tween.js/docs/user_guide.md

tween.js user guide

NOTE this is a work in progress, please excuse the gaps. Wherever you see something marked as TODO, it's not done yet.

What is a tween? How do they work? Why do you want to use them?

A tween (TODO: from in-between?) is a concept that allows you to change the values of the properties of an object in a smooth way. You just tell it which properties you want to change, which final values should they have when the tween finishes running, and how long should this take. For example, suppose you have a position object with x and y coordinates:

var position = { x: 100, y: 0 }

If you wanted to change the x value from 100 to 200 you'd do this:

// Create a tween for position first
var tween = new TWEEN.Tween(position);

// Then tell the tween we want to animate the x property over 1000 milliseconds
tween.to({ x: 200 }, 1000);

Actually this won't do anything yet. The tween has been created but it's not active. You need to start it:

// And set it to start
tween.start();

Finally in order to run as smoothly as possible you should call the TWEEN.update function in the same main loop you're using for animating. This generally looks like this:

animate();

function animate() {
	requestAnimationFrame(animate);
	// [...]
	TWEEN.update();
	// [...]
}

This will take care of updating all active tweens; after 1 second (i.e. 1000 milliseconds) position.x will be 200.

But unless you print the value of x to the console, you can't see its value changing. You might want to use the onUpdate callback:

tween.onUpdate(function() {
	console.log(this.x);
});

This function will be called each time the tween is updated; how often this happens depends on many factors--how fast (and how busy!) your computer or device is, for example.

So far we've only used tweens to print values to the console, but you could use it for things such as animating positions of three.js objects:

var tween = new TWEEN.Tween(cube.position);
		.to({ x: 100, y: 100, z: 100 }, 10000)
		.start();

animate();

function animate() {
	requestAnimationFrame(animate);
	TWEEN.update();

	threeRenderer.render(scene, camera);
}

In this case, because the three.js renderer will look at the object's position before rendering, you don't need to use an explicit onUpdate callback.

You might have noticed something different here too: we're chaining the tween function calls! Each tween function returns the tween instance, so you can rewrite the following code:

var tween = new TWEEN.Tween(position);
tween.to({ x: 200 }, 1000);
tween.start();

into this

var tween = new TWEEN.Tween(position)
	.to({ x: 200 }, 1000)
	.start();

You'll see this a lot in the examples, so it's good to be familiar with it! Check 04-simplest for a working example.

Animating with tween.js

Tween.js doesn't run by itself. You need to tell it when to run, by explicitly calling the update method. The recommended method is to do this inside your main animation loop, which should be called with requestAnimationFrame for getting the best graphics performance:

We've seen this example before:

animate();

function animate() {
	requestAnimationFrame(animate);
	// [...]
	TWEEN.update();
	// [...]
}

If called without parameters, update will determine the current time in order to find out how long has it been since the last time it ran.

However you can also pass an explicit time parameter to update. Thus,

TWEEN.update(100);

means "update with time = 100 milliseconds". You can use this to make sure that all the time-dependent functions in your code are using the very same time value. For example suppose you've got a player and want to run tweens in sync. Your animate code could look like this:

var currentTime = player.currentTime;
TWEEN.update(currentTime);

We use explicit time values for the unit tests. You can have a look at TestTweens to see how we call TWEEN.update() with different values in order to simulate time passing.

Controlling a tween

start and stop

So far we've learnt about the Tween.start method, but there are more methods that control individual tweens. Probably the most important one is the start counterpart: stop. If you want to cancel a tween, just call this method over an individual tween:

tween.stop();

Stopping a tween that was never started or that has already been stopped has no effect. No errors are thrown either.

