Physics system for A-Frame VR, built on Cannon.js
npm install aframe-physics-system-----------
dynamic-body and static-body
shape
constraint
spring
html
`
CDN builds for aframe-physics-system@v$npm_package_version:
- aframe-physics-system.js (development)
- aframe-physics-system.min.js (production)
$3
`
npm install --save aframe-physics-system
`
`javascript
// my-app.js
require('aframe-physics-system');
`
Once installed, you'll need to compile your JavaScript using something like Browserify or Webpack. Example:
`bash
npm install -g browserify
browserify my-app.js -o bundle.js
`
bundle.js may then be included in your page. See here for a better introduction to Browserify.
#### npm + webpack
When using webpack, you need to ensure that your loader for .js files includes this dependency. The example below assumes you're using Babel.
`js
{
test: /\.js$/,
include: ['src', require.resolve('aframe-physics-system') ],
use: {
loader: 'babel-loader', // or whatever loader you're using to parse modules
options: {}
}
}
`
> Note: You cannot use exclude: /node_modules for your .js loader. You must instead use include and pass an array of directories as dependencies to transpile.
Basics
`html
`
Components
$3
The dynamic-body and static-body components may be added to any that contains a mesh. Generally, each scene will have at least one static-body for the ground, and one or more dynamic-body instances that the player can interact with.
- dynamic-body: A freely-moving object. Dynamic bodies have mass, collide with other objects, bounce or slow during collisions, and fall if gravity is enabled.
- static-body: A fixed-position or animated object. Other objects may collide with static bodies, but static bodies themselves are unaffected by gravity and collisions.
| Property | Dependencies | Default | Description |
|----------------|------------------|---------|-----------------------------------------------------|
| shape | — | auto | auto, box, cylinder, sphere, hull, none |
| mass | dynamic-body | 5 | Simulated mass of the object, > 0. |
| linearDamping | dynamic-body | 0.01 | Resistance to movement. |
| angularDamping | dynamic-body | 0.01 | Resistance to rotation. |
| sphereRadius | shape:sphere | — | Override default radius of bounding sphere. |
| cylinderAxis | shape:cylinder | — | Override default axis of bounding cylinder. |
#### Body Shapes
Body components will attempt to find an appropriate CANNON.js shape to fit your model. When defining an object you may choose a shape or leave the default, auto. Select a shape carefully, as there are performance implications with different choices:
* None (none) – Does not add collision geometry. Use this when adding collision shapes manually, through the shape component or custom JavaScript.
* Auto (auto) – Chooses automatically from the available shapes.
* Box (box) – Great performance, compared to Hull or Trimesh shapes, and may be fitted to custom models.
* Cylinder (cylinder) – See box. Adds cylinderAxis option.
* Sphere (sphere) – See box. Adds sphereRadius option.
* Convex (hull) – Wraps a model like shrink-wrap. Convex shapes are more performant and better supported than Trimesh, but may still have some performance impact when used as dynamic objects.
* Primitives – Plane/Cylinder/Sphere. Used automatically with the corresponding A-Frame primitives.
Trimesh (mesh) – Deprecated.* Trimeshes adapt to fit custom geometry (e.g. a .OBJ or .DAE file), but have very minimal support. Arbitrary trimesh shapes are difficult to model in any JS physics engine, will "fall through" certain other shapes, and have serious performance limitations.
For more details, see the CANNON.js collision matrix.
Example using a bounding box for a custom model:
`html
`
$3
Compound shapes require a bit of work to set up, but allow you to use multiple primitives to define a physics shape around custom models. These will generally perform better, and behave more accurately, than mesh or hull shapes. For example, a chair might be modeled as a cylinder-shaped seat, on four long cylindrical legs.
Example:
`html
shape__main="shape: cylinder;
height: 0.36;
radiusTop: 0.24;
radiusBottom: 0.24;"
shape__handle="shape: box;
halfExtents: 0.15 0.18 0.04;
offset: 0.4 0 0;">
`
| Property | Shapes | Default | Description |
|-------------|------------|-----------|-------------|
|shape | — | box | box, sphere, or cylinder |
|offset | — | 0 0 0 | Position of shape relative to body. |
|orientation | — | 0 0 0 1 | Rotation of shape relative to body. |
|radius | sphere | 1 | Sphere radius. |
|halfExtents | box | 1 1 1 | Box half-extents. Use 0.5 0.5 0.5 for a 1x1x1 box. |
|radiusTop | cylinder | 1 | Cylinder upper radius. |
|radiusBottom | cylinder | 1 | Cylinder lower radius. |
|height | cylinder | 1 | Cylinder height. |
|numSegments | cylinder | 8 | Cylinder subdivisions. |
$3
The constraint component is used to bind physics bodies together using hinges, fixed distances, or fixed attachment points.
