黑客效果特效

<canvas id="canvas" style="background:#111;width:100%;"></canvas>
	<script>
		window.onload = function(){
			//获取画布对象
			var canvas = document.getElementById("canvas");
			//获取画布的上下文
			var context = canvas.getContext("2d");
			//获取浏览器的宽高
			var W = window.innerWidth;
			var H = window.innerHeight;
			//设置canvas的宽高
			canvas.width = W;
			canvas.height = H;
			//每个文字的大小
			var fontSize = 16;
			//计算浏览器一共可以放置多少列
			var colunms = Math.floor(W / fontSize);
			//记录每列文字的Y坐标
			var drops = [];
			//给每一个文字初始化一个起始点的位置
			for(var i = 0;i<colunms;i++){
				drops.push(0);
			}

			//要滚动的文字
			var str = "javascript html5 canvas";
			//4:fillText(str,x,y);原理就是去更改y的坐标位置
			//绘画的函数
			function draw(){
				context.fillStyle = "rgba(0,0,0,0.05)";
				context.fillRect(0,0,W,H);
				//给字体设置样式
				context.font = "700 "+fontSize+"px 微软雅黑";
				//给字体添加颜色
				context.fillStyle = "#00cc33";
				//写入画布中
				for(var i=0;i<colunms;i++){
					var index = Math.floor(Math.random() * str.length);
					var x = i*fontSize;
					var y = drops[i]*fontSize;
					context.fillText(str[index],x,y);
					//如果要改变时间，肯定就是改变每次他的起点
					if( y >= canvas.height && Math.random() > 0.99){
						drops[i] = 0;
					}
					drops[i]++;
				}
			};
			function randColor(){
				var r = Math.floor(Math.random() * 256);
				var g = Math.floor(Math.random() * 256);
				var b = Math.floor(Math.random() * 256);
				return "rgb(" + r +"," + g +"," + b +")";
			}

			draw();
			setInterval(function(){
				draw()
			},30)
		}
	</script>