TODO: explain that start accepts a time parameter too.

update

Individual tweens also have an update method---this is in fact called by TWEEN.update. You generally don't need to call this directly, but might be useful if you're doing crazy hacks.

chain

Things get more interesting when you sequence different tweens in order, i.e. setup one tween to start once a previous one has finished. We call this chaining tweens, and it's done with the chain method. Thus, to make tweenB start after tweenA finishes:

tweenA.chain(tweenB);

Or, for an infinite chain, set tweenA to start once tweenB finishes:

tweenA.chain(tweenB);
tweenB.chain(tweenA);

Check Hello world to see an example of these infinite chains.

repeat

If you wanted a tween to repeat forever you could chain it to itself, but a better way is to use the repeat method. It accepts a parameter that describes how many repetitions you want:

tween.repeat(10); // repeats 10 times and stops
tween.repeat(Infinity); // repeats forever

Check the Repeat example.

yoyo

This function only has effect if used along with repeat. When active, the behaviour of the tween will be like a yoyo, i.e. it will bounce to and from the start and end values, instead of just repeating the same sequence from the beginning.

delay

More complex arrangements might require delaying a tween before it actually starts running. You can do that using the delay method:

tween.delay(1000);
tween.start();

will start executing 1 second after the start method has been called.

Controlling all the tweens

The following methods are found in the TWEEN global object, and you generally won't need to use most of them, except for update.

TWEEN.update(time)

We've already talked about this method. It is used to update all the active tweens.

If time is not specified, it will use the current time.

TWEEN.getAll and TWEEN.removeAll

Used to get a reference to the active tweens array and to remove all of them from the array with just one call, respectively.

TWEEN.add(tween) and TWEEN.remove(tween)

Used to add a tween to the list of active tweens, or to remove an specific one from the list, respectively.

These methods are usually used internally only, but are exposed just in case you want to do something funny.

Changing the easing function (AKA make it bouncy)

Tween.js will perform the interpolation between values (i.e. the easing) in a linear manner, so the change will be directly proportional to the elapsed time. This is predictable but also quite uninteresting visually wise. Worry not--this behaviour can be easily changed using the easing method. For example:

tween.easing(TWEEN.Easing.Quadratic.In);

This will result in the tween slowly starting to change towards the final value, accelerating towards the middle, and then quickly reaching its final value. In contrast, TWEEN.Easing.Quadratic.Out would start changing quickly towards the value, but then slow down as it approaches the final value.

Available easing functions: TWEEN.Easing

There are a few existing easing functions provided with tween.js. They are grouped by the type of equation they represent: Linear, Quadratic, Cubic, Quartic, Quintic, Sinusoidal, Exponential, Circular, Elastic, Back and Bounce, and then by the easing type: In, Out and InOut.

Probably the names won't be saying anything to you unless you're familiar with these concepts already, so it is probably the time to check the Graphs example, which graphs all the curves in one page so you can compare how they look at a glance.

Credit where credit is due: these functions are derived from the original set of equations that Robert Penner graciously made available as free software a few years ago, but have been optimised to play nicely with JavaScript.

Using a custom easing function

Not only can you use any of the existing functions, but you can also provide your own, as long as it follows a couple of conventions:

• it must accept one parameter:
  • k: the easing progress, or how far along the duration of the tween we are. Allowed values are in the range [0, 1].
• it must return a value based on the input parameters.

The easing function is only called once per tween on each update, no matter how many properties are to be changed. The result is then used with the initial value and the difference (the deltas) between this and the final values, as in this pseudocode:

easedElapsed = easing(k);
for each property:
	newPropertyValue = initialPropertyValue + propertyDelta * easedElapsed;

For the performance obsessed people out there: the deltas are calculated only when start() is called on a tween.

So let's suppose you wanted to use a custom easing function that eased the values but appplied a Math.floor to the output, so only the integer part would be returned, resulting in a sort of step-ladder output:

function tenStepEasing(k) {
	return Math.floor(k * 10) / 10;
}

And you could use it in a tween by simply calling its easing method, as we've seen before:

tween.easing(tenStepEasing);

Check the graphs for custom easing functions example to see this in action (and also some metaprogramming for generating step functions).

Chaining

Repeating

Callbacks

• start
• stop
• update
• complete

Advanced tweening

• relative values
• arrays - and which interpolation function? (linear, bezier, catmull-rom)

Getting the best performance

• CSS properties which shouldn't be used (e.g. marginLeft? use transform instead?)
  • or use animations/transitions instead
  • When to use tween -> for more complex arrangements
• Be good to the GC
• Reusing tweens

Crazy tweening

• Using the tweening functions outside of tween.js