threejs样例：GPU拾取
文章链接：https://threejs.org/manual/#zh/indexed-textures
案例：https://threejs.org/manual/examples/picking-gpu.html
这是因为 JavaScript 无法通过简单的查看纹理和材质，就推测出你的对象是否存在一部分是透明的或者不透明。
对于这些问题的解决方案，就是使用基于GPU的拾取方法。可惜，该方法概念上简单，但是相比于射线追踪法，用起来就复杂了。
为了完成GPU拾取，对每一个对象使用唯一的颜色进行离屏渲染。然后，检查鼠标位置关联的像素的颜色。这个颜色就能告诉我们哪个对象被选中。
这能解决上面的问题2，3。至于问题1的速度问题，这取决于业务场景。每个对象会被绘制两次，一次用于观看，一次用于拾取。也许存在开脑洞的解决方案，可以只绘制一次就完成查看和拾取，此处我们不会尝试。
但是有一件事值得去做，因为拾取时我们只需读取1px，所以我们可以设置摄像机，只绘制1px，通过 PerspectiveCamera.setViewOffset 方法，可以告诉THREE.js 计算出一个摄像机 只呈现一个大矩形的一个很小的部分。这应该能节省一些运行时间。
此时，要在THREE.js中实现这种拾取方式，需要创建两个场景。一个使用正常的网格对象填充。另外一个使用“拾取材质”的网格对象填充。
<!-- Licensed under a BSD license. See license.html for license -->
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
<title>Three.js - Picking - RayCaster w/Transparency</title>
<style>
html, body {
height: 100%;
margin: 0;
}
#c {
width: 100%;
height: 100%;
display: block;
}
</style>
</head>
<body>
<canvas id="c"></canvas>
</body>
<script type="importmap">
{
"imports": {
"three": "../../build/three.module.js"
}
}
</script>
<script type="module">
import * as THREE from 'three';
function main() {
const canvas = document.querySelector( '#c' );
const renderer = new THREE.WebGLRenderer( { antialias: true, canvas } );
const fov = 60;
const aspect = 2; // the canvas default
const near = 0.1;
const far = 200;
const camera = new THREE.PerspectiveCamera( fov, aspect, near, far );
camera.position.z = 30;
const scene = new THREE.Scene();
scene.background = new THREE.Color( 'white' );
const pickingScene = new THREE.Scene();
pickingScene.background = new THREE.Color( 0 );
// put the camera on a pole (parent it to an object)
// so we can spin the pole to move the camera around the scene
const cameraPole = new THREE.Object3D();
scene.add( cameraPole );
cameraPole.add( camera );
{
const color = 0xFFFFFF;
const intensity = 3;
const light = new THREE.DirectionalLight( color, intensity );
light.position.set( - 1, 2, 4 );
camera.add( light );
}
const boxWidth = 1;
const boxHeight = 1;
const boxDepth = 1;
const geometry = new THREE.BoxGeometry( boxWidth, boxHeight, boxDepth );
function rand( min, max ) {
if ( max === undefined ) {
max = min;
min = 0;
}
return min + ( max - min ) * Math.random();
}
function randomColor() {
return `hsl(${rand( 360 ) | 0}, ${rand( 50, 100 ) | 0}%, 50%)`;
}
const loader = new THREE.TextureLoader();
const texture = loader.load( 'resources/images/frame.png' );
const idToObject = {};
const numObjects = 100;
for ( let i = 0; i < numObjects; ++ i ) {
const id = i + 1;
const material = new THREE.MeshPhongMaterial( {
color: randomColor(),
map: texture,
transparent: true,
side: THREE.DoubleSide,
alphaTest: 0.5,
} );
const cube = new THREE.Mesh( geometry, material );
scene.add( cube );
idToObject[ id ] = cube;
cube.position.set( rand( - 20, 20 ), rand( - 20, 20 ), rand( - 20, 20 ) );
cube.rotation.set( rand( Math.PI ), rand( Math.PI ), 0 );
cube.scale.set( rand( 3, 6 ), rand( 3, 6 ), rand( 3, 6 ) );
const pickingMaterial = new THREE.MeshPhongMaterial( {
emissive: new THREE.Color().setHex( id, THREE.NoColorSpace ),
color: new THREE.Color( 0, 0, 0 ),
specular: new THREE.Color( 0, 0, 0 ),
map: texture,
transparent: true,
side: THREE.DoubleSide,
alphaTest: 0.5,
blending: THREE.NoBlending,
} );
const pickingCube = new THREE.Mesh( geometry, pickingMaterial );
pickingScene.add( pickingCube );
pickingCube.position.copy( cube.position );
pickingCube.rotation.copy( cube.rotation );
pickingCube.scale.copy( cube.scale );
}
function resizeRendererToDisplaySize( renderer ) {
const canvas = renderer.domElement;
const width = canvas.clientWidth;
const height = canvas.clientHeight;
