这是用于在[page:DirectionalLight 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 THREE.WebGLRenderer();
renderer.shadowMap.enabled = true;
renderer.shadowMap.type = THREE.PCFSoftShadowMap; // default THREE.PCFShadowMap
//Create a DirectionalLight and turn on shadows for the light
const light = new THREE.DirectionalLight( 0xffffff, 1 );
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 THREE.SphereGeometry( 5, 32, 32 );
const sphereMaterial = new THREE.MeshStandardMaterial( { color: 0xff0000 } );
const sphere = new THREE.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 THREE.PlaneGeometry( 20, 20, 32, 32 );
const planeMaterial = new THREE.MeshStandardMaterial( { color: 0x00ff00 } )
const plane = new THREE.Mesh( planeGeometry, planeMaterial );
plane.receiveShadow = true;
scene.add( plane );
//Create a helper for the shadow camera (optional)
const helper = new THREE.CameraHelper( light.shadow.camera );
scene.add( helper );
创建一个新的[name]，不能直接调用-它是在[page:DirectionalLight DirectionalLights]内部调用使用
参阅[page:LightShadow 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。
Read-only flag to check if a given object is of type [name].
有关常用方法，请参阅基础[page:LightShadow LightShadow]类。
[link:https://github.com/mrdoob/three.js/blob/master/src/lights/[name].js src/lights/[name].js]