[FIXED] Matplotlib patches contains_point() returns inconsistent results

Issue

Why does changing the plotting parameters of matplotlib.patches contains_point() change the value to be nonsensical? For example:

import matplotlib.patches as mpatches
print(mpatches.Circle((0,0), radius=1, ec='r').contains_point((0,1.5)))
print(mpatches.Circle((0,0), radius=1).contains_point((0,1.5)))

returns True for the first line, and False for the second - when obviously a unit circle centered at (0,0) does not contain (0, 1.5).

Solution

In the contains_point function you could specify a radius parameter so that all points enclosed between the borders of the patch and half the radius specified are also considered in the area. When you specify an edgecolor a default linewidth is set to 1 and this is going to modify the radius parameter in the contains_point function. In the source code of this function in fact you can see that there's a call to a _process_radius function that modify the radius parameter of contains_point to linewidth. This behaviour is documented in the docs of the contains_point function of the general path object:

The path is extended tangentially by radius/2; i.e. if you would draw the path with a linewidth of radius, all points on the line would still be considered to be contained in the area. Conversely, negative values shrink the area: Points on the imaginary line will be considered outside the area.

From this we can understand the behaviour you are experimenting. To solve you can:

1. set_edgecolor after checked for points
2. always specify radius=0 in the contains_point function

###### Solution 1 ######
import matplotlib.patches as mpatches
import matplotlib.pyplot as plt

circle_ec = mpatches.Circle((0,0), radius=1)
circle = mpatches.Circle((0,0), radius=1)
point = (0,1.5)
print(circle_ec.contains_point(point, radius = 0))         # prints False
print(circle.contains_point(point))                        # prints False
circle_ec.set_edgecolor('r')

###### Solution 2 #######
import matplotlib.patches as mpatches
import matplotlib.pyplot as plt

circle_ec = mpatches.Circle((0,0), radius=1, ec='r')
circle = mpatches.Circle((0,0), radius=1)
point = (0,1.5)
print(circle_ec.contains_point(point, radius = 0))         # prints False
print(circle.contains_point(point))                        # prints False

Anyway I think that is an unwanted behaviour since linewidth is specified in points and not in data coords. Opened an issue here.

Answered By - Aelius