function pageLoaded(){


            var canvas = document.getElementById('testcanvas');//得到画布
            var context = canvas.getContext('2d');//得到绘图环境  参数为绘图环境的类型

            //画一个实心矩形
            var oldFillStyle = context.fillStyle;
            context.fillStyle = 'red';
            context.fillRect(0,0,100,100);
            context.fillStyle = oldFillStyle;

            context.save();//保存context的当前状态
            //画一个空心矩形
           // var oldStrokeStyle = context.strokeStyle;
            context.strokeStyle = 'rgb(155,0,0)'
            context.strokeRect(0,101,100,100);
            //context.strokeStyle = oldStrokeStyle;
            context.restore();//还原context的状态

            //清除指定的矩形区域 使透明
            context.clearRect(25,25,50,50);

            //闭合路径
            context.beginPath();//开始绘制一个新路径
            context.moveTo(105,0);//将画笔移到指定点
            context.lineTo(200,50);//从当前点绘制一条直线到指定点
            context.lineTo(105,100);
            context.closePath();//闭合路径
            context.stroke();//绘制空心形状


            context.beginPath();
            context.moveTo(105,105);
            context.lineTo(200,155);
            context.lineTo(105,205);
            context.closePath();//闭合路径
            context.fill();//填充形状

            context.beginPath();
            context.arc(160,300,50,0,Math.PI,true);//画弧
            context.stroke();

            oldStrokeStyle = context.strokeStyle;
            context.beginPath();
            context.fillStyle = 'rgba(155,0,0,0.6)';//设置颜色 同时设置透明度
            context.arc(160,300,30,0,2*Math.PI,true);//画弧
            context.fill();

            context.font = '30px Arial';
            context.strokeText('how are you', 405,20);

            context.font = '30px Arial';
            context.fillText('how are you', 405,60);

            var p_w_picpath = document.getElementById('tank');
            context.drawImage(p_w_picpath,600,0);

            context.drawImage(p_w_picpath,600,0,100,100);
            context.drawImage(p_w_picpath,200,200,50,50,600,100,100,100);


            context.translate(650,250);
            context.rotate(Math.PI/2);
            context.drawImage(p_w_picpath,-50,-50,100,100);
            context.rotate(-Math.PI/2);
            context.translate(-650,-250);

        }