angular2 繪製星空背景圖

製做一個星空背景動圖

參考代碼：http://www.htmleaf.com/Demo/201707294657.html

import {Component, OnInit, HostListener} from "@angular/core";

@Component({
  selector: 'star-background',
  template: `
    <canvas id="canvas" style="position: absolute"></canvas>
  `
})

export class StarBackgroundComponent implements OnInit {


  ngOnInit(): void {
    this.draw();
  }

  draw() {
    var canvas: any = document.getElementById('canvas'),
      ctx = canvas.getContext('2d'),
      w = canvas.width = window.innerWidth,
      h = canvas.height = window.innerHeight,

      hue = 217,
      stars = [],
      count = 0,
      maxStars = 1400;

    // Thanks @jackrugile for the performance tip! http://codepen.io/jackrugile/pen/BjBGoM
    // Cache gradient
    var canvas2 = document.createElement('canvas'),
      ctx2 = canvas2.getContext('2d');
    canvas2.width = 100;
    canvas2.height = 100;
    var half = canvas2.width / 2,
      gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
    gradient2.addColorStop(0.025, '#fff');
    gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
    gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
    gradient2.addColorStop(1, 'transparent');

    ctx2.fillStyle = gradient2;
    ctx2.beginPath();
    ctx2.arc(half, half, half, 0, Math.PI * 2);
    ctx2.fill();

    // End cache

    function random(min, max?) {
      if (arguments.length < 2) {
        max = min;
        min = 0;
      }

      if (min > max) {
        var hold = max;
        max = min;
        min = hold;
      }

      return Math.floor(Math.random() * (max - min + 1)) + min;
    }

    function Star() {

      this.orbitRadius = random(w / 2 - 50);
      this.radius = random(100, this.orbitRadius) / 10;
      this.orbitX = w / 2;
      this.orbitY = h / 2;
      this.timePassed = random(0, maxStars);
      this.speed = random(this.orbitRadius) / 1000000;
      this.alpha = random(2, 10) / 10;

      count++;
      stars[count] = this;
    }

    Star.prototype.draw = function () {
      var x = Math.sin(this.timePassed + 1) * this.orbitRadius + this.orbitX,
        y = Math.cos(this.timePassed) * this.orbitRadius / 2 + this.orbitY,
        twinkle = random(10);

      if (twinkle === 1 && this.alpha > 0) {
        this.alpha -= 0.05;
      } else if (twinkle === 2 && this.alpha < 1) {
        this.alpha += 0.05;
      }

      ctx.globalAlpha = this.alpha;
      ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
      this.timePassed += this.speed;
    };

    for (var i = 0; i < maxStars; i++) {
      new Star();
    }

    function animation() {
      ctx.globalCompositeOperation = 'source-over';
      ctx.globalAlpha = 0.8;
      ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
      ctx.fillRect(0, 0, w, h);

      ctx.globalCompositeOperation = 'lighter';
      for (var i = 1, l = stars.length; i < l; i++) {
        stars[i].draw();
      }
      window.requestAnimationFrame(animation);
    }

    animation();
  }

  @HostListener("window:resize") //監聽窗口事件，用於自適應窗口大小
  canvasResize() {
    this.draw();
  }
}

效果如圖：