1、 canvas 旋转中心点 rotate translate
2、代码
<!DOCTYPE html>
<html>
<body>
<canvas id="myCanvas" width="500" height="500" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
var c=document.getElementById("myCanvas");
var ctx = c.getContext('2d');
var x = 100,
y = 100,
width = 200,
height = 150,
angle = 90*Math.PI/180,
rectCenterPoint = {x: x+width/2, y: y+height/2}; // 矩形中心点
// 旋转前(红色矩形)
ctx.fillStyle = '#f00';
ctx.fillRect(x, y, width, height);
// 旋转后(绿色矩形)
ctx.translate(rectCenterPoint.x, rectCenterPoint.y);
ctx.rotate(angle);
ctx.translate(-rectCenterPoint.x, -rectCenterPoint.y);
ctx.fillStyle = '#690';
ctx.fillRect(x, y, width, height);
</script>
</body>
</html>
3、效果图
4、测试地址 http://www.w3school.com.cn/tiy/t.asp?f=html5_canvas_fillrect