alphashapes & n<4

[ljwolf]

in theory, the "delaunay" triangulation for cases where the number of points is less than 4 is still "known." N=3 is the triangle, n=2 is the line, and n=1 is the point.

as it stands, Qhull will error out when passed a collection with less than four points. It'd be nice if we checked this & returned something sane in these cases, rather than bailing out from qhull.

[ljwolf]

I think this is sufficient to ensure that the output is always a polygon:

def safe_ashape(xys, **ashape_kws):
    if xys.shape[0] < 4:
        return ops.cascaded_union([geom.Point(xy) for xy in xys]).convex_hull
    return alpha_shape(xys, **ashape_kws)

[darribas]

I've ran into this issue too. I think that solution might do it. Just a question, would that return a polygon object?

[ljwolf]

Just checked all the input n, and we'd need a buffer(0) to ensure n=1 and n=2 are polygon output:

def safe_ashape(xys, **ashape_kws):
    if xys.shape[0] < 4:
        return ops.cascaded_union([geom.Point(xy) for xy in xys]).convex_hull.buffer(0)
    return alpha_shape(xys, **ashape_kws)

[jGaboardi]

Addressed in #115