基于JS实现漫画中大雨滂沱的效果

<canvas id='canvas' width='1280' height='720'></canvas>

body {
margin:0;
padding:0;
background: #000;
overflow:hidden;
}
canvas {
background:url('../img/maxresdefault.jpg');
background-size: cover;
background-repeat: no-repeat;
background-position: center center;  
}

background-size: length|percentage|cover|contain;

function _pexresize() {
			var cw = window.innerWidth;
			var ch = window.innerHeight;
			if (cw<=ch*stgw/stgh) {
				loffset = 0;
				toffset = Math.floor(ch-(cw*stgh/stgw))/2;
				canvas.style.width = cw + "px";
				canvas.style.height = Math.floor(cw*stgh/stgw) + "px";
			} else {
				loffset = Math.floor(cw-(ch*stgw/stgh))/2;
				toffset = 0;
				canvas.style.height = ch + "px";
				canvas.style.width = Math.floor(ch*stgw/stgh) + "px";
			}
			canvas.style.marginLeft = loffset +"px";
			canvas.style.marginTop = toffset +"px";
		}

for (var l=0;l<lcount;l++) {
			ctx.clearRect(0,0,stgw,stgh);
			for (var i=0;i<count*(lcount-l)/1.5;i++) {
				var myx = Math.floor(Math.random()*stgw);
				var myy = Math.floor(Math.random()*stgh);
				var myh = l*6+8;
				var myw = myh/10;
				ctx.beginPath();
				ctx.moveTo(myx,myy);
				ctx.lineTo(myx+myw,myy+myh);
				ctx.arc(myx, myy+myh, myw, 0, 1 * Math.PI);
				ctx.lineTo(myx-myw,myy+myh);
				ctx.closePath();
				ctx.fill();
			}
			layer[l] = new Image();
			layer[l].src = canvas.toDataURL("image/png");
			layery[l] = 0;
		}



		var stt = 0;
		var str = Date.now()+Math.random()*4000;
		var stact = false;

		function animate() {
			ctx.clearRect(0,0,stgw,stgh);

			for (var l=0;l<lcount;l++) {
				layery[l] += (l+1.5)*5;
				if (layery[l]>stgh) {

					layery[l] =layery[l]-stgh;
				}
				ctx.drawImage(layer[l],0,layery[l]);
				ctx.drawImage(layer[l],0,layery[l]-stgh);
			}
			if (Date.now()>str) {
				stact = true;
			}
			if (stact) {
				stt++;
				if (stt<5+Math.random()*10) {
					var ex = stt/30;
				} else {
					var ex = (stt-10)/30;
				}
				if (stt>20) {
					stt = 0;
					stact = false;
					str = Date.now()+Math.random()*8000+2000;
				}

				ctx.fillStyle = "rgba(255,255,255,"+ex+")";
				ctx.fillRect(0,0,stgw,stgh);
			}
			window.requestAnimationFrame(animate);
		}