Canvas Scaling

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2023-12-01

HTML5画布提供了scale(x, y)方法，用于增加或减少画布网格中的单位。这可用于绘制缩小或放大的形状和位图。

该方法采用两个参数，其中x是水平方向上的比例因子，y是垂直方向上的比例因子。两个参数都必须是正数。

小于1.0的值会减小单位大小，大于1.0的值会增加单位大小。将缩放系数精确设置为1.0不会影响单位大小。

例子 (Example)

以下是一个简单的例子，它使用螺旋形函数绘制具有不同缩放因子的九个形状。

<!DOCTYPE HTML>
<html>
   <head>
      <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.strokeStyle = "#fc0";
               ctx.lineWidth = 1.5;
               ctx.fillRect(0,0,300,300);
               // Uniform scaling
               ctx.save()
               ctx.translate(50,50);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(100,0);
               ctx.scale(0.75,0.75);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(133.333,0);
               ctx.scale(0.75,0.75);
               drawSpirograph(ctx,22,6,5);
               ctx.restore();
               // Non uniform scaling (y direction)
               ctx.strokeStyle = "#0cf";
               ctx.save()
               ctx.translate(50,150);
               ctx.scale(1,0.75);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(100,0);
               ctx.scale(1,0.75);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(100,0);
               ctx.scale(1,0.75);
               drawSpirograph(ctx,22,6,5);
               ctx.restore();
               // Non uniform scaling (x direction)
               ctx.strokeStyle = "#cf0";
               ctx.save()
               ctx.translate(50,250);
               ctx.scale(0.75,1);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(133.333,0);
               ctx.scale(0.75,1);
               drawSpirograph(ctx,22,6,5);
               ctx.translate(177.777,0);
               ctx.scale(0.75,1);
               drawSpirograph(ctx,22,6,5);
               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 onload = "drawShape();">
      <canvas id = "mycanvas" width = "400" height = "400"></canvas>
   </body>
</html>