ulikoehler/UliEngineering

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Geometry functions, mainly for 2D coordinates.
"""
import numpy as np

__all__ = ["polygon_lines", "polygon_area"]

def polygon_lines(coords, closed=True):
    """
    Given a (n,2) set of XY coordinates as numpy array,
    creates an (n,2,2) array of coordinate pairs.
    If the given coordinate array represents the points of a polygon,
    the return value represents pairs of coordinates to draw lines between
    to obtain the full polygon image.

    If closed==True, a line is included between the last and the first point.
    Note that the last->first pair appears first in the list.

    Algorithm: http://stackoverflow.com/a/42407359/2597135
    """
    if len(coords.shape) != 2 or coords.shape[1] != 2:
        raise ValueError("Wrong shape for polygon lines input (expect (n,2)): {}".format(
            coords.shape))
    shifted = np.roll(coords, 1, axis=0)
    ret = np.transpose(np.dstack([shifted, coords]), axes=(0, 2, 1))
    return ret if closed else ret[1:]

def polygon_area(coords):
    """
    Compute the area of a polygon using the Shoelace formula

    Numpy hints from http://stackoverflow.com/a/30408825/2597135

    Parameters
    ----------
    coords : numpy array-like
        The coords in a (n,2) array
    """
    if len(coords.shape) != 2 or coords.shape[1] != 2:
        raise ValueError("Wrong shape for polygon area input (expect (n,2)): {}".format(
            coords.shape))
    x = coords[:,0]
    y = coords[:,1]
    return 0.5 * np.abs(np.dot(x, np.roll(y, 1)) - np.dot(y, np.roll(x, 1)))