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In two-dimensional computer graphics, the non-zero winding rule is a means of determining whether a given point falls within an enclosed curve. Unlike the similar even-odd rule, it relies on knowing the direction of stroke for each part of the curve.
For a given curve C and a given point P: construct a ray heading out from P in any direction towards infinity. Find all the intersections of C with this ray. Score up the winding number as follows: for every clockwise intersection subtract 1; for every counter-clockwise intersection add 1. If the total winding number is zero, P is outside C; otherwise, it is inside.
The winding number is effectively a count of how many full counter-clockwise revolutions the curve makes around P without doubling back on itself.
Some implementations instead score up the number of clockwise revolutions, so that clockwise crossings are awarded +1, counter-clockwise crossings -1. The result is the same.