JS实现大屏旋转星空背景

1.1 应用场景

大屏场景下给决策报表添加一个酷炫的旋转星空效果。

1.2 实现思路

采用插入动态 GIF 背景图的方式是无法进行自适应的，所以直接用 JS 代码来制作效果更好。

以FineReport.frm为例，给 body 组件添加一个初始化后事件，JS 代码如下：

function background() {
	$("body").prepend('<canvas id="canvas" style="position:absolute;z-index:-2;"></canvas>');
	//在body中插入canvas画布，并置于底层
	var canvas = document.getElementById('canvas'),
		ctx = canvas.getContext('2d'),
		w = canvas.width = window.innerWidth,
		h = canvas.height = window.innerHeight,

		hue = 217,
		stars = [],
		count = 0,
		maxStars = 800;
	//设置星星数量

	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();

	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;
	}

	var Star = function() {

		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) / 500000;
		//设置旋转速度
		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();
}
background();
$(window).resize(function() {
	background();
})

保存决策报表，点击PC端预览，效果图如应用场景中所示。

注：不支持移动端。

已完成模板可参见：%FR_HOME%\webapps\webroot\WEB-INF\reportlets\doc\JS\表单JS实例\大屏旋转星空背景图.frm

点击下载模板：大屏旋转星空背景图.frm