画布翻译（Canvas Translation）

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2023-12-01

HTML5画布提供translate(x, y)方法，该方法用于将画布及其原点移动到网格中的不同点。

这里参数x是画布向左或向右移动的量，y是向上或向下移动的量。

例子 (Example)

以下是一个简单的例子，利用上述方法绘制各种螺旋形图 -

<!DOCTYPE HTML>
<html>
   <head>
      <style>
         #test {
            width:100px;
            height:100px;
            margin:0px auto;
         }
      </style>
      <script type = "text/javascript">
         function drawShape() {
            // get the canvas element using the DOM
            var canvas = document.getElementById('mycanvas');
            // Make sure we don't execute when canvas isn't supported
            if (canvas.getContext) {
               // use getContext to use the canvas for drawing
               var ctx = canvas.getContext('2d');
               ctx.fillRect(0,0,300,300);
               for (i = 0;i<3;i++) {
                  for (j = 0;j<3;j++) {
                     ctx.save();
                     ctx.strokeStyle = "#FF0066";
                     ctx.translate(50+j*100,50+i*100);
                     drawSpirograph(ctx,10*(j+3)/(j+2),-2*(i+3)/(i+1),10);
                     ctx.restore();
                  }
               }
            } else {
               alert('You need Safari or Firefox 1.5+ to see this demo.');
            }
         }
         function drawSpirograph(ctx,R,r,O) {
            var x1 = R-O;
            var y1 = 0;
            var i  = 1;
            ctx.beginPath();
            ctx.moveTo(x1,y1);
            do {
               if (i>20000) break;
               var x2 = (R+r)*Math.cos(i*Math.PI/72) - (r+O)*Math.cos(((R+r)/r)*(i*Math.PI/72))
               var y2 = (R+r)*Math.sin(i*Math.PI/72) - (r+O)*Math.sin(((R+r)/r)*(i*Math.PI/72))
               ctx.lineTo(x2,y2);
               x1 = x2;
               y1 = y2;
               i++;
            } while (x2 != R-O && y2 != 0 );
            ctx.stroke();
         }
      </script>
   </head>
   <body id = "test" onload = "drawShape();">
      <canvas id = "mycanvas" width = "400" height = "400"></canvas>
   </body>
</html>