HTML5 canvas如何绘制酷炫能量线条效果（附代码）

// UTILconst PI = Math.PI,  TWO_PI = Math.PI * 2;const Util = {};Util.timeStamp = function() {  return window.performance.now();};Util.random = function(min, max) {  return min + Math.random() * (max - min);};Util.map = function(a, b, c, d, e) {  return (a - b) / (c - b) * (e - d) + d;};Util.lerp = function(value1, value2, amount) {  return value1 + (value2 - value1) * amount;};Util.clamp = function(value, min, max) {  return Math.max(min, Math.min(max, value));};// Vectorclass Vector {  constructor(x, y) {    this.x = x || 0;    this.y = y || 0;  }  set(x, y) {    this.x = x;    this.y = y;  }  reset() {    this.x = 0;    this.y = 0;  }  fromAngle(angle) {    let x = Math.cos(angle),      y = Math.sin(angle);    return new Vector(x, y);  }  add(vector) {    this.x += vector.x;    this.y += vector.y;  }  sub(vector) {    this.x -= vector.x;    this.y -= vector.y;  }  mult(scalar) {    this.x *= scalar;    this.y *= scalar;  }  p(scalar) {    this.x /= scalar;    this.y /= scalar;  }  dot(vector) {    return vector.x * this.x + vector.y * this.y;  }  limit(limit_value) {    if (this.mag() > limit_value) this.setMag(limit_value);  }  mag() {    return Math.hypot(this.x, this.y);  }  setMag(new_mag) {    if (this.mag() > 0) {      this.normalize();    } else {      this.x = 1;      this.y = 0;    }    this.mult(new_mag);  }  normalize() {    let mag = this.mag();    if (mag > 0) {      this.x /= mag;      this.y /= mag;    }  }  heading() {    return Math.atan2(this.y, this.x);  }  setHeading(angle) {    let mag = this.mag();    this.x = Math.cos(angle) * mag;    this.y = Math.sin(angle) * mag;  }  dist(vector) {    return new Vector(this.x - vector.x, this.y - vector.y).mag();  }  angle(vector) {    return Math.atan2(vector.y - this.y, vector.x - this.x);  }  copy() {    return new Vector(this.x, this.y);  }}// Init canvaslet canvas = document.createElement("canvas"),  ctx = canvas.getContext("2d"),  H = (canvas.height = window.innerHeight),  W = (canvas.width = window.innerWidth);document.body.appendChild(canvas);// Mouselet mouse = {  x: W/2,  y: H/2};canvas.onmousemove = function(event) {  mouse.x = event.clientX - canvas.offsetLeft;  mouse.y = event.clientY - canvas.offsetTop;};document.body.onresize = function(event){  H = (canvas.height = window.innerHeight);  W = (canvas.width = window.innerWidth);}// Let&#39;s goclass Arrow {  constructor(x, y, target) {    this.position = new Vector(x, y);    this.velocity = new Vector().fromAngle(Util.random(0,TWO_PI));    this.acceleration = new Vector(0, 0);    this.target = target;    this.travelled_distance = 0;    this.min_size = 1;    this.max_size = 6;    this.size = Util.random(this.min_size, this.max_size);    this.zone = this.size * 4;    this.topSpeed = Util.map(this.size,this.min_size,this.max_size,40,10);    let tailLength = Math.floor(Util.map(this.size, this.min_size, this.max_size, 4, 16));    this.tail = [];    for (let i = 0; i < tailLength; i++) {      this.tail.push({        x: this.position.x,        y: this.position.y      });    }    this.wiggle_speed = Util.map(this.size, this.min_size, this.max_size, 2 , 1.2);    this.blink_offset = Util.random(0, 100);    this.alpha = Util.random(0.1,1)  }  render() {    this.update();    this.draw();  }  update() {    let old_position = this.position.copy();    // Focus on target    let t = new Vector(this.target.x, this.target.y),      angle = this.position.angle(t);    let d_f_target = t.dist(this.position);      let f = new Vector().fromAngle(angle);      f.setMag(Util.map(Util.clamp(d_f_target,0,400), 0, 