Determine if a point is inside a given polygon or not Polygon is a list of (x,y) points. Uses the 'Ray Casting' algorithm

point_in_polygon.py

def is_inside(polygon, point):
    """Determine if a point is inside a given polygon or not
       Polygon is a list of (x,y) points.
       Uses the 'Ray Casting' algorithm"""

    x, y = point

    n = len(polygon)
    inside = False

    p1x, p1y = polygon[0]
    for i in range(n + 1):
        p2x, p2y = polygon[i % n]
        if y > min(p1y, p2y) and y <= max(p1y, p2y) and x <= max(p1x, p2x):
            if p1y != p2y:
                xints = (y-p1y)*(p2x-p1x)/(p2y-p1y)+p1x
            if p1x == p2x or x <= xints:
                inside = not inside
        p1x, p1y = p2x, p2y

    return inside

def test():
    polygon = [(0,10),(10,10),(10,0),(0,0)]
    assert is_inside(polygon, (5, 5))
    assert is_inside(polygon, (0.1, 0.1))
    assert not is_inside(polygon, (0, 0))
    assert not is_inside(polygon, (11, 4))
    assert is_inside(polygon, (9.9, 9.9))
    assert is_inside(polygon, (9, 10))
    assert is_inside(polygon, (10, 9))

    polygon = [(0, 0), (2, 0), (2, 1)]
    assert is_inside(polygon, (1, 0.4))
    assert not is_inside(polygon, (1, 0.6))

test()