threejs 空心圆柱

你可以使用Three.js的CylinderGeometry对象来创建一个圆柱体，然后将其转换为空心圆柱。具体步骤如下：

1. 创建CylinderGeometry对象，设置其半径、高度、分段数和是否闭合等参数。例如：

const radiusTop = 1;
const radiusBottom = 1;
const height = 2;
const radialSegments = 32;
const heightSegments = 1;
const openEnded = true;

const cylinderGeometry = new THREE.CylinderGeometry(
  radiusTop,
  radiusBottom,
  height,
  radialSegments,
  heightSegments,
  openEnded
);

2. 将圆柱体的顶部和底部面的法向量翻转，使其朝向内部。这样圆柱体就成了空心的。例如：

const faceVertexUvs = cylinderGeometry.faceVertexUvs[0];
const n = faceVertexUvs.length;

for (let i = 0; i < n; i++) {
  const uvs = faceVertexUvs[i];

  const face = cylinderGeometry.faces[i];
  if (face.normal.y === 1) {
    uvs[0].set(0, 0);
    uvs[1].set(0, 1);
    uvs[2].set(1, 1);
    face.normal.multiplyScalar(-1);
    face.vertexNormals.forEach((vn) => vn.multiplyScalar(-1));
  } else if (face.normal.y === -1) {
    uvs[0].set(1, 1);
    uvs[1].set(0, 1);
    uvs[2].set(0, 0);
    face.normal.multiplyScalar(-1);
    face.vertexNormals.forEach((vn) => vn.multiplyScalar(-1));
  }
}

3. 创建Mesh对象，将CylinderGeometry对象和Material对象传入构造函数中。例如：

const material = new THREE.MeshBasicMaterial({ color: 0xff0000 });
const cylinderMesh = new THREE.Mesh(cylinderGeometry, material);

4. 将Mesh对象添加到场景中。例如：

scene.add(cylinderMesh);

这样就创建了一个空心圆柱体。你可以根据需要设置其位置、旋转等属性。