Geo Fencing - point inside/outside polygon

Question

I would like to determine a polygon and implement an algorithm which would check if a point is inside or outside the polygon.

Does anyone know if there is any exampl

Answer 1

the polygon is defined as a sequential list of point pairs A, B, C .... A. no side A-B, B-C ... crosses any other side

Determine box Xmin, Xmax, Ymin, Ymax

case 1 the test point P lies outside the box

case 2 the test point P lies inside the box:

Determine the 'diameter' D of the box {[Xmin,Ymin] - [Xmax, Ymax]} ( and add a little extra to avoid possible confusion with D being on a side)

Determine the gradients M of all sides

Find a gradient Mt most different from all gradients M

The test line runs from P at gradient Mt a distance D.

Set the count of intersections to zero

For each of the sides A-B, B-C test for the intersection of P-D with a side from its start up to but NOT INCLUDING its end. Increment the count of intersections if required. Note that a zero distance from P to the intersection indicates that P is ON a side

An odd count indicates P is inside the polygon