Physijs物理引擎

概念

Physijs建立在ammo.js之上

使用

五步

1. 导入physi.js
2. 配置Physijs.scripts.worker和Physijs.scripts.ammo
3. 用于Physijs.Scene代替THREE.Scene
4. THREE.Mesh，选择的Physijs替换
5. 调用scene.simulate方法，当您渲染或每当你想迭代物理设置

基本形状

Physijs.PlaneMesh - 零厚度平面
Physijs.BoxMesh - THREE.CubeGeometry
Physijs.SphereMesh - THREE.SphereGeometry
Physijs.CylinderMesh - THREE.CylinderGeometry
Physijs.ConeMesh- THREE.CylinderGeometry（锥形）
Physijs.CapsuleMesh- THREE.CylinderGeometry，除了两端半球
Physijs.ConvexMesh - 具有的任何凸几何
Physijs.ConcaveMesh - 任何凹面几何，即任意网格
Physijs.HeightfieldMesh - 匹配z坐标中给出的高度值的常规网格

更新和回调

因为Physijs在与主应用程序不同的线程上运行，所以不能保证每次调用时都可以迭代场景scene.simulate。因此，您可以将事件侦听器附加到运行物理模拟时触发的场景。

var scene = new Physijs.scene;
scene.addEventListener( 'update', function() {
    // the scene's physics have finished updating
});

添加一个对象

var readyHandler = function() {
    // object has been added to the scene
};
var mesh = new Physijs.SphereMesh( geometry, material );
mesh.addEventListener( 'ready', readyHandler );
scene.add( mesh );

碰撞检测

实例代码：

var mesh = new Physijs.SphereMesh(
    new THREE.SphereGeometry( 3 ),
    new THREE.MeshBasicMaterial({ color: 0x888888 })
);
mesh.addEventListener( 'collision', function( other_object, relative_velocity, relative_rotation, contact_normal ) {
    // `this` has collided with `other_object` with an impact speed of `relative_velocity` and a rotational force of `relative_rotation` and at normal `contact_normal`
});

其中碰撞事件的四个参数分别是：
相撞物体,速度差，角速度的差,接触之间

Motion Clamping(运动限制?)

后续研究

// Enable CCD if the object moves more than 1 meter in one simulation frame
//在一个模拟框架中，如果物体移动超过1米，就启用CCD
mesh.setCcdMotionThreshold(1);

// Set the radius of the embedded sphere such that it is smaller than the object
//设置嵌入球体的半径，使其小于物体的半径。
mesh.setCcdSweptSphereRadius(0.2);

复合形状

复合形状是将复杂几何添加到场景中的有效方法，并通过将Physijs中的可用形状拼接在一起来创建更大，更复杂的几何体来创建。

var parent = new Physijs.BoxMesh( new THREE.CubeGeometry( 5, 5, 5 ), new THREE.MeshBasicMaterial({ color: 0x888888 }) );

var child = new Physijs.SphereMesh( new THREE.SphereGeometry( 2.5 ), new THREE.MeshBasicMaterial({ color: 0x888888 }) );
child.position.z = 5;

parent.add( child );
scene.add( parent );

约束

点对点

var constraint = new Physijs.PointConstraint(
    physijs_mesh_a, // First object to be constrained
    physijs_mesh_b, // OPTIONAL second object - if omitted then physijs_mesh_1 will be constrained to the scene
    new THREE.Vector3( 0, 10, 0 ) // point in the scene to apply the constraint
);
scene.addConstraint( constraint );

铰链约束

var constraint = new Physijs.HingeConstraint(
    physijs_mesh_a, // First object to be constrained
    physijs_mesh_b, // OPTIONAL second object - if omitted then physijs_mesh_1 will be constrained to the scene
    new THREE.Vector3( 0, 10, 0 ), // point in the scene to apply the constraint
    new THREE.Vector3( 1, 0, 0 ) // Axis along which the hinge lies - in this case it is the X axis
);
scene.addConstraint( constraint );
constraint.setLimits(
    low, // minimum angle of motion, in radians
    high, // maximum angle of motion, in radians
    bias_factor, // applied as a factor to constraint error
    relaxation_factor, // controls bounce at limit (0.0 == no bounce)
);
constraint.enableAngularMotor( target_velocity, acceration_force );
constraint.disableMotor();

滑块约束

var constraint = new Physijs.SliderConstraint(
    physijs_mesh_a, // First object to be constrained
    physijs_mesh_b, // OPTIONAL second object - if omitted then physijs_mesh_1 will be constrained to the scene
    new THREE.Vector3( 0, 10, 0 ), // point in the scene to apply the constraint
    new THREE.Vector3( 1, 0, 0 ) // Axis along which the hinge lies - in this case it is the X axis
);
scene.addConstraint( constraint );
constraint.setLimits(
    linear_lower, // lower limit of linear movement, expressed in world units
    linear_upper, // upper limit of linear movement, expressed in world units
    angular_lower, // lower limit of angular movement, expressed in radians
    angular_upper // upper limit of angular movement, expressed in radians
);
constraint.setRestitution(
    linear, // amount of restitution when reaching the linear limits
    angular // amount of restitution when reaching the angular limits
);
constraint.enableLinearMotor( target_velocity, acceration_force );
constraint.disableLinearMotor();
constraint.enableAngularMotor( target_velocity, acceration_force );
constraint.disableAngularMotor();

