geometry clip 的实现

    const geometry = new THREE.BoxGeometry( 10, 10, 10 );
    var mat = new THREE.ShaderMaterial({
        uniforms: {},
        defaultAttributeValues : {},
        vertexShader: `
            varying float flag; 

            void main(){
                float a = 3.141592653589 / 4.0;
                vec3 axis = vec3(1.0,0.0,0.0);
                mat3 rMat= mat3(
                    cos(a), -sin(a), 0.0,
                    sin(a), cos(a), 0.0,
                    0.0, 0.0, 1.0
                );
                flag = dot(vec3(position.x - 5.0,position.yz),rMat * axis);
                gl_Position = projectionMatrix * modelViewMatrix * vec4(position,1.0);
            }
        `,
        fragmentShader: `
            varying float flag; 

            void main(){
                if(flag > 0.0){
                    discard;
                }
                gl_FragColor = vec4(1.0,1.0,1.0,0.5);
            }
        `,
        transparent: true,
        side:THREE.DoubleSide,
        depthTest:true,
        depthWrite:false,
    });
    const cube = new THREE.Mesh( geometry, mat );
    cube.position.y = 20;
    scene.add( cube );