python - Find holes in a union of rectangles? -


i have number of random rectangles (black) in , around unit square (red) , need extract polygonal regions inside unit square not covered rectangle.

enter image description here

it looks can done shapely , i've gotten point when have union of rectangles (green) i'm not sure how subtract unit square , retrieve list of polygons.

here code generate test data:

import pylab import random matplotlib import pyplot shapely.geometry import point, polygon shapely.ops import cascaded_union descartes import polygonpatch  def make_square(x, y, size1, size2):     dx = [size1, -size1, -size1, size1, size1]     dy = [size2, size2, -size2, -size2, size2]     return [(x+sx, y+sy) sx, sy in zip(dx, dy)]   pylab.figure()  square = make_square(0.5, 0.5, 1.0, 1.0) a, b = zip(*square) pylab.plot(a, b, 'r-') polygons = []  in xrange(10):     x = random.random()     y = random.random()     s1 = random.random()     s2 = random.random()      square = make_square(x, y, s1, s2)     polygons.append(polygon(square))     a, b = zip(*square)     pylab.plot(a, b, 'k-')  u = cascaded_union(polygons) patch2b = polygonpatch(u, fc='#00ff00', ec='#00ff00', alpha=0.5, zorder=2) pylab.gca().add_patch(patch2b)   pylab.show()

basically, want take difference of "unioned" polygons large square , polygonize result individual, separate polygons. example:

#-- region not covered individual squares. uncovered_region = polygon(bigsquare).difference(union)  # in cases, result single polygon... if not isinstance(uncovered_region, multipolygon):     uncovered_region = [uncovered_region]  poly in polygonize(uncovered_region):     patch = polygonpatch(poly, fc='purple', alpha=0.5, zorder=2)     ax.add_patch(patch)

as full example, based on yours:

import matplotlib.pyplot plt import random shapely.geometry import polygon, multipolygon shapely.ops import cascaded_union, polygonize descartes import polygonpatch  def make_square(x, y, size1, size2):     dx = [size1, -size1, -size1, size1, size1]     dy = [size2, size2, -size2, -size2, size2]     return [(x+sx, y+sy) sx, sy in zip(dx, dy)]   fig, ax = plt.subplots()  bigsquare = make_square(0.5, 0.5, 1.0, 1.0) a, b = zip(*bigsquare) ax.plot(a, b, 'r-') polygons = []  in xrange(10):     x = random.random()     y = random.random()     s1 = random.random()     s2 = random.random()      square = make_square(x, y, s1, s2)     polygons.append(polygon(square))     a, b = zip(*square)     ax.plot(a, b, 'k-')  union = cascaded_union(polygons) patch2b = polygonpatch(union, fc='#00ff00', ec='#00ff00', alpha=0.5, zorder=2) ax.add_patch(patch2b)  #-- region not covered individual squares. uncovered_region = polygon(bigsquare).difference(union)  # in cases, result single polygon... if not isinstance(uncovered_region, multipolygon):     uncovered_region = [uncovered_region]  poly in polygonize(uncovered_region):     print poly     patch = polygonpatch(poly, fc='purple', alpha=0.5, zorder=2)     ax.add_patch(patch)  plt.margins(0.05) plt.show()

enter image description here


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