Complex shapes

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It’s possible to clear complex shaped regions by changing the globalCompositeOperation property.

// All pixels being drawn will be transparent
ctx.globalCompositeOperation = 'destination-out';

// Clear a triangular section
ctx.globalAlpha = 1;    // ensure alpha is 1
ctx.fillStyle = '#000'; // ensure the current fillStyle does not have any transparency
ctx.moveTo(10, 0);
ctx.lineTo(0, 10);
ctx.lineTo(20, 10);

// Begin drawing normally again
ctx.globalCompositeOperation = 'source-over';

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