Fix: Friction disappears when manually applying gravity #224

readme.md

@@ -16,6 +16,7 @@ These minor changes and improvements were also made:
 - Add support for [Trigger bodies](https://pmndrs.github.io/cannon-es/examples/trigger). [#83](https://github.com/pmndrs/cannon-es/pull/83)
 - Deprecated properties and methods have been removed.
 - The [original cannon.js debugger](https://github.com/schteppe/cannon.js/blob/master/tools/threejs/CannonDebugRenderer.js), which shows the wireframes of each body, has been moved to its own repo [cannon-es-debugger](https://github.com/pmndrs/cannon-es-debugger).
+- Added optional property `World.frictionGravity: Vec3` which can be set to customize the force used when computing the friction between two colliding bodies. If `undefined`, `World.gravity` will be used. This property is useful to enable friction in zero gravity. This addresses issue [#224](https://github.com/schteppe/cannon.js/issues/224) and follows the [pattern established for p2.js](https://github.com/schteppe/p2.js/blob/master/src/world/World.js#L88-L92).
 
 If instead you're using three.js in a **React** environment with [react-three-fiber](https://github.com/pmndrs/react-three-fiber), check out [use-cannon](https://github.com/pmndrs/use-cannon)! It's a wrapper around cannon-es.
 

src/world/Narrowphase.test.ts

@@ -6,6 +6,7 @@ import { Sphere } from '../shapes/Sphere'
 import { Body } from '../objects/Body'
 import { World } from '../world/World'
 import { ContactEquation } from '../equations/ContactEquation'
+import { FrictionEquation } from '../equations/FrictionEquation'
 
 describe('Narrowphase', () => {
   test('sphere + sphere contact', () => {
@@ -63,6 +64,92 @@ describe('Narrowphase', () => {
 
     expect(result.length).toBe(1)
   })
+
+  test('should default to using global gravity to create friction equations', () => {
+    const gravity = new Vec3(0, -9.81, 0)
+    const world = new World({ gravity })
+    // No frictionGravity override.
+    expect(world.frictionGravity).toBeUndefined()
+
+    const narrowPhase = new Narrowphase(world)
+    const contacts: ContactEquation[] = []
+    const sphereShape = new Sphere(1)
+
+    const bodyA = new Body({ mass: 1 })
+    const bodyB = new Body({ mass: 1 })
+    bodyA.addShape(sphereShape)
+    bodyB.addShape(sphereShape)
+
+    narrowPhase.result = contacts
+    narrowPhase.sphereSphere(
+      sphereShape,
+      sphereShape,
+      new Vec3(0.5, 0, 0),
+      new Vec3(-0.5, 0, 0),
+      new Quaternion(),
+      new Quaternion(),
+      bodyA,
+      bodyB
+    )
+
+    expect(contacts.length).toBe(1)
+    const [contact] = contacts
+    const result: FrictionEquation[] = []
+
+    // No frictionGravity, should use global gravity.
+    narrowPhase.createFrictionEquationsFromContact(contact, result)
+    expect(result.length).toBe(2)
+    result.forEach((result) => {
+      expect(result.maxForce > 0).toBe(true)
+    })
+  })
+
+  test('if provided, should use frictionGravity to create friction equations', () => {
+    const gravity = new Vec3(0, 0, 0)
+    const frictionGravity = new Vec3(0, -9.81, 0)
+    const world = new World({ gravity, frictionGravity })
+
+    const narrowPhase = new Narrowphase(world)
+    const contacts: ContactEquation[] = []
+    const sphereShape = new Sphere(1)
+
+    const bodyA = new Body({ mass: 1 })
+    const bodyB = new Body({ mass: 1 })
+    bodyA.addShape(sphereShape)
+    bodyB.addShape(sphereShape)
+
+    narrowPhase.result = contacts
+    narrowPhase.sphereSphere(
+      sphereShape,
+      sphereShape,
+      new Vec3(0.5, 0, 0),
+      new Vec3(-0.5, 0, 0),
+      new Quaternion(),
+      new Quaternion(),
+      bodyA,
+      bodyB
+    )
+
+    expect(contacts.length).toBe(1)
+    const [contact] = contacts
+    const result: FrictionEquation[] = []
+
+    // No gravity, frictionGravity defined, friction.
+    narrowPhase.createFrictionEquationsFromContact(contact, result)
+    expect(result.length).toBe(2)
+    result.forEach((result) => {
+      expect(result.maxForce > 0).toBe(true)
+    })
+
+    // No gravity, no frictionGravity, no friction.
+    result.length = 0
+    world.frictionGravity = new Vec3(0, 0, 0)
+    narrowPhase.createFrictionEquationsFromContact(contact, result)
+    expect(result.length).toBe(2)
+    result.forEach((result) => {
+      expect(result.maxForce).toBe(0)
+    })
+  })
 })
 
 function createHeightfield() {

src/world/Narrowphase.ts

@@ -224,7 +224,8 @@ export class Narrowphase {
 
     if (friction > 0) {
       // Create 2 tangent equations
-      const mug = friction * world.gravity.length()
+      // Users may provide a force different from global gravity to use when computing contact friction.
+      const mug = friction * (world.frictionGravity || world.gravity).length()
       let reducedMass = bodyA.invMass + bodyB.invMass
       if (reducedMass > 0) {
         reducedMass = 1 / reducedMass

src/world/World.test.ts

@@ -259,4 +259,21 @@ describe('World', () => {
   test('using ObjectCollisionMatrix', () => {
     testCollisionMatrix(ObjectCollisionMatrix)
   })
+
+  test('frictionGravity: should be undefined by default', () => {
+    const gravity = new Vec3(0, -9.81, 0)
+    const world = new World({ gravity })
+
+    expect(world.gravity).toEqual(gravity)
+    expect(world.frictionGravity).toBeUndefined()
+  })
+
+  test('frictionGravity: should be configurable', () => {
+    const gravity = new Vec3(0, 0, 0)
+    const frictionGravity = new Vec3(0, -9.81, 0)
+    const world = new World({ gravity, frictionGravity })
+
+    expect(world.gravity).toEqual(gravity)
+    expect(world.frictionGravity).toEqual(frictionGravity)
+  })
 })

src/world/World.ts

@@ -79,6 +79,13 @@ export class World extends EventTarget {
    */
   gravity: Vec3
 
+  /**
+   * Gravity to use when approximating the friction max force (mu*mass*gravity).
+   * If undefined, global gravity will be used.
+   * Use to enable friction in a World with a null gravity vector (no gravity).
+   */
+  frictionGravity?: Vec3
+
   /**
    * The broadphase algorithm to use.
    * @default NaiveBroadphase
@@ -170,6 +177,11 @@ export class World extends EventTarget {
        * The gravity of the world.
        */
       gravity?: Vec3
+      /**
+       * Gravity to use when approximating the friction max force (mu*mass*gravity).
+       * If undefined, global gravity will be used.
+       */
+      frictionGravity?: Vec3
       /**
        * Makes bodies go to sleep when they've been inactive.
        * @default false
@@ -215,6 +227,10 @@ export class World extends EventTarget {
     if (options.gravity) {
       this.gravity.copy(options.gravity)
     }
+    if (options.frictionGravity) {
+      this.frictionGravity = new Vec3()
+      this.frictionGravity.copy(options.frictionGravity)
+    }
 
     this.broadphase = options.broadphase !== undefined ? options.broadphase : new NaiveBroadphase()
     this.bodies = []