globalAlpha 示例

代码实例：
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>canvas</title>
    <script>
        function draw() {
            var canvas = document.getElementById('canvas');
            if (canvas.getContext){
                var ctx = canvas.getContext('2d');
                ctx.fillStyle='#FD0';
                ctx.fillRect(0,0,75,75);
                ctx.fillStyle='#6C0';
                ctx.fillRect(75,0,75,75);
                ctx.fillStyle='#09F';
                ctx.fillRect(0,75,75,75);
                ctx.fillStyle = '#F30';
                ctx.fillRect(75,75,75,75);
                ctx.fillStyle = '#FFF';

                ctx.globalAlpha=0.2;

                for(var i=0;i<7;i++) {
                    ctx.beginPath();
                    ctx.arc(75,75,10+10*i,0,Math.PI*2,true);
                    ctx.fill();
                }

            }
        }
    </script>
</head>
<body  onload=" draw() ;">
<canvas  id="canvas"  width="400"   height="400"></canvas>
</body>
</html>
　　效果：
2017-09-09 12:37:29