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79 lines
2.3 KiB
79 lines
2.3 KiB
const WIDTH = window.innerWidth
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const HEIGHT = window.innerHeight
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const settings = {
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r: 1400, // 圆形轨迹半径
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height: 260, // 围绕旋转的圆露出的圆弧的高度
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alpha: 0.0, // 当前透明度
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maxAlpha: 1 // 最大透明度
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}
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function Star () {
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// 圆的轨迹方程式为：(x-a)²+(y-b)²=r²
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// 因此，已知x, 则y = Math.sqrt(r² - (x-a)²) + b
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// 其中，圆心是(a, b)
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// 在本例子中
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// 圆心坐标是(WIDTH/2, HEIGHT - 600 + r)
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var a = WIDTH/2, b = HEIGHT - settings.height + settings.r;
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// 因此，已知横坐标随机
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this.x = Math.floor(Math.random() * WIDTH);
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// 纵坐标需要在圆弧以上
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// 越往上，越稀疏
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this.offsety = getMinRandom() * (HEIGHT - settings.height);
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this.y = b - Math.sqrt(settings.r * settings.r - (this.x - a) * (this.x - a)) - this.offsety;
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this.vx = Math.random() * 0.05 + 0.05; // 水平偏移，也是移动速度
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// 星星的尺寸
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this.particleSize = 0.5 + (Math.random() + 0.1 / 4);
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this.alpha = 0.0;
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this.maxAlpha = 0.2 + (this.y/HEIGHT) * Math.random() * 0.8;
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this.alphaAction = 1;
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}
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Star.prototype.draw = function (context) {
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// 横坐标移动
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this.x += this.vx;
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// 根据切线方向进行偏移
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// y坐标
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this.y = HEIGHT - settings.height + settings.r - Math.sqrt(settings.r * settings.r - (this.x - WIDTH/2) * (this.x - WIDTH/2)) - this.offsety;
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// 透明度慢慢起来
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if (this.alphaAction == 1) {
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if (this.alpha < this.maxAlpha ) {
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this.alpha += 0.005;
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} else {
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this.alphaAction = -1;
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}
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} else {
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if (this.alpha > 0.2 ) {
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this.alpha -= 0.002;
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} else {
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this.alphaAction = 1;
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}
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}
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if (this.x + (this.particleSize*2) >= WIDTH) {
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// x到左侧
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this.x = this.x - WIDTH;
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}
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// 绘制星星
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context.beginPath();
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context.fillStyle="rgba(255,255,255," + this.alpha.toString() + ")";
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context.arc(this.x, this.y, this.particleSize, 0, Math.PI * 2, true);
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context.closePath();
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context.fill();
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}
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// 挑选一个随机数
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function getMinRandom () {
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const rand = Math.random()
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// step的大小决定了星星靠近地球的聚拢程度，
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// step = Math.ceil(2 / (1 - rand))就聚拢很明显
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const step = Math.ceil(1 / (1 - rand))
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let rands = []
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for (let i = 0; i < step; i++) {
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rands.push(Math.random())
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}
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return Math.min.apply(null, rands)
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}
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export default Star
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