const needResize = canvas.width !== width || canvas.height !== height;
if ( needResize ) {
renderer.setSize( width, height, false );
}
return needResize;
}
class GPUPickHelper {
constructor() {
// create a 1x1 pixel render target
this.pickingTexture = new THREE.WebGLRenderTarget( 1, 1 );
this.pixelBuffer = new Uint8Array( 4 );
this.pickedObject = null;
this.pickedObjectSavedColor = 0;
}
pick( cssPosition, scene, camera, time ) {
const { pickingTexture, pixelBuffer } = this;
// restore the color if there is a picked object
if ( this.pickedObject ) {
this.pickedObject.material.emissive.setHex( this.pickedObjectSavedColor );
this.pickedObject = undefined;
}
// set the view offset to represent just a single pixel under the mouse
const pixelRatio = renderer.getPixelRatio();
camera.setViewOffset(
renderer.getContext().drawingBufferWidth, // full width
renderer.getContext().drawingBufferHeight, // full top
cssPosition.x * pixelRatio | 0, // rect x
cssPosition.y * pixelRatio | 0, // rect y
1, // rect width
1, // rect height
);
// render the scene
renderer.setRenderTarget( pickingTexture );
renderer.render( scene, camera );
renderer.setRenderTarget( null );
// clear the view offset so rendering returns to normal
camera.clearViewOffset();
//read the pixel
renderer.readRenderTargetPixels(
pickingTexture,
0, // x
0, // y
1, // width
1, // height
pixelBuffer );
const id =
( pixelBuffer[ 0 ] << 16 ) |
( pixelBuffer[ 1 ] << 8 ) |
( pixelBuffer[ 2 ] );
const intersectedObject = idToObject[ id ];
if ( intersectedObject ) {
// pick the first object. It's the closest one
this.pickedObject = intersectedObject;
// save its color
this.pickedObjectSavedColor = this.pickedObject.material.emissive.getHex();
// set its emissive color to flashing red/yellow
this.pickedObject.material.emissive.setHex( ( time * 8 ) % 2 > 1 ? 0xFFFF00 : 0xFF0000 );
}
}
}
const pickPosition = { x: 0, y: 0 };
const pickHelper = new GPUPickHelper();
clearPickPosition();
function render( time ) {
time *= 0.001; // convert to seconds;
if ( resizeRendererToDisplaySize( renderer ) ) {
const canvas = renderer.domElement;
camera.aspect = canvas.clientWidth / canvas.clientHeight;
camera.updateProjectionMatrix();
}
cameraPole.rotation.y = time * .1;
pickHelper.pick( pickPosition, pickingScene, camera, time );
renderer.render( scene, camera );
requestAnimationFrame( render );
}
requestAnimationFrame( render );
function getCanvasRelativePosition( event ) {
const rect = canvas.getBoundingClientRect();
return {
x: ( event.clientX - rect.left ) * canvas.width / rect.width,
y: ( event.clientY - rect.top ) * canvas.height / rect.height,
};
}
function setPickPosition( event ) {
const pos = getCanvasRelativePosition( event );
pickPosition.x = pos.x;
pickPosition.y = pos.y;
}
function clearPickPosition() {
// unlike the mouse which always has a position
// if the user stops touching the screen we want
// to stop picking. For now we just pick a value
// unlikely to pick something
pickPosition.x = - 100000;
pickPosition.y = - 100000;
}
window.addEventListener( 'mousemove', setPickPosition );
window.addEventListener( 'mouseout', clearPickPosition );
window.addEventListener( 'mouseleave', clearPickPosition );
window.addEventListener( 'touchstart', ( event ) => {
// prevent the window from scrolling
event.preventDefault();
setPickPosition( event.touches[ 0 ] );
}, { passive: false } );
window.addEventListener( 'touchmove', ( event ) => {
setPickPosition( event.touches[ 0 ] );
} );
window.addEventListener( 'touchend', clearPickPosition );
}
main();
</script>
</html>
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