subregion-fill-paint.html

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<!DOCTYPE html>

<html>

<body>

<svg style="display: block; width: 0; height: 0">

  <defs>

    <filter id="merge" primitiveUnits="objectBoundingBox">

      <feMerge x="25%" y="25%" width="50%" height="50%">

        <feMergeNode in="SourceGraphic"/>

        <feMergeNode in="FillPaint"/>

      </feMerge>

    </filter>

  </defs>

</svg>

<canvas id="canvas" width="100" height="100"></canvas>

<script>

var canvas = document.getElementById('canvas');

var ctx = canvas.getContext('2d');

ctx.filter = 'url(#merge)';

ctx.fillStyle = '#0f0';

ctx.arc(50, 50, 25, 0, Math.PI * 2, true);

ctx.fill();

</script>

</body>

</html>