import numpy as np

def remove_colinear( xy, threshold ):
    """
    This isn't a great algorithm but does a pretty good
    first-pass kind of job

    Computes the area of triangle for 3 points. If area is too big then
    we keep the middle point, otherwise we move onto the next one.
    """
    retval = [xy[0]]  # always keep the 1st point
    for ii in xrange( 2, len(xy) ): # Polygon must have 3 points!

        x0 = retval[-1][0]  # The last saved point
        y0 = retval[-1][1]

        x2 = xy[ii][0]  # The current end-point
        y2 = xy[ii][1]

        x1 = xy[ii-1][0]  # The middle point
        y1 = xy[ii-1][1]

        area = np.abs(0.5*((x1-x0)*(y2-y0)-(x2-x0)*(y1-y0)))
        if area > threshold:
            # If area is too big, then keep last point
            retval.append( xy[ii-1] )
    return retval