How to plot half-ellipses in Python?

Question

I need to plot half-ellipses (or to be precise, half-elliptical disks, hence I can't use matplotlib to plot an elliptical arc).

I found OpenCV can do this with this syntax: cv2.Ellipse(img, center, axes, angle, start_angle, end_angle, color, thickness=1, lineType=8, shift=0), but there is one thing that really bothers me. I want to use standard x and y coordinates, not pixels. Also, I will need to plot half ellipses with non-integer radius, and it seem OpenCV can't do that (for the same reason of having pixel coordinates).

So, I need a code that can do what OpenCV does (with the same angle + start_angle + end_angle structure), but without requiring me to work in pixels.

score 4 · Accepted Answer · edited May 23 '17 at 11:54

4

You can use matplotlib arc is you didn't want them filled in. For a filled arc, you can use the accepted solution here where a generic patch is defined and combine with the matplotlib ellipse example,

import matplotlib.patches as mpatches
import matplotlib.pyplot as plt
import numpy.random as rnd
import numpy as np

def arc_patch(xy, width, height, theta1=0., theta2=180., resolution=50, **kwargs):

    # generate the points
    theta = np.linspace(np.radians(theta1), np.radians(theta2), resolution)
    points = np.vstack((width*np.cos(theta)  + xy[0], 
                        height*np.sin(theta) + xy[1]))
    # build the polygon and add it to the axes
    poly = mpatches.Polygon(points.T, closed=True, **kwargs)

    return poly

NUM = 10
arcs = []
for i in range(NUM):
    r = rnd.rand()*360.
    arcs.append(arc_patch(xy=rnd.rand(2)*10, width=1., 
                height=1., theta1=r, theta2=r+180.))

# axis settings
fig, ax = plt.subplots(1,1)
for a in arcs:
    ax.add_artist(a)
    a.set_clip_box(ax.bbox)
    a.set_alpha(0.5)
    a.set_facecolor(rnd.rand(3))

ax.set_xlim(0, 10)
ax.set_ylim(0, 10)

plt.show()

Which looks like,

edited May 23 '17 at 11:54

Community

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answered Apr 22 '16 at 11:06

Ed Smith

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FYI, `matplotlib.patches.Arc` can make filled half-ellipses. – Syrtis Major Mar 14 '17 at 14:02
Do you have a reference for this @Syrtis Major? The official docs for `matplotlib.patches.Arc` here: http://matplotlib.org/api/patches_api.html#module-matplotlib.patches explicitly says "Because it performs various optimizations, it can not be filled." – Ed Smith Mar 17 '17 at 10:01
Ah, I didn't notice the docs, but just tried it (see answer below). I'm not sure why, maybe the docs is a bit outdated? – Syrtis Major Mar 17 '17 at 12:32
I just realize that `Arc`s can not fill themselves indeed, but the `PatchCollection` can fill them. That's why my solution works. – Syrtis Major Apr 01 '17 at 06:05

Syrtis Major · Answer 2 · 2017-04-05T02:15:14.990

Use matplotlib.patches.Arc can make half-ellipses, just specify the keywords theta1=0.0, theta2=180.0 (or 90 to 270). I've written a wrapper function named arcs for making a scatter plot of Arcs. It uses the PatchCollection, should have better performance and enable colorbar. You can find it at gist (link).

Here's an example:

a = np.arange(11)
arcs(a, a, w=4, h=a, rot=a*30, theta1=0.0, theta2=180.0, 
     c=a, alpha=0.5, edgecolor='none')
plt.colorbar()

A brief implementation of arcs is posted below for completeness as Ed Smith suggested.

def arcs(x, y, w, h, rot=0.0, theta1=0.0, theta2=360.0,
         c='b', **kwargs):
    import numpy as np
    import matplotlib.pyplot as plt
    from matplotlib.patches import Arc
    from matplotlib.collections import PatchCollection

    if np.isscalar(c):
        kwargs.setdefault('color', c)
        c = None

    zipped = np.broadcast(x, y, w, h, rot, theta1, theta2)
    patches = [Arc((x_, y_), w_, h_, rot_, t1_, t2_)
               for x_, y_, w_, h_, rot_, t1_, t2_ in zipped]
    collection = PatchCollection(patches, **kwargs)

    if c is not None:
        c = np.broadcast_to(c, zipped.shape).ravel()
        collection.set_array(c)

    ax = plt.gca()
    ax.add_collection(collection)
    return collection

The full version can be found at gist (link).

Your solution looks good. Is it possible to extract out the minimal code for an example of using PatchCollections so as to give a self contained answer here? — Ed Smith, Apr 03 '17 at 15:50

How to plot half-ellipses in Python?

2 Answers2