【瞎折腾/3D】无父物体下物体的旋转与移动

说在前面

测试环境：Microsoft Edge 120.0.2210.91
three.js版本：0.160.0
其他：本篇文章中只探讨了无父对象下的移动与旋转，有父对象的情况将在下篇文章中讨论
新年快乐！

移动

World Space

在世界坐标系中移动时，直接对position进行运算即可
```
const translationSpeed = new THREE.Vector3(0.5, 0, 0);
// delta 为时间间隔
cubeB.position.x += translationSpeed.x*delta;
cubeB.position.y += translationSpeed.y*delta;
cubeB.position.z += translationSpeed.z*delta;
```
在three.js中，position为局部坐标(local position)。当物体没有父物体时，局部坐标与世界坐标相等，所以此时可以直接操作position，达到在世界坐标系中移动的效果。
在下面的例子中，我们让物体先让Z轴旋转60°，再沿着世界坐标轴X移动

Local Space

在局部坐标系中移动时，由于物体本身可能会旋转，所以需要进行一定的转换，在three.js中，我们可以使用translateX方法：

translateOnAxis( axis, distance ) {
	// translate object by distance along axis in object space
	// axis is assumed to be normalized
	// 也就是应用了物体本身的旋转角度(local space)
	_v1.copy( axis ).applyQuaternion( this.quaternion );
	this.position.add( _v1.multiplyScalar( distance ) );
	return this;
}

translateX( distance ) {
	return this.translateOnAxis( _xAxis, distance );
}

所以我们可以这样写：

const translationSpeed = new THREE.Vector3(0.5, 0, 0);

cubeB.translateX(translationSpeed.x*delta);
cubeB.translateY(translationSpeed.y*delta);
cubeB.translateZ(translationSpeed.z*delta);

在下面的例子中，我们让物体先让Z轴旋转60°，再沿着局部坐标轴X移动

旋转

World Space

在three.js中，旋转提供了方法供我们使用：
```
rotateOnWorldAxis( axis, angle ) {
	// rotate object on axis in world space
	// axis is assumed to be normalized
	// method assumes no rotated parent
	_q1.setFromAxisAngle( axis, angle );
	this.quaternion.premultiply( _q1 );
	return this;
}
```
我们可以使用四元数来表示旋转，那么世界坐标系中的旋转公式为：
$Q_{new}=Q_{delta}Q_{old}$
注意公式里变化量在前
故而，世界坐标系下的旋转代码如下：
```
const rotationSpeed = new THREE.Vector3(0, Math.PI/3, 0);

cubeB.rotateOnWorldAxis(_xAxis, rotationSpeed.x*delta);
cubeB.rotateOnWorldAxis(_yAxis, rotationSpeed.y*delta);
cubeB.rotateOnWorldAxis(_zAxis, rotationSpeed.z*delta);
```
在下面的例子中，我们让物体先让Z轴旋转60°，再绕着世界坐标轴Y旋转

Local Space

在three.js中，旋转提供了方法供我们局部坐标轴旋转：
```
rotateOnAxis( axis, angle ) {
	// rotate object on axis in object space
	// axis is assumed to be normalized
	_q1.setFromAxisAngle( axis, angle );
	this.quaternion.multiply( _q1 );
	return this;
}
```
同样，我们可以使用四元数来表示旋转，局部坐标系中的旋转公式为：
$Q_{new}=Q_{old}Q_{delta}$
注意公式里变化量在后
因此，局部坐标系下的旋转代码可以是：
```
const rotationSpeed = new THREE.Vector3(0, Math.PI/3, 0);

cubeB.rotateX(rotationSpeed.x*delta);
cubeB.rotateY(rotationSpeed.y*delta);
cubeB.rotateZ(rotationSpeed.z*delta);
```
在下面的例子中，我们让物体先让Z轴旋转60°，再绕着局部坐标轴Y旋转

代码

import * as THREE from 'three';
import { OrbitControls } from 'three/addons/controls/OrbitControls.js';

let camera, scene, renderer;

scene = new THREE.Scene();

renderer = new THREE.WebGLRenderer({ antialias: true });
renderer.setPixelRatio(window.devicePixelRatio);
renderer.setSize(window.innerWidth, window.innerHeight);
document.body.appendChild(renderer.domElement);

// camera
camera = new THREE.PerspectiveCamera(40, window.innerWidth / window.innerHeight, 1, 1000);
camera.position.set(15, 20, 30);
scene.add(camera);

// controls
const controls = new OrbitControls(camera, renderer.domElement);
controls.minDistance = 20;
controls.maxDistance = 50;
controls.maxPolarAngle = Math.PI / 2;

// ambient light
scene.add(new THREE.AmbientLight(0x666666));

// point light
const light = new THREE.PointLight(0xffffff, 3, 0, 0);
camera.add(light);

// helper
scene.add(new THREE.AxesHelper(20));

// textures
const loader = new THREE.TextureLoader();
const texture = loader.load('textures/sprites/disc.png');
texture.colorSpace = THREE.SRGBColorSpace;

// CubeA
const meshA = new THREE.BoxGeometry(1, 1, 1);
const mateA = new THREE.MeshNormalMaterial();
const cubeA = new THREE.Mesh(meshA, mateA);
scene.add(cubeA);

// CubeB
const meshB = new THREE.BoxGeometry(1, 1, 1);
const mateB = new THREE.MeshNormalMaterial();
const cubeB = new THREE.Mesh(meshA, mateA);
scene.add(cubeB);

cubeB.position.x = 2;
cubeB.rotateZ(Math.PI/3);
cubeB.add(new THREE.AxesHelper(4));

window.addEventListener('resize', onWindowResize);

const translationSpeed = new THREE.Vector3(0.5, 0, 0);
const rotationSpeed = new THREE.Vector3(0, Math.PI/3, 0);

let preTime = Date.now();
let curTime = preTime;
let delta;

const _xAxis = /*@__PURE__*/ new THREE.Vector3( 1, 0, 0 );
const _yAxis = /*@__PURE__*/ new THREE.Vector3( 0, 1, 0 );
const _zAxis = /*@__PURE__*/ new THREE.Vector3( 0, 0, 1 );

animate();

function onWindowResize() {

    camera.aspect = window.innerWidth / window.innerHeight;
    camera.updateProjectionMatrix();

    renderer.setSize(window.innerWidth, window.innerHeight);

}

function animate() {

    requestAnimationFrame(animate);

    curTime = Date.now();
    delta = (curTime - preTime)/1000;
    preTime = curTime;

    // cubeB.position.x += translationSpeed.x*delta;
    // cubeB.position.y += translationSpeed.y*delta;
    // cubeB.position.z += translationSpeed.z*delta;

    cubeB.translateX(translationSpeed.x*delta);
    cubeB.translateY(translationSpeed.y*delta);
    cubeB.translateZ(translationSpeed.z*delta);

    // cubeB.rotateX(rotationSpeed.x*delta);
    // cubeB.rotateY(rotationSpeed.y*delta);
    // cubeB.rotateZ(rotationSpeed.z*delta);

    cubeB.rotateOnWorldAxis(_xAxis, rotationSpeed.x*delta);
    cubeB.rotateOnWorldAxis(_yAxis, rotationSpeed.y*delta);
    cubeB.rotateOnWorldAxis(_zAxis, rotationSpeed.z*delta);

    render();

}

function render() {

    renderer.render(scene, camera);

}