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<femerge x="0" y="0" width="50%" height="40%">
<femergenode in="SourceGraphic"></femergenode>
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var canvas = document.createElement('canvas');
canvas.width = 100;
canvas.height = 100;
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ctx.filter = 'url(#crop)';
ctx.fillStyle = '#0f0';
ctx.fillRect(0, 0, 90, 90);
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