Pythons mplcursors with matplotlib.pyplot patches

Question

I am currently adding patches on to an image and would like to annotate them with mplcursors.

However, I can't get mplcursors to selectively react to the drawn patches instead to the whole image (that is, each pixel): Here is a minimal working example that I am working on, any tips on how to solve this issue?

import matplotlib.pyplot as plt
import numpy as np
import mplcursors
from matplotlib.patches import Circle, Rectangle

fig, ax = plt.subplots(1,1)
im = np.zeros((300, 300))
imgplot = ax.imshow(im, interpolation = 'none')

rect = Rectangle((50,100),40,30,linewidth=1,edgecolor='r',facecolor='none')
label = ['a', 'b', 'c', 'd', 'e', 'f']
cursor = mplcursors.cursor(ax.patches, hover = True).connect('add',
    lambda sel: sel.annotation.set(text = label[sel.index]))
ax.add_patch(rect)

plt.show()

JohanC · Accepted Answer · 2022-03-21 18:35:45Z

1

ax.patches is still empty if the cursor is created before adding the rectangle to the plot. So, it would help to call ax.add_patch(rect) earlier. The annotation function has access to the properties of the selected element ("artist"), so you could e.g. add the information as a label. As mplcursors doesn't seem to work for circles, you could create a regular polygon to approximate one.

import matplotlib.pyplot as plt
import numpy as np
import mplcursors
from matplotlib.patches import Rectangle, Polygon

fig, ax = plt.subplots(1, 1)
im = np.zeros((300, 300))
img = ax.imshow(im, interpolation='none')

ax.add_patch(Rectangle((50, 100), 40, 30, linewidth=3, edgecolor='r', facecolor='none', label='first rectangle'))
ax.add_patch(Rectangle((150, 100), 40, 30, linewidth=3, edgecolor='y', facecolor='none', label='second rectangle'))
th = np.linspace(0, 2 * np.pi, 32)
rad, cx, cy = 50, 130, 200
ax.add_patch(Polygon(rad * np.c_[np.cos(th), np.sin(th)] + np.array([cx, cy]), closed=True, linewidth=3, edgecolor='g',
                     facecolor='none', label='a polygon\napproximating a circle'))

cursor = mplcursors.cursor(ax.patches, hover=True)
cursor.connect('add', lambda sel: sel.annotation.set(text=sel.artist.get_label()))

plt.show()

answered Mar 21, 2022 at 18:35

JohanC

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1 Comment

nebroth Over a year ago

Thank you very much, I thought I was somehow 'linking' the cursor to ax.patches and not passing it's values to cursor once. But that makes it obvious why it didn't work before. And thank you very much for the circle approximation, I actually will need that for my project

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