -Wpedantic -Werror. Also compiles as C++.
• MIT licensed.

Quick start

Copy ease.h into your project. In exactly one .c or .cpp file, define EASE_IMPLEMENTATION before including it:

#define EASE_IMPLEMENTATION
#include "ease.h"

Every other file that needs the declarations just includes it normally:

#include "ease.h"

A minimal example, animating a value over 60 frames using ease_in_out_cubic_f:

#define EASE_IMPLEMENTATION
#include "ease.h"
#include <stdio.h>

int main(void)
{
    const int total_frames = 60;

    for (int frame = 0; frame <= total_frames; frame++)
    {
        float t = (float)frame / (float)total_frames;
        float eased = ease_in_out_cubic_f(t);
        printf("frame %2d: t=%.3f  eased=%.3f\n", frame, t, eased);
    }

    return 0;
}

On Linux/glibc, link against libm:

gcc -std=c99 -o demo demo.c -lm

Function reference

Every function takes a single progress value x, typically in [0, 1], and returns the eased progress. back and elastic intentionally overshoot outside [0, 1] partway through the curve, everything else stays within [0, 1].

Each row lists the float variant; the double variant has the same name with _d instead of _f.

Function	Family	Notes
ease_in_sine_f	Sine
ease_out_sine_f	Sine
ease_in_out_sine_f	Sine
ease_in_quad_f	Quad
ease_out_quad_f	Quad
ease_in_out_quad_f	Quad
ease_in_cubic_f	Cubic
ease_out_cubic_f	Cubic
ease_in_out_cubic_f	Cubic
ease_in_quart_f	Quart
ease_out_quart_f	Quart
ease_in_out_quart_f	Quart
ease_in_quint_f	Quint
ease_out_quint_f	Quint
ease_in_out_quint_f	Quint
ease_in_expo_f	Expo
ease_out_expo_f	Expo
ease_in_out_expo_f	Expo
ease_in_circ_f	Circ
ease_out_circ_f	Circ
ease_in_out_circ_f	Circ
ease_in_back_f	Back	overshoots
ease_out_back_f	Back	overshoots
ease_in_out_back_f	Back	overshoots
ease_in_elastic_f	Elastic	overshoots
ease_out_elastic_f	Elastic	overshoots
ease_in_out_elastic_f	Elastic	overshoots
ease_in_bounce_f	Bounce
ease_out_bounce_f	Bounce
ease_in_out_bounce_f	Bounce

Configuration macros

Define these before including ease.h. If you include the header in more than one translation unit, define the same macros in each one.

Macro	Effect
EASE_IMPLEMENTATION	Includes function definitions, not just declarations. Define this in exactly one .c/.cpp file.
EASE_FLOAT	Builds only the _f (float) functions.
EASE_DOUBLE	Builds only the _d (double) functions.
EASE_STATIC	Marks every function static instead of extern. Use this to include the header with EASE_IMPLEMENTATION directly in a single .c file without multiple-definition errors. Every including translation unit gets its own private copy.
EASE_NO_PREFIX	Drops the ease_ prefix from every function name, e.g. in_out_cubic_f instead of ease_in_out_cubic_f. Watch for name collisions with other libraries if you use this.

If neither EASE_FLOAT nor EASE_DOUBLE is defined, both are built. This is the default.

Naming convention

ease_<in|out|in_out>_<family>_<f|d>(x)

• in / out / in_out is the direction of the curve.
• family is one of sine, quad, cubic, quart, quint, expo, circ, back, elastic, bounce.
• f / d is float or double precision.

With EASE_NO_PREFIX defined, the leading ease_ is dropped and the pattern becomes <in|out|in_out>_<family>_<f|d>(x).

Building and testing

ease.h has no dependencies beyond <math.h>. It compiles clean under both C and C++ with strict warnings enabled:

gcc  -std=c99   -Wall -Wextra -Wpedantic -Werror -c test.c -lm
g++  -std=c++17 -Wall -Wextra -Wpedantic -Werror -c test.cpp -lm

Every function has been checked against a reference implementation of Penner's easing equations, confirming f(0) == 0 and f(1) == 1 for all 30 functions in both precisions, plus midpoint values matched to several decimal places.

Credits

The easing equations implemented here are standard, widely-published formulas originating from Robert Penner's easing equations, catalogued today at easings.net, created by Andrey Sitnik and Ivan Solovev.

License

MIT. See LICENSE.