Testing if a Point is Inside a Polygon in Python

Written by Taro Sato, on . Tagged: algo python

Finally got around to finding this out by Googling. It’s a useful function so I reproduce it here for copy & paste:

def inside_polygon(x, y, points):
    Return True if a coordinate (x, y) is inside a polygon defined by
    the list of verticies [(x1, y1), (x2, y2), ... , (xN, yN)].

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

It is a clever algorithm if you think through it.

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