Search code examples
javascripthtmlcanvashtml5-animation

Canvas arc clearing

How do I overwrite an HTML5 canvas arc? I presumed this code would work but it leaves a border around it despite the fact that its exactly the same values and just a different colour.. Is there a border property I'm missing??

<!DOCTYPE html>
<html>
  <head>
    <title>Test</title>
  </head>
  <body>
  <canvas id="surface" width="300" height="300"></canvas>

  <script type="text/javascript">
      var canvas = document.getElementById('surface');
      var ctx = canvas.getContext('2d');

      ctx.fillStyle = 'black';
      ctx.beginPath();
      ctx.arc(100, 100, 20, 0, Math.PI * 2, true);
      ctx.fill();

      ctx.fillStyle = 'white';
      ctx.beginPath();
      ctx.arc(100, 100, 20, 0, Math.PI * 2, true);
      ctx.fill();
  </script>
  </body>
</html>
Solution

  • This black edge is a side affect of anti-aliasing.

    The easiest solution is to increase the radius of the arc slightly.