## target / current

This interpolation do not requires any timer. It is similar to a constant deceleration. The idea is to gradually reduce the difference between a target and a current value. The code is simplest:

```float target = 0;
float current = 0;
...
target = 3;
...
// (in an animation for instance)
current += (target-current) * 0.1;
```

At each loop, the 10% of the difference is added to current. To avoid an infinite process, the addition can be controlled by 2 tests, one to apply the operation, and another to stop it if the difference is too small ( 0.0000001 in this case ).

```if ( current != target ) {
current += (target-current) * 0.1;
if ( abs( target-current ) < 1e-7 ) {
current = target;
}
}
```

## speed / current

This interpolation do not requires any timer either. It is similar to an elastic out interpolation. The idea here is to modify the speed of current in order to reach a given value. Once accelerated, the current value might go too far, inverting the direction of the motion (elastic). The speed is then adapted to go in the other direction, until the requested value is reached. Code is a bit less obvious then the target / current:

```float target = 0;
float speed = 0;
float current = 0;
...
target = 3;
...
// (in an animation for instance)
speed = ( (target - current) - speed_roll ) * 0.35;
current += speed_roll * 0.1;
```

At each loop, the speed is increased by 35% of the difference of the difference between target and current value and previous speed. A lower percentage of this speed (10%) is then added to current. This method might ends up with high speed. It very simple to cap it using 2 tests:

```speed = ( (target - current) - speed_roll ) * 0.35;
if (speed_roll > 1) {
speed_roll = 1;
} else if (speed_roll < -1) {
speed_roll = -1;
}
current += speed_roll * 0.1;
```

See Camera rig example in engine for an example.

## Resources

• Tween, previous study of interpolation using timers. See bitbucket for code or vimeo for a demo video.
1. Elastic out interpolation visualisation in tween.js, also called overshoot, see Realistic Bounce and Overshoot

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