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#canvas{background: url(https://image-static.segmentfault.com/464/827/464827894-6363d955ba2a5_fix732);
background-size: 100%;
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你的浏览器不反对 canvas，请降级你的浏览器。</canvas>
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ctx.fillStyle = '#9c9fb3';
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.globalCompositeOperation = 'destination-out';
ctx.lineWidth = 60;
ctx.lineCap = 'round';
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ctx.moveTo(x, y);
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canvas.onmousemove = function(e) {
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var x = event.clientX;
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ctx.lineTo(x, y);
ctx.stroke();}
</script>
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