第633页 21

第632页讲canvas处，rotateAbout函数应为逆时针旋转theta弧度，而不是顺时针旋转。英文原文也错了，应为couterclockwise, 而不是clockwise.
以下为验证(另外通过数学变换也可推出)：


<canvas id="my_canvas_id" width=1000 height=1000></canvas>
<script>
        var canvas = $('#my_canvas_id')[0];
        var c = canvas.getContext('2d');
        function rotateAbout(c, theta, x, y) {
	        var ct = Math.cos(theta), st = Math.sin(theta);
		c.transform(ct, -st, st, ct, -x*ct-y*st+x, x*st-y*ct+y);
                /* 等同如下变换序列
                c.translate(x, y);
                c.rotate(theta);
                c.tranlate(-x, -y);
                */
	 }
		    
	rotateAbout(c, Math.PI, 500, 500); 
	c.beginPath();
	c.moveTo(300, 300);
	c.lineTo(200, 200);
	c.lineTo(100, 200);
	c.stroke();
</script>