javascript – 如何旋转此形状？

我刚开始用帆布.我做的：

http://www.kingoslo.com/instruments/

转速计的箭头是使用画布绘制的.现在我试图制作一个动画,围绕转速计的输入(而不是自身)旋转它.这是我到目前为止所写的：

<script type="text/javascript">
  var img = new Image(); 

  function init(){  
   img.src = 'background.png'; 
   setTimeout(draw,4000);   
  }  

  function draw() {  
   var ctx = document.getElementById('canvas').getContext('2d');   
   img.onload = function(){  
    ctx.drawImage(img,0); 
    ctx.beginPath();
    ctx.moveto(247,308);
    ctx.bezierCurveto(51,408,51,410,411);
    ctx.bezierCurveto(53,413,52,412,249,313);
    ctx.fillStyle = "red";
    ctx.fill();
   }  
  }
 </script>

我不知道如何进一步发展.此外,是否有涵盖这些内容的任何地方的综合文档？

谢谢.

亲切的问候,
马吕斯

解决方法

你见过这个 https://github.com/vjt/canvas-speedometer吗？

我写了你的代码：

确保仪表的中心点必须位于图像的中心位置,否则它将无法工作.

你可以处理i的值来旋转你的引脚.

在这里,我拍了两张照片

> mt.png //用于背景仪表
http://i.stack.imgur.com/qbWeO.png
> pin3.png //用于仪表针
http://i.stack.imgur.com/SQEv6.png

码：

<script type="text/javascript">
            function startup() {
                    var canvas = document.getElementById('canvas');
                    var context = canvas.getContext('2d');
                    var meter = new Image();
                    meter.src = 'Mt.png';    //meter background image
                    var pin = new Image();
                    pin.src = 'pin3.png';    //meter pin image                 

                    var x=meter.width/2;     // center point.X (must be the center of image)
                    var y=meter.height/2;    // center point.Y 
                    var width = meter.width; 
                    var height = meter.height;

                    var i=120;             // angle of rotation in degrees
                    var min =-115;         // maximum angle = 6 RPM in meter 
                    var max =100;          // minimum angle = 0 RPM in meter 

                    i=(i < min)? min:(i > max)? max:i;  //to check i must be within min & max range
                    var angleInradians = Math.PI * i/180;  //converting degree to radians
                    meter.onload = function()
                    {
                        context.translate(x,y);    //setting center at x,y                        
                        context.drawImage(meter,-width / 2,-height / 2,width,height);  //drawing meter background 
                        context.rotate(angleInradians); //rotating by angle
                        context.drawImage(pin,0-pin.width/2,0-pin.height+pin.width/2,pin.width,pin.height);  //adjusting pin center at meter center

                    }
            }
    </script>

更新：
如果您想要处理具有rpm值的仪表
更换

var i=120;             // angle of rotation in degrees
                    var min =-115;         // maximum angle = 6 RPM in meter 
                    var max =100;          // minimum angle = 0 RPM in meter

同

var r=1.7;               //handle here
                    var min =-114;           
                    var max =99;                                  
                    var span = (max-min)/6;  // dividing rotation angle by meter scale
                    var i=r*span+min;        //angle of rotation from value of r and span

输出：