canvas中绘制image,如何将image的宽高适应整个canvas的高宽？

img


<!DOCTYPE html>
<html>
<body>
<canvas id='test_canvas' width='640px' height='480px' style='border:1px solid #d3d3d3'>
</canvas>
<script language="JavaScript">
    var ctx = document.getElementById('test_canvas').getContext('2d');
    var img = new Image();
    img.src = "03.png";
    img.onload = function() {
        ctx.drawImage(img, 0, 0);
    };
    window.setInterval("refreshCanvas()", 10);
    function refreshCanvas(){
        ctx.drawImage(img, 0, 0);
    };
</script>
</body>
</html>

ctx.drawImage(img, 0, 0,640,480)


ctx.drawImage(img, 0, 0,200,200); x,y后面就是 width,height