TBC
A bigger circle is subtracted from a smaller one. Even though there are no resulting shapes, the distance field still contains traces of the original circle.
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<html>
<head>
<meta charset="utf-8">
<meta name="description" content="Distance fields (empty space)" />
<title>Distance fields (empty space)</title>
<link rel="stylesheet" href="index.css">
<script src="https://d3js.org/d3.v3.min.js"></script>
</head>
<body>
<canvas id="left" width="480" height="500"></canvas>
<canvas id="right" width="480" height="500"></canvas>
<script src="index.js"></script>
</body>
</html>
Modified http://d3js.org/d3.v3.min.js to a secure url
https://d3js.org/d3.v3.min.js