使用 Three.js 的 ExtrudeGeometry 创建挤压几何体，实现顶面、底面和侧面的不同颜色分离效果。

class ThreeColorExtrudeGeometry extends THREE.ExtrudeGeometry {
    constructor(shape, options) {
        super(shape, options);
        this.regroupFaces();
    }

    regroupFaces() {
        const groups = this.groups;
        const index = this.index;
        
        if (!index) return;
        
        groups.length = 0;
        const position = this.attributes.position;
        const faceCount = index.count / 3;
        
        for (let i = 0; i < faceCount; i++) {
            const a = index.getX(i * 3);
            const b = index.getX(i * 3 + 1);
            const c = index.getX(i * 3 + 2);
            
            const vA = new THREE.Vector3().fromBufferAttribute(position, a);
            const vB = new THREE.Vector3().fromBufferAttribute(position, b);
            const vC = new THREE.Vector3().fromBufferAttribute(position, c);
            
            const normal = new THREE.Vector3();
            const cb = new THREE.Vector3();
            const ab = new THREE.Vector3();
            
            cb.subVectors(vC, vB);
            ab.subVectors(vA, vB);
            cb.cross(ab);
            normal.copy(cb).normalize();
            
            let materialIndex = 2;
            
            if (Math.abs(normal.y) > 0.9) {
                materialIndex = normal.y > 0 ? 0 : 1;
            }
            
            const start = i * 3;
            const count = 3;
            
            let groupIndex = groups.findIndex(g => g.materialIndex === materialIndex);
            if (groupIndex === -1) {
                groups.push({
                    start: start,
                    count: count,
                    materialIndex: materialIndex
                });
            } else {
                groups[groupIndex].count += count;
            }
        }
        
        groups.sort((a, b) => a.materialIndex - b.materialIndex);
        
        let currentStart = 0;
        for (let i = 0; i < groups.length; i++) {
            groups[i].start = currentStart;
            currentStart += groups[i].count;
        }
    }
}

const materials = [
    new THREE.MeshBasicMaterial({ color: 0xffff00 }), // 顶面
    new THREE.MeshBasicMaterial({ color: 0x00ff00 }), // 底面
    new THREE.MeshBasicMaterial({ color: 0x0000ff })  // 侧面
];