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.
来源：https://stackoverflow.com/questions/6175042/canvas-arc-clearing