Even odd method for determining inside points in a Polygon: This method is used to determine whether a point is inside the polygon or outside. To do this, simply construct a line segment from the point in question to point outside the polygon. Now, we draw a line segment from point 1 to a outside point and see that the line intersects the polygon at 4 places. That’s even, hence the point is outside the polygon.

Same goes with point 2. Now, Point 3 makes an odd intersection of 1, hence it is inside the polygon. If the intersection point is vertex of polygon then, if the other end points of the two segments which meets at this vertex lie on same side of constructed line then point is taken as having even number of intersections. If they lie on the opposite side of constructed line then point is counted to have a single intersection.

Winding number method: In this method, it is imagined that a elastic is stretched between the point in question and a point outside the polygon for one rotation. Now the point is checked for how many times the elastic is wind to it. If it is wind once, point is inside. If there’s no winding then point is outside.

Sonal Thapa says

thanks….This defination and diagram is good and easy for understand even-odd method

Devang Sharma says

Thank you. You explained this much better than my lecturer

Thet nong phu says

I do not understand about winding.