Developing with three.js

Note: This documentation is for the old 0.9.0 version of A-Frame. Check out the documentation for the current 1.6.0 version

Being a framework based on three.js, A-Frame provides full access to the three.js API. We’ll go over how to access the underlying three.js scene, objects, and API that lay underneath A-Frame.

Relationship Between A-Frame and three.js Scene Graphs

A-Frame’s <a-scene> maps one-to-one with a three.js scene.
A-Frame’s <a-entity> maps to one or more three.js objects.
three.js’s objects have a reference to their A-Frame entity via .el, which is set by A-Frame.

Parent-Child Relationships

When A-Frame entities are nested in parent-child relationships, so are their three.js objects. For example, take this A-Frame scene:

<a-scene>
  <a-box>
    <a-sphere></a-sphere>
    <a-light></a-light>
  </a-box>
</a-scene>

The three.js scene graph will correspond and look like:

THREE.Scene
  THREE.Mesh
    THREE.Mesh
    THREE.Light

Accessing the three.js API

three.js is available as a global object on the window:

console.log(THREE);

Working With three.js Objects

A-Frame is an abstraction on top of three.js, but we still operate with three.js underneath. A-Frame’s elements have doors that lead to three.js’s scene graph.

Accessing the three.js Scene

The three.js scene is accessible from the <a-scene> element as .object3D:

document.querySelector('a-scene').object3D;  // THREE.Scene

And every A-Frame entity also has a reference to <a-scene> via .sceneEl:

document.querySelector('a-entity').sceneEl.object3D;  // THREE.Scene

From a component, we access the scene through its entity (i.e., this.el):

AFRAME.registerComponent('foo', {
  init: function () {
    var scene = this.el.sceneEl.object3D;  // THREE.Scene
  }
});

Accessing an Entity’s three.js Objects

Every A-Frame entity (e.g., <a-entity>) has its own THREE.Object3D, more specifically a THREE.Group that contains different types of Object3Ds. The root THREE.Group of an entity is accessed via .object3D:

document.querySelector('a-entity').object3D;  // THREE.Group

Entities can be composed of multiple types of Object3Ds. For example, an entity can be both a THREE.Mesh and a THREE.Light by having both a geometry component and light component:

<a-entity geometry light></a-entity>

Components add the mesh and light under the entity’s root THREE.Group. References to the mesh and light are stored as different types of three.js objects in the entity’s .object3DMap.

console.log(entityEl.object3DMap);
// {mesh: THREE.Mesh, light: THREE.Light}

But we can access them through the entity’s .getObject3D(name) method:

entityEl.getObject3D('mesh');  // THREE.Mesh
entityEl.getObject3D('light');  // THREE.Light

Now let’s see how these three.js objects were set in the first place.

Setting an `Object3D` on an Entity

Setting an Object3D on an entity adds the Object3D to the entity’s Group, which makes the newly set Object3D part of the three.js scene. We set the Object3D with the entity’s .setObject3D(name) method where the name denotes the Object3Ds purpose.

For example, to set a point light from within a component:

AFRAME.registerComponent('pointlight', {
  init: function () {
    this.el.setObject3D('light', new THREE.PointLight());
  }
});
// <a-entity light></a-entity>

We set the light with the name light. To later access it, we can use the entity’s .getObject3D(name) method as described before:

entityEl.getObject3D('light');

And when we set a three.js object on an A-Frame entity, A-Frame will set a reference to the A-Frame entity from the three.js object via .el:

entityEl.getObject3D('light').el;  // entityEl

There’s also a .getOrCreateObject3D(name, constructor) method for creating and setting an Object3D if one has not been set with the name. This is commonly used in the case of THREE.Mesh when both the geometry and material components need to get or create a mesh. Whichever component gets initialized first creates the mesh, then the other component gets the mesh.

Removing an `Object3D` From an Entity

To remove an Object3D from an entity, and consequently the three.js scene, we can use the entity’s .removeObject3D(name) method. Going back to our example with the point light, we remove the light when the component is detached:

AFRAME.registerComponent('pointlight', {
  init: function () {
    this.el.setObject3D('light', new THREE.PointLight());
  },

  remove: function () {
    // Remove Object3D.
    this.el.removeObject3D('light');
  }
});

Transforming Between Coordinate Spaces

Every object and the scene (world) in general has their own coordinate space. A parent object’s position, rotation, and scale transformations are applied to its children’s position, rotation, and scale transformations. Consider this scene:

<a-entity id="foo" position="1 2 3">
  <a-entity id="bar" position="2 3 4"></a-entity>
</a-entity>

From the world’s reference point, foo has position (1,2,3) and bar has position (3, 5, 7) since foo’s transformations apply onto bar’s. From foo’s reference point, foo has position (0, 0, 0) and bar has position (2, 3, 4).

Often we will want to transform between these reference points and coordinate spaces. Above was a simple example, but we might want to do operations such as finding the world-space coordinate of bar’s position, or translate an arbitrary coordinate into foo’s coordinate space. In 3D programming, these operations are accomplished with matrices, but three.js provides helpers to make them easier.

Local to World Transforms

Normally, we’d need to call .updateMatrixWorld () on parent Object3Ds, but three.js defaults Object3D.matrixAutoUpdate to true. We can use three.js’s .getWorldPosition () and .getWorldRotation ().

To get the world position of an Object3D:

entityEl.object3D.getWorldPosition();

To get the world rotation of an Object3D:

entityEl.object3D.getWorldRotation();

three.js Object3D has more functions available for local-to-world transforms:

.localToWorld (vector)
.getWorldDirection ()
.getWorldQuaternion ()
.getWorldScale ()

World to Local Transforms

To obtain a matrix that transforms from world to an object’s local space, get the inverse of the object’s world matrix.

var worldToLocal = new THREE.Matrix4().getInverse(object3D.matrixWorld)

Then we can apply that worldToLocal matrix to anything we want to transform:

anotherObject3D.applyMatrix(worldToLocal);

A-Frame