问题:

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>

回答1:

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

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

回答2:

for this situation, it makes more sense to just redraw the the portion of the canvas that contained the arc. You can clear a section of the canvas with

ctx.clearRect(x, y, width, height);

or for simplicity you could just clear the entire canvas and redraw it completely:

ctx.clearRect(0, 0, canvas.width, canvas.height);

回答3:

In case you want something like undo feature you could copy the image data to a swap canvas before the arc drawing. Then copy the image data from swap canvas to the visible one if the undo is required.

Canvas arc clearing

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