desired language:

desired language:

# Move entities

To move, rotate or resize an entity in your scene, change the *position*, *rotation* and *scale* values stored in an entity’s `Transform`

component incrementally, frame by frame. This can be used on primitive shapes (cubes, spheres, planes, etc) as well as on 3D models (glTF).

You can easily perform these incremental changes by moving entities a small amount each time the `update()`

function of a system is called.

## Move

The easiest way to move an entity is to use the `translate()`

function to change the *position* value stored in the `Transform`

component.

```
export class SimpleMove {
update() {
let transform = myEntity.get(Transform)
let distance = Vector3.Forward.scale(0.1)
transform.translate(distance)
}
}
engine.addSystem(new SimpleMove())
const myEntity = new Entity()
myEntity.set(new Transform())
myEntity.set(new BoxShape())
engine.addEntity(myEntity)
```

In this example we’re moving an entity by 0.1 meters per frame.

`Vector3.Forward()`

returns a vector that faces forward and measures 1 meter in length. In this example we’re then scaling this vector down to 1/10 of its length with `scale()`

. If our scene has 30 frames per second, the entity is moving at 3 meters per second in speed.

## Adjust movement to delay time

Suppose that the user running your scene is struggling to keep up with the pace of the frame rate. That could result in the movement appearing jumpy, as not all frames are evenly timed but each moves the entity in the same amount.

You can compensate for this uneven timing by using the `dt`

parameter to adjust the scale the movement.

```
export class SimpleMove {
update(dt: number) {
let transform = myEntity.get(Transform)
let distance = Vector3.Forward.scale(dt * 3)
transform.translate(distance)
}
}
// (...)
```

The example above keeps movement at approximately the same speed as before, even if the frame rate drops. When running at 30 frames per second, the value of `dt`

is 1/30 .

## Rotate

The easiest way to rotate an entity is to use the `rotate()`

function to change the values in the Transform component incrementally, and run this as part of the `update()`

function of a system.

The `rotate()`

function takes two arguments:

- The direction in which to rotate (as a
*Vector3*) - The amount to rotate, in euler degrees (from 0 to 360)

```
export class SimpleRotate {
update() {
let transform = myEntity.get(Transform)
let distance = dt * 3
transform.rotate(Vector3.Left(), distance)
}
}
engine.addSystem(new SimpleRotate())
```

Tip: To make an entity always rotate to face the user, you can use the

`billboardMode`

setting. See Set entity poision for details.

## Rotate over a pivot point

When rotating an entity, the rotation is always in reference to the entity’s center coordinate. To rotate an entity using another set of coordinates as a pivot point, create a second (invisible) entity with the pivot point as its position and make it a parent of the entity you with to rotate.

When rotating the parent entity, its children will be all rotated using the parent’s position as a pivot point. Note that the `position`

of the child entity is in reference to that of the parent entity.

```
// Create entity you wish to rotate
const door = new Entity()
// Create the pivot entity
const pivot = new Entity()
// Position the pivot entity on the pivot point of the rotation
pivot.set(new Transform({
position: new Vector3(4, 1, 3)
}))
// Set pivot as the parent
door.setParent(pivot)
// Position child in reference to parent
door.set(new Transform({
position: new Vector3(0.5, 0, 0)
}))
// Rotate the parent. The child rotates using the parent's location as a pivot point.
pivot.get(Transform).rotation.setEuler(0, 90, 0)
// Add both entities to the engine
engine.addEntity(door)
engine.addEntity(pivot)
// Define a system that updates the rotation on every frame
export class PivotRotate {
update() {
let transform = myEntity.get(pivot)
let distance = dt * 3
transform.rotate(Vector3.Left(), distance )
}
}
// Add the system
engine.addSystem(new PivotRotate())
```

Note that in this example, the system is rotating the `pivot`

entity, that’s a parent of the `door`

entity.

## Move between two points

If you want an entity to move smoothly between two points, use the *lerp* (linear interpolation) algorithm. This algorithm is very well known in game development, as it’s really useful.

The `lerp()`

function takes three parameters:

- The vector for the origin position
- The vector for the target position
- The amount, a value from 0 to 1 that represents what fraction of the translation to do.

```
const originVector = Vector3.Zero()
const targetVector = Vector3.Forward()
let newPos = Vector3.Lerp(originVector, targetVector, 0.6)
```

The linear interpolation algorithm finds an intermediate point in the path between both vectors that matches the provided amount.

For example, if the origin vector is *(0, 0, 0)* and the target vector is *(10, 0, 10)*:

- Using an amount of 0 would return
*(0, 0, 0)* - Using an amount of 0.3 would return
*(3, 0, 3)* - Using an amount of 1 would return
*(10, 0, 10)*

To implement this `lerp()`

in your scene, we recommend creating a custom component to store the necessary information. You also need to define a system that implements the gradual movement in each frame.

```
@Component("lerpData")
export class LerpData {
origin: Vector3 = Vector3.Zero()
target: Vector3 = Vector3.Zero()
fraction: number = 0
}
// a system to carry out the movement
export class LerpMove {
update(dt: number) {
let transform = myEntity.get(Transform)
let lerp = myEntity.get(LerpData)
if (lerp.fraction < 1) {
transform.position = Vector3.Lerp(
lerp.origin,
lerp.target,
lerp.fraction
)
lerp.fraction += dt / 6
}
}
}
// Add system to engine
engine.addSystem(new LerpMove())
const myEntity = new Entity()
myEntity.set(new Transform())
myEntity.set(new BoxShape())
myEntity.set(new LerpData())
myEntity.get(LerpData).origin = new Vector3(1, 1, 1)
myEntity.get(LerpData).target = new Vector3(8, 1, 3)
engine.addEntity(myEntity)
```

## Rotate between two angles

To rotate smoothly between two angles, use the *slerp* (*spherical* linear interpolation) algorithm. This algorithm is very similar to a *lerp*, but it handles quaternion rotations.

