canvas-7global.html 

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <canvas id="canvas" style="margin:0 auto;border:1px #ddd solid">
        The current browser does not support Canvas, can replace the browser a try!
    </canvas>

    <script>

        window.onload = function(){
            var canvas = document.getElementById('canvas');

            canvas.width = 1024;
            canvas.height = 768;

            if(canvas.getContext('2d')){
                var context = canvas.getContext('2d');

                // 整个系统的透明度设置
                context.globalAlpha = 0.7;

                for(var i=0;i<100;i++){
                    var R = Math.floor(Math.random()*255);
                    var G = Math.floor(Math.random()*255);
                    var B = Math.floor(Math.random()*255);

                    context.fillStyle = "rgb(" + R + "," + G + "," + B + ")";

                    context.beginPath();
                        context.arc(Math.random()*canvas.width,Math.random()*canvas.height,Math.random()*100,0,2*Math.PI)
                    context.closePath();

                    context.fill();
                }

            }else{
                alert('当前游览器不支持Canvas，请更换游览器后再试！');
            }
        }

    </script>
</body>
<script>
    /*global

        globalAlpha

        globalCompositeOperation  = "source-over"(default)
                                    sourse-atop
                                    sourse-in
                                    sourse-out

                                    destination-over  //前面绘制的覆盖了后面绘制的图形
                                    destination-atop
                                    destination-in
                                    destination-out

                                    lighter
                                    copy
                                    xor
                
    */
</script>
</html>

 

posted @ 2015-11-23 15:07  Cynthia娆墨旧染  阅读(231)  评论(0)    收藏  举报