three.js+shader 制作粒子飞线，小蝌蚪那种。

学习three.js四个月了，最近终于利用shader粒子做出了我心心念念的蝌蚪状飞线，这个网上也是查不到什么资料的，2Dcanvas的可以查得到，3D的都藏着掖着很难找的到相关案例，最近我通过腾讯一个小姐姐发布几个three.js 案例受到了一点启发，终于花了一个晚上做了出来，也是第一次在three.js中使用shader。

下面是效果图：

先说说思路，第一步是制作一个蝌蚪状的粒子束，其实很简单就是让粒子一个一个从小到大排列就好，这部分主要利用shader处理，代码如下：

const vs:string = `

      attribute float size; // 顶点尺寸

      attribute vec4 colors; //顶点颜色

      varying float opacity; // 控制透明度

      varying vec3 vexColor; // 顶点颜色

      void main(){

          vexColor.x = colors.r;

          vexColor.y = colors.g;

          vexColor.z = colors.b;

          //w分量为透明度

          opacity = colors.w;

          vec4 mvPosition = modelViewMatrix * vec4(position,1.0); //?这里模型矩阵，坐标向量，和投影矩阵都是three给你注入的好像。

          gl_PointSize = size;

          gl_Position = projectionMatrix * mvPosition;

      }

`;

然后就是配置数据源，数据源可以利用three.js 给的curve3组件中的getPoints取得，代码如下

this.spline = new THREE.CatmullRomCurve3(vecs);

    this.pointNum = num;

    this.distance = this.spline.getLength();

    //初始化粒子

    this.points = this.spline.getPoints(num);

    const colorsLen = this.points.length * 4;

    const sizeLen = this.points.length;

    const colors:Float32Array = new Float32Array(colorsLen);

    const sizes:Float32Array = new Float32Array(sizeLen);

    this.geometry = new THREE.BufferGeometry().setFromPoints(this.points);

    for(let i=0,z=0;i<colorsLen;i+=4,z++){

      //color

      colors[i] = color.r;

      colors[i+1] = color.g;

      colors[i+2] = color.b;

      // opacity

      colors[i+3] = (i+3)/sizeLen;

      // size从小到大

      sizes[z] =size*(z/sizeLen);

    };

    this.geometry.addAttribute('colors',new THREE.BufferAttribute(colors,4));

    this.geometry.addAttribute('size',new THREE.BufferAttribute(sizes,1));

第二部分就是如何让粒子运动起来，这里我借助了tween.js，整体思路就是取一段固定的粒子数目+加上Curve3的getPointAt取点函数，不断的取出固定数目并不断前移的坐标，直接修改bufferGeometry的position数据即可。

下面贴出完整代码：

import * as THREE from "three";

import TWEEN from "@tweenjs/tween.js";

const fs:string = `

      uniform sampler2D texture;

      varying float opacity;

      varying vec3 vexColor;

      void main(){

          gl_FragColor = vec4(vexColor,opacity);

          gl_FragColor = gl_FragColor * texture2D(texture,gl_PointCoord);

      }     

`;

const vs:string = `

      attribute float size;

      attribute vec4 colors;

      varying float opacity;

      varying vec3 vexColor;

      void main(){

          vexColor.x = colors.r;

          vexColor.y = colors.g;

          vexColor.z = colors.b;

          //w分量为透明度

          opacity = colors.w;

          vec4 mvPosition = modelViewMatrix * vec4(position,1.0);

          gl_PointSize = size;

          gl_Position = projectionMatrix * mvPosition;

      }

`;

/**

* 粒子飞线

*/

export default class PointsFlyLine{

  //粒子位置

  geometry: THREE.BufferGeometry;

  //曲线

  spline: THREE.CatmullRomCurve3;

  //粒子系统

  particleSystem: THREE.Points;

  //粒子数目

  pointNum:number;

  //粒子间的总距离

  distance: number;

  points: THREE.Vector3[];

  tween: any;

  /**

  * 创建粒子系统

  * @param points 粒子

  * @param size 粒子大小

  * @param num 粒子数目

  * @param color 粒子颜色

  */

  constructor({ vecs, num, size, color }: { vecs: THREE.Vector3[]; num: number; size: number; color: THREE.Color; }){

    this.spline = new THREE.CatmullRomCurve3(vecs);

    this.pointNum = num;

    this.distance = this.spline.getLength();

    //初始化粒子

    this.points = this.spline.getPoints(num);

    const colorsLen = this.points.length * 4;

    const sizeLen = this.points.length;

    const colors:Float32Array = new Float32Array(colorsLen);

    const sizes:Float32Array = new Float32Array(sizeLen);

    this.geometry = new THREE.BufferGeometry().setFromPoints(this.points);

    for(let i=0,z=0;i<colorsLen;i+=4,z++){

      //color

      colors[i] = color.r;

      colors[i+1] = color.g;

      colors[i+2] = color.b;

      // opacity

      colors[i+3] = (i+3)/sizeLen;

      // size从小到大

      sizes[z] =size*(z/sizeLen);

    };

    this.geometry.addAttribute('colors',new THREE.BufferAttribute(colors,4));

    this.geometry.addAttribute('size',new THREE.BufferAttribute(sizes,1));

    const uniforms:object = {

      texture: {

        value: new THREE.CanvasTexture(this.createSpriteCanvas(size)),

      }

    };

    const shaderMaterial:THREE.ShaderMaterial = new THREE.ShaderMaterial({

      uniforms,

      vertexShader:vs,

      fragmentShader:fs,

      transparent:true,

      depthTest:false

    });

    this.particleSystem = new THREE.Points(this.geometry,shaderMaterial);

  }

  //飞线开始

  start(){

    const max = this.distance*10;

    const end:number = this.pointNum;

    const m = {start:0,end};

    this.tween = this.tweenAnimate(m,{start:max-end,end:max},2000,null,()=>{

      let pointArr:number[] = [];

      let s = Math.round(m.start),e = Math.floor(m.end);

      for (let i = s; i <= e && i<=max; i++) {

        pointArr = pointArr.concat(this.spline.getPointAt(i/max).toArray());

      }

      this.geometry.attributes.position = new THREE.BufferAttribute(new Float32Array(pointArr),3);

    });

    this.tween.repeat(Infinity).start();

  }

  stop(){

    this.tween.stop();

  }

  tweenAnimate(current:object, target:object, interval:number, animation?:TWEEN.Easing, onUpdate?:Function, complete?:Function) {

    var animate = animation ? animation : TWEEN.Easing.Linear.None;

    let tween = new TWEEN.Tween(current).to(target, interval).easing(animate);

    onUpdate && tween.onUpdate(() => onUpdate());

    complete && tween.onComplete(() => complete());

    return tween;

  }

  //创建圆形精灵贴图

  createSpriteCanvas(size:number){

    const canvas = document.createElement('canvas');

    canvas.width = canvas.height = size;

    const context = canvas.getContext('2d');

    if(context!=null){

      context.fillStyle='rgba(255,255,255,.0)';

      context.beginPath();

      context.arc(size/2,size/2,size/2,0,Math.PI*2);

      context.fillStyle = 'white';

      context.fill();

      context.closePath();

    }

    return canvas;

  }

}