How to get the points that outline a shape from a matplotlib artist path in python?

Question

I have create an ellipse as follows

x=0
y=0
ells=Ellipse(xy=(x,y), #create ellipse
             width =1, 
             height =2,
             edgecolor='b',
             fc='g',
             alpha=0.3,
             zorder=0)

I can't find out how to get reference to those points so I can directly assign them to variable. I read the docs on path, but I didn't see anything there that helped. The reason I need access to the points is that I want to apply a specific tranform to them that is more complex than I can do with the transform attribute of paths. I also tried indexing them like

ells[0] #and got
TypeError: 'Path' object does not support indexing

How do I get the points as an array?

Answer 1

You should use Ellipse.get_path() , and work from that Path object. Below you can find a fully working example, based on this post .

import matplotlib.pyplot as plt
from matplotlib.patches import Ellipse
from matplotlib.path import Path
from matplotlib.patches import PathPatch

img = plt.imread("image.jpg")

fig, ax = plt.subplots(1)
ax.set_aspect('equal')

ax.imshow(img)

# Create the base ellipse
ellipse = Ellipse((300, 300), width=400, height=100,
                  edgecolor='white', facecolor='none', linewidth=2)

# Get the path
path = ellipse.get_path()
# Get the list of path codes
codes = path.codes
# Get the list of path vertices
vertices = path.vertices.copy()
# Transform the vertices so that they have the correct coordinates
vertices = ellipse.get_patch_transform().transform(vertices)

# Do your transforms here
vertices[0] = vertices[0] - 10

# Create a new Path
modified_path = Path(vertices, codes)
# Create a new PathPatch
modified_ellipse = PathPatch(modified_path, edgecolor='white',
                             facecolor='none', linewidth=2)

# Add the modified ellipse
ax.add_patch(modified_ellipse)

plt.show()

Results before and after modifying the ellipse:

My example only moves one vertice, but you can apply any transform you want. If you change the number of vertices, however, make sure to update codes accordingly.