27.canvas获得和写入图片.html

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>canvas获得和写入图片</title>
	<style type="text/css">
		*{
			padding: 0;
			margin: 0;
		}
		canvas{
			display: block;
			background: #ccc;
			margin: 50px auto;
		}
	</style>
</head>
<body>

	<canvas id="canvas" height="500" width="800"></canvas>
	<script type="text/javascript">
	
			var canvas = document.getElementById("canvas");
			var cv = canvas.getContext('2d');
			var pic = document.createElement("img");
			pic.src = "./file/p.png";

			pic.onload = function(){
				alert("aaa");
				cv.drawImage(pic,20,20);


				//为什么无效？？？

				//获得画布图像信息
				var partPic = cv.getImageData(40,40,60,60);

				//写入画布信息
				cv.putImageData(partPic,500,500);
			}

	</script>
</body>
</html>