这是用于在DirectionalLights内部计算阴影
与其他阴影类不同，它是使用OrthographicCamera来计算阴影，而不是PerspectiveCamera。这是因为来自DirectionalLight的光线是平行的。
//Create a WebGLRenderer and turn on shadows in the renderer
const renderer = new v3d.WebGLRenderer();
renderer.shadowMap.enabled = true;
renderer.shadowMap.type = v3d.PCFSoftShadowMap; // default v3d.PCFShadowMap
//Create a DirectionalLight and turn on shadows for the light
const light = new v3d.DirectionalLight(0xffffff, 1, 100);
light.position.set(0, 1, 0); //default; light shining from top
light.castShadow = true; // default false
scene.add(light);
//Set up shadow properties for the light
light.shadow.mapSize.width = 512; // default
light.shadow.mapSize.height = 512; // default
light.shadow.camera.near = 0.5; // default
light.shadow.camera.far = 500; // default
//Create a sphere that cast shadows (but does not receive them)
const sphereGeometry = new v3d.SphereBufferGeometry(5, 32, 32);
const sphereMaterial = new v3d.MeshStandardMaterial({ color: 0xff0000 });
const sphere = new v3d.Mesh(sphereGeometry, sphereMaterial);
sphere.castShadow = true; //default is false
sphere.receiveShadow = false; //default
scene.add(sphere);
//Create a plane that receives shadows (but does not cast them)
const planeGeometry = new v3d.PlaneBufferGeometry(20, 20, 32, 32);
const planeMaterial = new v3d.MeshStandardMaterial({ color: 0x00ff00 })
const plane = new v3d.Mesh(planeGeometry, planeMaterial);
plane.receiveShadow = true;
scene.add(plane);
//Create a helper for the shadow camera (optional)
const helper = new v3d.CameraHelper(light.shadow.camera);
scene.add(helper);
创建一个新的DirectionalLightShadow，不能直接调用-它是在DirectionalLights内部调用使用
参阅LightShadow类来了解常用的基本属性
在光的世界里。这用于生成场景的深度图;从光的角度来看，其他物体背后的物体将处于阴影中。
有关常用方法，请参阅基础LightShadow类。
src/lights/DirectionalLightShadow.js