400, 0, this.topSpeed * 0.1));      this.addForce(f);        // Update position and velocity    this.velocity.add(this.acceleration);    if(d_f_target < 800){       this.velocity.limit(Util.map(Util.clamp(d_f_target,0,800), 0, 800, this.topSpeed*0.4, this.topSpeed));       }else{         this.velocity.limit(this.topSpeed);       }    this.position.add(this.velocity);    // Reset acceleration for the next loop    this.acceleration.mult(0);    this.travelled_distance += old_position.dist(this.position);      let wiggle =        Math.sin(frame * this.wiggle_speed) *        Util.map(this.velocity.mag(), 0, this.topSpeed, 0, this.size);      let w_a = this.velocity.heading() + Math.PI / 2;      let w_x = this.position.x + Math.cos(w_a) * wiggle,        w_y = this.position.y + Math.sin(w_a) * wiggle;      this.travelled_distance = 0;      let from = this.tail.length - 1,        to = 0;          let n = new Vector().fromAngle(Util.random(0,TWO_PI));      n.setMag(Math.random()*this.size);          var tail = { x: w_x+ n.x, y: w_y + n.y};      this.tail.splice(from, 1);      this.tail.splice(to, 0, tail);      }  draw() {            let energy = Util.map(this.velocity.mag(),0,this.topSpeed,0.1,1);        let color =      "hsl("+Math.sin((frame + this.blink_offset) * 0.1) * 360+",50%,"+                Util.map(this.velocity.mag(),0,this.topSpeed,40,100) * this.alpha        +"%)";    ctx.globalAlpha = this.alpha;    ctx.strokeStyle = color;    for (let i = 0; i < this.tail.length - 1; i++) {      let t = this.tail[i],        next_t = this.tail[i + 1];      ctx.lineWidth = Util.map(i, 0, this.tail.length - 1, this.size, 1);      ctx.beginPath();      ctx.moveTo(t.x, t.y);      ctx.lineTo(next_t.x, next_t.y);      ctx.closePath();      ctx.stroke();    }        let gradient_size = 140 * energy;var grd = ctx.createRadialGradient(  this.position.x,this.position.y , 5,  this.position.x,this.position.y, gradient_size);grd.addColorStop(0, "rgba(255,255,255,0.01)");grd.addColorStop(0.1, "rgba(255,120,200,0.02)");grd.addColorStop(0.9, "rgba(255,255,120,0)");grd.addColorStop(1, "rgba(0,0,0,0)");// Fill with gradientctx.fillStyle = grd;ctx.fillRect(this.position.x - gradient_size / 2 ,this.position.y - gradient_size / 2 , gradient_size, gradient_size);         ctx.globalAlpha = energy+0.2;    ctx.fillStyle = "white";    for(let i = 0; i < 4; i++){      let n = new Vector().fromAngle(Util.random(0,TWO_PI));      n.setMag(Math.random()*energy*100);      n.add(this.position);      ctx.beginPath();      ctx.arc(n.x,n.y,Math.random(),0,TWO_PI)      ctx.fill();    }      }  addForce(vector) {    this.acceleration.add(vector);  }  avoid(others) {    others.forEach(other => {      if (other !== this) {        let dist = this.position.dist(other.position),          max_dist = this.zone + other.size;        if (max_dist - dist >= 0) {          let angle = other.position.angle(this.position);          let force = new Vector().fromAngle(angle);          force.setMag(Util.map(dist, 0, max_dist, 2, 0));          this.addForce(force);        }      }    });  }}let arrows = [];for (let i = 0; i < 100; i++) {  arrows.push(new Arrow(W / 2, H / 2, mouse));}let frame = 0;ctx.strokeStyle = "white";function loop() {  ctx.fillStyle="black";  ctx.globalCompositeOperation = "source-over";  ctx.globalAlpha = 0.2;  ctx.fillRect(0, 0, W, H);  ctx.globalAlpha = 1;  ctx.globalCompositeOperation = "lighter";  arrows.forEach(a => {    a.avoid(arrows);  });  arrows.forEach(a => {    a.render();  });  frame += 1;  requestAnimationFrame(loop);}ctx.lineCap = "round";ctx.lineJoin = "round";loop();