canvas_doubleBuffering

<!DOCTYPE html>
<html>
<head>
    <title>canvas 26</title>
    <style type="text/css">
        #can1 {
            border:dotted black 1px;
        }
    </style>
    <script>
        
            var snowArr = [];
            var maxSnow = 200;
            var canvas = null;
            var ctx = null;
            var buffCanvas = null;
            var buffCtx = null;
            var interv = null;
            var interv2 = null;
            
            /*constructor to Snow object*/
            function Snow()   {
                this.x = Math.round(Math.random() * ctx.canvas.width);
                this.y = -10;//above the top of the canvas
                this.drift = Math.random();
                this.speed = Math.round(Math.random() * 5)+1;//the +1 is to avoid 0
                this.width = (Math.random() * 3)+2;//width is between 2 and  5
                this.height = this.width;
            }
            
            window.onload = function () {
                canvas = document.getElementById('can1');
                ctx = canvas.getContext('2d');
                
                buffCanvas = document.createElement("canvas");
                buffCtx = buffCanvas.getContext('2d');
                buffCtx.canvas.width = ctx.canvas.width;
                buffCtx.canvas.height = ctx.canvas.height;
                
                interv = setInterval(addSnow2Arr,200);
                
                Draw();
                
                interv2 = setInterval(animate,30);
            }
            
            function addSnow2Arr()    {
                snowArr[snowArr.length] = new Snow();
                
                if (snowArr.length == maxSnow)
                    clearInterval(interv);
            }
            
            function animate()  {
                Update();//changing parameters values
                Draw();//moving snow objects and copying to the real canvas
            }
            
            function Update()   {
                for (var i=0; i<snowArr.length; i++)  {
                    if(snowArr[i].y < ctx.canvas.height)  {
                        snowArr[i].y += snowArr[i].speed;
                        if (snowArr[i].y > ctx.canvas.height)
                            snowArr[i].y = -5;
                        snowArr[i].x += snowArr[i].drift;
                        if(snowArr[i].x > ctx.canvas.width)
                            snowArr[i].x = 0;
                    }
                }
            }
            
            function Draw()   {
                ctx.save();
                
                //create a clipping region
                buffCtx.beginPath();
                buffCtx.fillStyle = "#d8e3e8";
                buffCtx.fillRect(0,0,buffCanvas.width,buffCanvas.height);
                buffCtx.arc(buffCanvas.width/2, buffCanvas.height/2,buffCanvas.height * 0.4, 0, 2*Math.PI);
                buffCtx.clip();
                
                coverBgr();
                
                for (var i=0; i<snowArr.length; i++)  {
                    if(snowArr[i].y < ctx.canvas.height)  {
                        buffCtx.fillStyle = "white";
                        buffCtx.fillRect(snowArr[i].x, snowArr[i].y, snowArr[i].width, snowArr[i].height);
                    }
                }
                
                //copy the entire rendered img from the buffer canvas to the visible canvas on screen
                ctx.drawImage(buffCanvas, 0,0, buffCanvas.width, buffCanvas.height);
                ctx.restore();
            }
            
            function coverBgr()    {
                buffCtx.fillStyle = "#1e3d6c";
                buffCtx.fillRect(0,0,ctx.canvas.width,ctx.canvas.height);
            }
            
            function onCanvasClick() {                
                //rectX = 0;
                clearInterval(interv);
                clearInterval(interv2);
            }
        
    </script>
</head>
<body>
  <div>
    <header>
      <h1>canvas 26</h1>
    </header>

<div>
      <canvas id="can1" onclick="onCanvasClick()" width="600" height="400">
          Fallback Content
      </canvas>
    </div>