下面代码 canvas rotate旋转 旋转的坐标点在哪，为什么不在元素的中心点？_html/css_WEB-ITnose

下面代码 canvas rotate旋转 旋转的坐标点在哪，为什么不在元素的中心点？

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 <script> <br /> <br /> function init() <br /> { <br /> var canvas = document.getElementById('canvas'); <br /> var cxt = canvas.getContext('2d'); <br /> cxt.strokeStyle = "red"; <br /> cxt.strokeRect(300,300,100,100); <br /> cxt.strokeStyle = "blue"; <br /> cxt.rotate(10*Math.PI/180); <br /> cxt.strokeRect(300,300,100,100); <br /> cxt.strokeStyle = "green"; <br /> cxt.rotate(20*Math.PI/180); <br /> cxt.strokeRect(300,300,100,100); <br /> } <br /> </script>
 
 
 
 
  

回复讨论(解决方案)

canvas旋转其实就是画布的旋转，旋转中心点是画布原点，即0,0点。
如果想中心旋转的话设置画布原点即可。

function init() {	 var canvas = document.getElementById('canvas');	 var cxt = canvas.getContext('2d');	 cxt.strokeStyle = "red";	 cxt.strokeRect(0,0,100,100);	 cxt.strokeStyle = "blue";	 cxt.rotate(10*Math.PI/180);	 cxt.strokeRect(0,0,100,100);	 cxt.strokeStyle = "green";	 cxt.rotate(20*Math.PI/180);	 cxt.strokeRect(0,0,100,100);	 cxt.rotate(-30*Math.PI/180);//将画布旋转恢复到初始状态	 cxt.translate(50,50);//将画布原点移动到旋转对象的中心位置	 cxt.rotate(45*Math.PI/180);//旋转画布	 cxt.translate(-50,-50);//将画布原点恢复到初始状态	 cxt.strokeStyle = "yellow";	 cxt.strokeRect(0,0,100,100); }