CODEWITHSUNDEEP
pythonmatplotlib
MrDrFenner
MrDrFenner

Reputation: 1150

Coloring Intersection of Circles/Patches

The following code:

# in ipython notebook, enable inline plotting with:
# %pylab inline --no-import-all
import matplotlib.pyplot as plt

# create some circles
circle1 = plt.Circle((-.5,0), 1, color='r', alpha=.2)
circle2 = plt.Circle(( .5,0), 1, color='b', alpha=.2)

# add them to the plot (bad form to use ;, but saving space)
# and control the display a bit
ax = plt.gca()
ax.add_artist(circle1); ax.add_artist(circle2)
ax.set_xlim(-2, 2); ax.set_ylim(-2, 2)
ax.set_aspect('equal')

# display it
plt.plot()

Produces the following plot:

VennishDiagram

I would like to specify the colors of the four regions (1) the background (currently white), (2 and 3) each individual event (the non-overlapping areas, currently blue and red), and (4) the intersection event (currently blended to purple). For example, I might color them red, green, blue, yellow -or- I might give them four different, precisely specified grayscale values (the later is more likely). [The colors will be generated based on characteristics of the underlying data.]

I specifically do not want to use alpha blending to "infer" a color in the intersection. I need to explicitly control the colors of all four regions.

I can think of a few strategies to solve this:

Upvotes: 14

Views: 7783

Answers (1)

hitzg
hitzg

Reputation: 12691

I'm not 100% sure but I think matplotlib does not have the functionality to intersect polygons. But you could use shapely:

import shapely.geometry as sg
import matplotlib.pyplot as plt
import descartes

# create the circles with shapely
a = sg.Point(-.5,0).buffer(1.)
b = sg.Point(0.5,0).buffer(1.)

# compute the 3 parts
left = a.difference(b)
right = b.difference(a)
middle = a.intersection(b)

# use descartes to create the matplotlib patches
ax = plt.gca()
ax.add_patch(descartes.PolygonPatch(left, fc='b', ec='k', alpha=0.2))
ax.add_patch(descartes.PolygonPatch(right, fc='r', ec='k', alpha=0.2))
ax.add_patch(descartes.PolygonPatch(middle, fc='g', ec='k', alpha=0.2))

# control display
ax.set_xlim(-2, 2); ax.set_ylim(-2, 2)
ax.set_aspect('equal')
plt.show()

enter image description here

Upvotes: 18

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