I’m trying to find the distance between a point and a given line segment.

I’m working on line simplification algorithms. I chose the Ramer–Douglas–Peucker algorithm. I found this Python implementation but I didn’t understand how they calculate the distance between a point and a line segment in the pldist function. How does this function work?

def pldist(point, start, end):
    """
    Calculates the distance from ``point`` to the line given
    by the points ``start`` and ``end``.

    :param point: a point
    :type point: numpy array
    :param start: a point of the line
    :type start: numpy array
    :param end: another point of the line
    :type end: numpy array
    """
    if np.all(np.equal(start, end)):
        return np.linalg.norm(point - start)

    return np.divide(
            np.abs(np.linalg.norm(np.cross(end - start, start - point))),
            np.linalg.norm(end - start))