Tween

Rigid motion often appears unnatural. This library provides easing and interpolation functions widely used in game development to create natural looking or stylized tweens ("inbetweens").

Example usage:

const tween = @import("tween");
const lerp = tween.interp.lerp;
const ease = tween.ease;

// ...

pos = lerp(start, end, ease.bounceOut(t));

Interpolation

The following interpolation functions are provided:

• lerp
• ilerp
• remap
• clamp01
• damp

Linear interpolation differs from the implementation in the standard library in that it returns exact results at 0 and 1.

Additionally, it accepts a larger variety of types:

• Floats
• Vectors
• Arrays
• Structures containing other supported types

ilerp is the inverse of lerp, and only accepts floats. remap uses a combination of ilerp and lerp to remap a value from one range to another.

clamp01 is what it sounds like, and is provided for convenience. You can apply it to a t value getting passed to lerp to create a clamped lerp.

damp is useful for framerate independent lerping.

If you're unfamiliar with this class of functions, I recommend viewing The Simple Yet Powerful Math We Don't Talk About by Freya Holmer.

Ease

The following easing functions are supported, most of these originated with Robert Penner:

• linear
• sine
• quad
• cubic
• quart
• quint
• exp
• circ
• back
• elastic
• bounce
• step
• smoothstep
• smootherstep

I've opted to not include GIFs demonstrating the easing styles here, as easings.net already has a great visualizer for almost all of these.

(Note that the links above are provided for convenient reference, the actual implementations may not be 100% identical or may have slightly different parameters.)

In, out, and in-out variations of each are provided, functions are exact at 0 and 1 for 32 bit floats unless otherwise noted.

Easing functions operate on the t value given to lerp. For example:

const result = lerp(a, b, ease.smootherstep(t));

If you're new to easing and not sure which to use, smootherstep is a reasonable default to slap on everything to start.

You can adapt easing functions with mix, combine, reflect, and reverse.

The provided easing functions are exact at 0 and 1 for 32 bit floats unless otherwise noted.

When designing your own easing functions, I highly recommend testing them in Desmos.

Build Configuration

If you're shipping binaries, you probably have a min spec CPU in mind. I recommend making sure muladd is enabled for your baseline so it doesn't end up getting emulated in software, more info here. On x86 this means enabling fma, on arm/aarch64 this means enabling either neon or vfp4.