threejs如何写物体反射

要在Three.js中实现物体反射，需要使用渲染到纹理 (Render To Texture) 的技术。具体步骤如下：

1. 创建一个场景和摄像机，以及一个正常的物体。

var scene = new THREE.Scene();
var camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 1000);
var renderer = new THREE.WebGLRenderer();
renderer.setSize(window.innerWidth, window.innerHeight);
document.body.appendChild(renderer.domElement);

var geometry = new THREE.BoxGeometry(1, 1, 1);
var material = new THREE.MeshBasicMaterial({ color: 0x00ff00 });
var cube = new THREE.Mesh(geometry, material);
scene.add(cube);

2. 创建一个反射平面，并将其放置在场景中，使其与物体相对。

var mirrorPlane = new THREE.Plane(new THREE.Vector3(0, 1, 0), -10);
var mirrorGeometry = new THREE.PlaneGeometry(20, 20, 1);
var mirrorMaterial = new THREE.MeshBasicMaterial({ color: 0xff0000, side: THREE.DoubleSide });
var mirrorMesh = new THREE.Mesh(mirrorGeometry, mirrorMaterial);
mirrorMesh.rotation.x = -Math.PI / 2;
mirrorMesh.position.y = -10;
scene.add(mirrorMesh);

3. 创建一个渲染器，并创建一个纹理，将反射平面渲染到纹理上。

var mirrorRenderer = new THREE.WebGLRenderer({ antialias: true });
mirrorRenderer.setSize(512, 512);
var mirrorTexture = new THREE.WebGLRenderTarget(512, 512);
var mirrorCamera = new THREE.PerspectiveCamera(75, 1, 0.1, 1000);
mirrorCamera.position.set(0, -5, 0);
mirrorCamera.lookAt(0, 0, -1);

mirrorRenderer.render(scene, mirrorCamera, mirrorTexture);

4. 将反射纹理应用到反射平面上，并设置其属性以使其看起来像一个反射镜面。

var mirrorMaterial = new THREE.MeshBasicMaterial({ map: mirrorTexture.texture });
mirrorMaterial.side = THREE.DoubleSide;
mirrorMaterial.transparent = true;
mirrorMaterial.opacity = 0.5;
mirrorMesh.material = mirrorMaterial;

5. 在渲染循环中，对物体进行渲染，并使用反射平面的纹理进行反射。

function animate() {
  requestAnimationFrame(animate);

  cube.rotation.x += 0.01;
  cube.rotation.y += 0.01;

  mirrorRenderer.render(scene, mirrorCamera, mirrorTexture);
  renderer.render(scene, camera);
}

animate();

这样，你就可以在Three.js中实现物体反射了。