HTML & Primitives - Aframe

This section will go over the concepts of A-Frame's primitive elements and their relation to the entity-component framework. If you're looking for a guide on using HTML and primitives, check out the Building a Basic Scene guide.

HTML

A-Frame is based on top of HTML and the DOM using a polyfill for Custom Elements. HTML is the building block of the Web, providing one of the most accessible computing languages around. There are no installations or build steps required, creating with HTML involves just text in an HTML file and opening the HTML file in a browser. Since most of the Web was built on top of HTML, most existing tools and libraries work with A-Frame including React, Vue.js, Angular, d3.js, and jQuery.

If you don't have too much experience with HTML, no problem! It's fairly easy to pick up and perhaps even easier to grasp than 2D HTML. Once you pick up the general structure or syntax of HTML (opening tag, attributes, closing tag), then you're good to go! Read an introduction to HTML on MDN.

Primitives

While the HTML layer looks basic, HTML and the DOM are only the outermost abstraction layer of A-Frame. Underneath, A-Frame is an entity-component framework for three.js that is exposed declaratively.

A-Frame provides a handful of elements such as <a-box> or <a-sky> called primitives that wrap the entity-component pattern to make it appealing for beginners. At the bottom of the documentation navigation sidebar, we can see every primitive that A-Frame provides out of the box. Developers can create their own primitives as well.

Example

Below is the Hello, WebVR example that uses a few basic primitives. A-Frame provides primitives to create meshes, render 360° content, customize the environment, place the camera, etc.

html

<html>
  <head>
    <script src="https://aframe.io/releases/1.7.1/aframe.min.js"></script>
  </head>
  <body>
    <a-scene>
      <a-box position="-1 0.5 -3" rotation="0 45 0" color="#4CC3D9"></a-box>
      <a-sphere position="0 1.25 -5" radius="1.25" color="#EF2D5E"></a-sphere>
      <a-cylinder position="1 0.75 -3" radius="0.5" height="1.5" color="#FFC65D"></a-cylinder>
      <a-plane position="0 0 -4" rotation="-90 0 0" width="4" height="4" color="#7BC8A4"></a-plane>
      <a-sky color="#ECECEC"></a-sky>
    </a-scene>
  </body>
</html>

Under the Hood

Primitives act as a convenience layer (i.e., syntactic sugar) primarily for newcomers. Keep in mind for now that primitives are <a-entity>s under the hood that:

Have a semantic name (e.g., <a-box>)
Have a preset bundle of components with default values
Map or proxy HTML attributes to component data

Primitives are similar to prefabs in Unity. Some literature on the entity-component-system pattern refer to them as assemblages. They abstract the core entity-component API to:

Pre-compose useful components together with prescribed defaults
Act as a shorthand for complex-but-common types of entities (e.g., <a-sky>)
Provide a familiar interface for beginners since A-Frame takes HTML in a new direction

Under the hood, this <a-box> primitive:

html

<a-box color="red" width="3"></a-box>

represents this entity-component form:

html

<a-entity geometry="primitive: box; width: 3" material="color: red"></a-entity>

<a-box> defaults the geometry.primitive property to box. And the primitive maps the HTML width attribute to the underlying geometry.width property as well as the HTML color attribute to the underlying material.color property.

Attaching Components to Primitives

Primitives are just <a-entity>s under the hood. This means primitives have the same API as entities such as positioning, rotating, scaling, and attaching components.

Example

Let's attach community physics components to primitives. We include the source for Don McCurdy's aframe-physics-system and attach the physics components via HTML attributes:

html

<html>
  <head>
    <script src="https://aframe.io/releases/1.7.1/aframe.min.js"></script>
    <script src="https://unpkg.com/@c-frame/[email protected]/dist/aframe-physics-system.min.js"></script>
  </head>
  <body>
    <a-scene physics>
      <a-box position="-1 4 -3" rotation="0 45 0" color="#4CC3D9" dynamic-body></a-box>
      <a-plane position="0 0 -4" rotation="-90 0 0" width="4" height="4" color="#7BC8A4" static-body></a-plane>
      <a-sky color="#ECECEC"></a-sky>
    </a-scene>
  </body>
</html>

Registering a Primitive

We can register our own primitives (i.e., register an element) using AFRAME.registerPrimitive(name, definition). name is a string and must contain a dash (i.e. 'a-foo'). definition is a JavaScript object defining these properties:

Property	Description	Example
defaultComponents	Object specifying default components of the primitive. The keys are the components' names and the values are the components' default data.	`{geometry: {primitive: 'box'}}`
mappings	Object specifying mapping between HTML attribute name and component property names. Whenever the HTML attribute name is updated, the primitive will update the corresponding component property. The component property is defined using a dot syntax `${componentName}.${propertyName}`.	`{depth: 'geometry.depth', height: 'geometry.height', width: 'geometry.width'}`

Example

For example, below is A-Frame's registration for the <a-box> primitive:

var extendDeep = AFRAME.utils.extendDeep;

// The mesh mixin provides common material properties for creating mesh-based primitives.
// This makes the material component a default component and maps all the base material properties.
var meshMixin = AFRAME.primitives.getMeshMixin();

AFRAME.registerPrimitive('a-box', extendDeep({}, meshMixin, {
  // Preset default components. These components and component properties will be attached to the entity out-of-the-box.
  defaultComponents: {
    geometry: {primitive: 'box'}
  },

  // Defined mappings from HTML attributes to component properties (using dots as delimiters).
  // If we set `depth="5"` in HTML, then the primitive will automatically set `geometry="depth: 5"`.
  mappings: {
    depth: 'geometry.depth',
    height: 'geometry.height',
    width: 'geometry.width'
  }
}));

For example, aframe-extras package includes an <a-ocean> primitive that wraps his ocean component. Here is the definition for <a-ocean>.

AFRAME.registerPrimitive('a-ocean', {
  // Attaches the `ocean` component by default.
  // Defaults the ocean to be parallel to the ground.
  defaultComponents: {
    ocean: {},
    rotation: {x: -90, y: 0, z: 0}
  },

  // Maps HTML attributes to the `ocean` component's properties.
  mappings: {
    width: 'ocean.width',
    depth: 'ocean.depth',
    density: 'ocean.density',
    color: 'ocean.color',
    opacity: 'ocean.opacity'
  }
});

With the <a-ocean> primitive registered, we'd be able to create oceans using a line of traditional HTML:

html

<a-ocean color="aqua" depth="100" width="100"></a-ocean>