HTML5 Canvas:如何通过以度为单位旋转图像来制作加载微调器？

Question

我正在使用html5画布制作一个加载微调器.我在画布上有我的图形但是当我旋转它时,图像从画布上旋转.如何告诉它在中心点旋转图形？

<!DOCTYPE html>
<html>
 <head>
  <title>Canvas test</title>
  <script type="text/javascript">
   window.onload = function() {
    var drawingCanvas = document.getElementById('myDrawing');
    // Check the element is in the DOM and the browser supports canvas
    if(drawingCanvas && drawingCanvas.getContext) {
     // Initaliase a 2-dimensional drawing context
     var context = drawingCanvas.getContext('2d');

     //Load the image object in JS, then apply to canvas onload     
     var myImage = new Image();
     myImage.onload = function() {
      context.drawImage(myImage, 0, 0, 27, 27);
     }
     myImage.src = "img/loading.png";

     context.rotate(45);
    }
   }
  </script>
 </head>
 <body>
  <canvas id="myDrawing" width="27" height="27">
  </canvas>
 </body>
</html>

Answer 1

这是完整的工作示例:)

<!DOCTYPE html>
<html>
    <head>
        <title>Canvas Cog</title>
        <script type="text/javascript">
            var cog = new Image();
            function init() {
                cog.src = 'data:image/png;base64,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'; // Set source path
                setInterval(draw,10);
            }
            var rotation = 0;
            function draw(){
                var ctx = document.getElementById('myCanvas').getContext('2d');
                ctx.globalCompositeOperation = 'destination-over';
                ctx.save();
                ctx.clearRect(0,0,27,27);
                ctx.translate(13.5,13.5); // to get it in the origin
                rotation +=1;
                ctx.rotate(rotation*Math.PI/64); //rotate in origin
                ctx.translate(-13.5,-13.5); //put it back
                ctx.drawImage(cog,0,0);
                ctx.restore();
            }
            init();
        </script>
    </head>
    <body>
        <canvas width="27" height="27" id="myCanvas"></canvas>
    </body>
</html>

Answer 2

rotate将画布(？)转到当前位置,即0,0开始.你需要"移动"到你想要的中心点,你可以完成

context.translate(x,y);

移动参考点后,您希望将图像置于该点的中心位置.你可以通过电话来做到这一点

context.drawImage(myImage, -(27/2), -(27/2), 27, 27);

这告诉浏览器开始从当前参考点的上方和左侧绘制图像,具有图像的大小,而在您从参考点开始并完全在下方和右侧绘制之前(所有方向相对)到画布的旋转).

由于您的画布是图像的大小translate,因此对于x和y坐标,您的调用将使用相同的测量值(27/2).

所以,把它们放在一起

// initialization:
context.translate(27/2, 27/2);

// onload: 
context.rotate(Math.PI * 45 / 180);
context.drawImage(myImage, -(27/2), -(27/2), 27, 27);

编辑:另外,旋转单位是弧度,因此您需要将度数转换为代码中的弧度.

编辑重新排列的东西.