The `slerp()`

function takes three parameters:

- The quaternion angle for the origin rotation
- The quaternion angle for the target rotation
- The amount, a value from 0 to 1 that represents what fraction of the translation to do.

Tip: You can pass rotation values in euler degrees (from 0 to 360) by using

`Quaternion.Euler()`

.

```
const originRotation = Quaternion.Euler(0, 90, 0)
const targetRotation = Quaternion.Euler(0, 0, 0)
let newRotation = Scalar.Lerp(originRotation, targetRotation, 0.6)
```

To implement this in your scene, we recommend storing the data that goes into the `Slerp()`

function in a custom component. You also need to define a system that implements the gradual rotation in each frame.

```
@Component('slerpData')
export class SlerpData {
originRot: Quaternion = Quaternion.Euler(0, 90, 0)
targetRot: Quaternion = Quaternion.Euler(0, 0, 0)
fraction: number = 0
}
// a system to carry out the rotation
export class SlerpRotate implements ISystem {
update(dt: number) {
let slerp = myEntity.get(SlerpData)
let transform = myEntity.get(Transform)
slerp.fraction += dt
let rot = Quaternion.Slerp(slerp.originRot, slerp.targetRot, slerp.fraction)
transform.rotation = rot
}
}
// Add system to engine
engine.addSystem(new SlerpRotate())
const myEntity = new Entity()
myEntity.set(new Transform())
myEntity.set(new BoxShape())
myEntity.set(new SlerpData())
myEntity.get(SlerpData).originRot = Quaternion.Euler(0, 90, 0)
myEntity.get(SlerpData).targetRot = Quaternion.Euler(0, 0, 0)
engine.addEntity(myEntity)
```

Note: You could instead represent the rotation with

`Vector3`

values and use a`Lerp()`

function, but that would imply a conversion from`Vector3`

to`Quaternion`

on each frame. Rotation values are internally stored as quaternions in the`Transform`

component, so it’s more efficient to work with quaternions.

## Change scale between two sizes

If you want an entity to change size smoothly and without changing its proportions, use the *lerp* (linear interpolation) algorithm of the `Scalar`

object.

Otherwise, if you want to change the axis in different proportions, use `Vector3`

to represent the origin scale and the target scale, and then use the *lerp* function of the `Vector3`

.

The `lerp()`

function of the `Scalar`

object takes three parameters:

- A number for the origin scale
- A number for the target scale
- The amount, a value from 0 to 1 that represents what fraction of the scaling to do.

```
const originScale = 1
const targetScale = 10
let newScale = Scalar.Lerp(originScale, targetScale, 0.6)
```

To implement this lerp in your scene, we recommend creating a custom component to store the necessary information. You also need to define a system that implements the gradual scaling in each frame.

## Move at irregular speeds between two points

While using the lerp method, you can make the movement speed non-linear. In the previous example we increment the lerp amount by a given amount each frame, but we could also use a mathematical function to increase the number exponentially or in other measures that give you a different movement pace.

You could also use a function that gives recurring results, like a sine function, to describe a movement that comes and goes.

```
@Component("lerpData")
export class LerpData {
origin: Vector3 = Vector3.Zero()
target: Vector3 = Vector3.Zero()
fraction: number = 0
time: number = 0
}
export class LerpMove {
update(dt: number) {
let transform = lerpEntity.get(Transform)
let lerp = lerpEntity.get(LerpData)
lerp.time += dt / 6
lerp.fraction = Math.sin(lerp.time)
transform.position = Vector3.Lerp(
lerp.origin,
lerp.target,
lerp.fraction
)
}
}
```

The example above adds a `time`

field to the custom component. The `time`

field is incremented on every frame, and then `fraction`

is set to the *sin* of that value. Because of the nature of the *sin* operation, the entity will lerp back and forth between both points.

## Follow a path

A `Path3`

object stores a series of vectors that describe a path. You can have an entity loop over the list of vectors, performing a lerp movement between each.

```
const point1 = new Vector3(1, 1, 1)
const point2 = new Vector3(8, 1, 3)
const point3 = new Vector3(8, 4, 7)
const point4 = new Vector3(1, 1, 7)
const myPath = new Path3D([point1, point2, point3, point4])
@Component("pathData")
export class PathData {
origin: Vector3 = myPath.path[0]
target: Vector3 = myPath.path[1]
fraction: number = 0
nextPathIndex: number = 1
}
export class PatrolPath {
update(dt: number) {
let transform = myEntity.get(Transform)
let path = myEntity.get(PathData)
if (path.fraction < 1) {
transform.position = Vector3.Lerp(
path.origin,
path.target,
path.fraction
)
path.fraction += dt / 6
} else {
path.nextPathIndex += 1
if (path.nextPathIndex >= myPath.path.length) {
path.nextPathIndex = 0
}
path.origin = path.target
path.target = myPath.path[path.nextPathIndex]
path.fraction = 0
}
}
}
engine.addSystem(new PatrolPath())
const myEntity = new Entity()
myEntity.set(new Transform())
myEntity.set(new BoxShape())
myEntity.set(new PathData())
engine.addEntity(myEntity)
```

The example above defines a 3D path that’s made up of four 3D vectors. We also define a custom `PathData`

component, that includes the same data used by the custom component in the *lerp* example above, but adds a `nextPathIndex`

field to keep track of what vector to use next from the path.

The system is very similar to the system in the *lerp* example, but when a lerp action is completed, it sets the `target`

and `origin`

fields to new values. If we reach the end of the path, we return to the first value in the path.