锥形约束

var constraint = new Physijs.ConeTwistConstraint(
    physijs_mesh_a, // First object to be constrained
    physijs_mesh_b, // Second object to be constrained
    new THREE.Vector3( 0, 10, 0 ), // point in the scene to apply the constraint
);
scene.addConstraint( constraint );
constraint.setLimit( x, y, z ); // rotational limit, in radians, for each axis
constraint.setMotorMaxImpulse( max_impulse ); // float value of the maximum impulse the motor can apply toward its target
constraint.setMotorTarget( target ); // target is the desired rotation for the constraint and can be expressed by a THREE.Vector3, THREE.Matrix4, or THREE.Quaternion
constraint.enableMotor();
constraint.disableMotor();

自由度约束

var constraint = new Physijs.DOFConstraint(
    physijs_mesh_a, // First object to be constrained
    physijs_mesh_b, // OPTIONAL second object - if omitted then physijs_mesh_1 will be constrained to the scene
    new THREE.Vector3( 0, 10, 0 ), // point in the scene to apply the constraint
);
scene.addConstraint( constraint );
constraint.setLinearLowerLimit( new THREE.Vector3( -10, -5, 0 ) ); // sets the lower end of the linear movement along the x, y, and z axes.
constraint.setLinearUpperLimit( new THREE.Vector3( 10, 5, 0 ) ); // sets the upper end of the linear movement along the x, y, and z axes.
constraint.setAngularLowerLimit( new THREE.Vector3( 0, -Math.PI, 0 ) ); // sets the lower end of the angular movement, in radians, along the x, y, and z axes.
constraint.setAngularUpperLimit( new THREE.Vector3( 0, Math.PI, 0 ) ); // sets the upper end of the angular movement, in radians, along the x, y, and z axes.
constraint.configureAngularMotor(
    which, // which angular motor to configure - 0,1,2 match x,y,z
    low_limit, // lower limit of the motor
    high_limit, // upper limit of the motor
    velocity, // target velocity
    max_force // maximum force the motor can apply
);
constraint.enableAngularMotor( which ); // which angular motor to configure - 0,1,2 match x,y,z
constraint.disableAngularMotor( which ); // which angular motor to configure - 0,1,2 match x,y,z

冻结一个对象

可以使用两种方法来使对象冻结或不可移动。

1. 如果对象始终是静态的，例如地面，则可以0使用第三个参数创建网格时将其设置为质量：new Physijs.BoxMesh( geometry, material, 0)。任何具有质量的对象0将永远是静态的。
2. 用于对象在某些时候是静态的，并且在其他方​​面是动态的。
   The second method can be used for objects when they will be static at some times and dynamic at others, like in the jenga example. If you call object.setAngularFactor
   and object.setLinearFactor
   with a THREE.Vector3( 0, 0, 0 )
   then no energy will be applied to the object. You can use object.setAngularVelocity
   and object.setLinearVelocity
   in the same way to clear any velocities the object may already have. At a later point you can reset the object's linear and angular factors to ( 1, 1, 1 )
   , again as it's done in the jenga example.

材质Materials

在THREE材质基础上增加了摩擦度和恢复度

var friction = 0.8; // 摩擦度
var restitution = 0.3; // 恢复度
var material = Physijs.createMaterial(
    new THREE.MeshBasicMaterial({ color: 0x888888 }),
    friction,
    restitution
);
var mesh = new Physijs.BoxMesh(
    new THREE.CubeGeometry( 5, 5, 5 ),
    material
);

暂停/恢复模拟

var render = function() {
    if (!isPaused) {
        scene.simulate();
    }
    renderer.render();
};
var unpauseSimulation = function() {
    isPaused = false;
    scene.onSimulationResume();
};

恢复模拟需要调用场景的onSimulationResume方法.

场景配置

var scene = new Physijs.Scene({ reportsize: 50, fixedTimeStep: 1 / 60 });

1. fixedTimeStep default=1/60 此数字确定模拟步骤的模拟时间。数字越小，模拟越准确
2. broadphase 指定将使用哪个宽带，选择是dynamic和sweepprune。
3. reportsize default 50 作为优化，包含对象位置的世界报告基于此数字预先初始化。最好将其设置为您的场景将具有的对象数量。
4. setGravity方法 default ( 0, -10, 0 ) 设定重力的数量和方向
5. setFixedTimeStep 在构造函数中default 1 / 60 重置fixedTimeStep给定的值

更新对象的位置和旋转

有一个方面，无法与three.js进行无缝集成：更改对象的位置和/或旋转。如果这样做，您必须将该对象__dirtyPosition或__dirtyRotation标志设置为true，否则将从模拟中的最后一个已知值覆盖。

var mesh = new Physijs.BoxMesh( geometry, material );
scene.add( mesh );

var render = function() {
    // Change the object's position
    mesh.position.set( 0, 0, 0 );
    mesh.__dirtyPosition = true;

    // Change the object's rotation
    mesh.rotation.set(0, 90, 180);
    mesh.__dirtyRotation = true;
    
    // You may also want to cancel the object's velocity
    mesh.setLinearVelocity(new THREE.Vector3(0, 0, 0));
    mesh.setAngularVelocity(new THREE.Vector3(0, 0, 0));
    
    scene.simulate();
    renderer.render();
};