Example:
`html
`
| Property | Dependencies | Default | Description |
| --- | --- | --- | --- |
| type | — | lock | Type of constraint. Options: lock, distance, hinge, coneTwist, pointToPoint. |
| target | — | — | Selector for a single entity to which current entity should be bound. |
| maxForce | — | 1e6 | Maximum force that may be exerted to enforce this constraint. |
| collideConnected | — | true | If true, connected bodies may collide with one another. |
| wakeUpBodies | — | true | If true, sleeping bodies are woken up by this constraint. |
| distance | type:distance | auto | Distance at which bodies should be fixed. Default, or 0, for current distance. |
| pivot | type: pointToPoint, coneTwist, hinge | 0 0 0 | Offset of the hinge or point-to-point constraint, defined locally in this element's body. |
| targetPivot | type: pointToPoint, coneTwist, hinge | 0 0 0 | Offset of the hinge or point-to-point constraint, defined locally in the target's body. |
| axis | type: coneTwist, hinge | 0 0 1 | An axis that each body can rotate around, defined locally to this element's body. |
| targetAxis | type: coneTwist, hinge | 0 0 1 | An axis that each body can rotate around, defined locally to the target's body. |
$3
The spring component connects two bodies, and applies forces as the bodies become farther apart.
Example:
`html
spring="target: #anchor;
damping: 0.25;
stiffness: 25;">
`
| Property | Default | Description |
| ------------ | --- | -------------------------------------------------------------- |
| target | — | Target (other) body for the constraint. |
| restLength | 1 | Length of the spring, when no force acts upon it. |
| stiffness | 100 | How much will the spring suppress force. |
| damping | 1 | Stretch factor of the spring. |
| localAnchorA | — | Where to hook the spring to body A, in local body coordinates. |
| localAnchorB | — | Where to hook the spring to body B, in local body coordinates. |
Using the CANNON.js API
For more advanced physics, use the CANNON.js API with custom JavaScript and A-Frame components. The CANNON.js documentation and source code offer good resources for learning to work with physics in JavaScript.
In A-Frame, each entity's CANNON.Body instance is exposed on the el.body property. To apply a quick push to an object, you might do the following:
`html
`
`javascript
var el = sceneEl.querySelector('#nyan');
el.body.applyImpulse(
/ impulse / new CANNON.Vec3(0, 1, -1),
/ world position / new CANNON.Vec3().copy(el.getComputedAttribute('position'))
);
`
Events
| event | description |
|-------|-------------|
| body-loaded | Fired when physics body (el.body) has been created. |
| collide | Fired when two objects collide. Touching objects may fire collide events frequently. Unavailable with driver: worker. |
$3
> NOTE: Collision events are currently only supported with the local driver, and will not be fired with physics="driver: worker" enabled.
CANNON.js generates events when a collision is detected, which are propagated onto the associated A-Frame entity. Example:
`javascript
var playerEl = document.querySelector('[camera]');
playerEl.addEventListener('collide', function (e) {
console.log('Player has collided with body #' + e.detail.body.id);
e.detail.target.el; // Original entity (playerEl).
e.detail.body.el; // Other entity, which playerEl touched.
e.detail.contact; // Stats about the collision (CANNON.ContactEquation).
e.detail.contact.ni; // Normal (direction) of the collision (CANNON.Vec3).
});
`
Note that CANNON.js cannot perfectly detect collisions with very fast-moving bodies. Doing so requires Continuous Collision Detection, which can be both slow and difficult to implement. If this is an issue for your scene, consider (1) slowing objects down, (2) detecting collisions manually (collisions with the floor are easy – position.y - height / 2 <= 0), or (3) attempting a PR to CANNON.js. See: Collision with fast bodies.
System Configuration
Contact materials define what happens when two objects meet, including physical properties such as friction and restitution (bounciness). The default, scene-wide contact materials may be configured on the scene element:
`html
`
> NOTE: It is possible to run physics on a Web Worker using the physics="driver: worker" option.
> Using a worker is helpful for maintaining a smooth framerate, because physics simulation does
> not block the main thread. However, scenes needing highly-responsive interaction (for example,
> tossing and catching objects) may prefer to run physics locally, where feedback from the physics
> system will be immediate.
| Property | Default | Description |
|---------------------------------|---------|----------------------------------------------------|
| debug | true | Whether to show wireframes for debugging. |
| gravity | -9.8 | Force of gravity (in m/s^2). |
| iterations | 10 | The number of solver iterations determines quality of the constraints in the world. The more iterations, the more correct simulation. More iterations need more computations though. If you have a large gravity force in your world, you will need more iterations. |
| maxInterval | 0.0667 | Maximum simulated time (in milliseconds) that may be taken by the physics engine per frame. Effectively prevents weird "jumps" when the player returns to the scene after a few minutes, at the expense of pausing physics during this time. |
| friction | 0.01 | Coefficient of friction. |
| restitution | 0.3 | Coefficient of restitution (bounciness). |
| contactEquationStiffness | 1e8 | Stiffness of the produced contact equations. |
| contactEquationRelaxation | 3 | Relaxation time of the produced contact equations. |
| frictionEquationStiffness | 1e8 | Stiffness of the produced friction equations. |
| frictionEquationRegularization | 3 | Relaxation time of the produced friction equations |
| driver | local | [local, worker] |
| workerFps | 60 | Steps per second to be used in physics simulation on worker. |
| workerInterpolate | true | Whether the main thread should interpolate physics frames from the worker. |
| workerInterpBufferSize | 2 | Number of physics frames to be 'padded' before displaying. Advanced. |
| workerDebug | false | If true, the worker codepaths are used on the main thread. This is slow, because physics snapshots are needlessly serialized, but helpful for debugging. |
More advanced configuration, including specifying different collision behaviors for different objects, is available through the CANNON.js JavaScript API.
Resources:
* CANNON.World
* CANNON.ContactMaterial
Examples
To help demonstrate the features and capabilities of aframe-physics-system` a