Tween

Tween is a small library to perform tweening in Go. It has a minimal interface, and it comes with several easing functions.

Quick start

Tween usage

// tween from 0 to 1 in 3 seconds
tw := tween.NewTween(0, 1, 3, ease.Linear)

// advance by 1.5 seconds
tw.Update(1.5)

// get tween value at 1.5 seconds using Value()
fmt.Println(tw.Value()) // 0.5

Sequence usage

// merge multiple tweens into a sequence
sequence := tween.NewSequence(
	tween.NewTween(0, 100, 3, ease.InCubic),
	tween.NewTween(100, 40, 2, ease.OutCubic),
	tween.NewTween(4, 100, 20, ease.InOutBounce),
)

// advance by 7.5 seconds
sequence.Update(7.5)

// get sequence value at 7.5 seconds using Value()
fmt.Println(sequence.Value()) // 5.3125

See examples folder for more examples.

Easing functions

Easing functions are functions that express how slow/fast the interpolation happens in tween.

The easing functions can be found in the ease package.

They can be divided into several families:

• linear is the simplest easing function, straight from one value to the other.
• quad, cubic, quart, quint, expo, sine and circle are all "smooth" curves that will make transitions look natural.
• The back family starts by moving the interpolation slightly "backwards" before moving it forward.
• The bounce family simulates the motion of an object bouncing.
• The elastic family simulates inertia in the easing, like an elastic gum.

Each family (except linear) has 4 variants:

• In starts slow, and accelerates at the end
• Out starts fast, and decelerates at the end
• InOut starts and ends slow, but it's fast in the middle
• OutIn starts and ends fast, but it's slow in the middle

family	in	out	inOut	outIn
Linear	Linear	Linear	Linear	Linear
Quad	InQuad	OutQuad	InOutQuad	OutInQuad
Cubic	InCubic	OutCubic	InOutCubic	OutInCubic
Quart	InQuart	OutQuart	InOutQuart	OutInQuart
Quint	InQuint	OutQuint	InOutQuint	OutInQuint
Expo	InExpo	OutExpo	InOutExpo	OutInExpo
Sine	InSine	OutSine	InOutSine	OutInSine
Circ	InCirc	OutCirc	InOutCirc	OutInCirc
Back	InBack	OutBack	InOutBack	OutInBack
Bounce	InBounce	OutBounce	InOutBounce	OutInBounce
Elastic	InElastic	OutElastic	InOutElastic	OutInElastic

Custom easing functions

You are not limited to gween's easing functions; if you pass a function parameter in the easing, it will be used.

The passed function will need to suite the TweenFunc interface: func(t, b, c, d float64) float64

• t (time): starts in 0 and usually moves towards duration
• b (begin): initial value of the of the property being eased.
• c (change): ending value of the property - starting value of the property
• d (duration): total duration of the tween

And must return the new value after the interpolation occurs.

Here's an example using a custom easing.

tw := tween.NewTween(0, 300, 4, func(t, b, c, d) float64 {
  return c*t/d + b // linear ease
})

Credits

tween is an independent fork of the gween package. The API is different.