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);
}