In Matplotlib, how do you add an Imagedraw object to a PyPlot? - image

Question

I need to add a shape to a preexisting image generated using a pyplot (plt). The best way I know of to generate basic shapes quickly is using Imagedraw's predefined shapes. The original data has points with corresponding colors in line_holder and colorholder. I need to add a bounding box (or in this case ellipse) to the plot to make it obvious to the user whether the data is in an acceptable range.
import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection
from PIL import Image
...
lines = LineCollection(mpl.line_holder, colors=mpl.colorholder , linestyle='solid')
plt.axes().add_collection(lines)
plt.axes().set_aspect('equal', 'datalim')
plt.axes().autoscale_view(True,True,True)
plt.draw()
plt.show()
I tried inserting this before the show():
image = Image.new('1',(int(ceil(disc/conv))+2,int(ceil(disc/conv))+1), 1)
draw = ImageDraw.Draw(image)
box=(1, 1, int(ceil(disc/conv)), int(ceil(disc/conv))) #create bounding box
draw.ellipse(box, 1, 0) #draw circle in black
but I cannot find a way to then add this ellipse to the pyplot. Does anyone know how one would go about getting the images together? If it is not possible to add an imagedraw object to a pyplot, are there good alternatives for performing this type of operation?

Answer 1

Matplotlib has several patches (shapes) that appear to meet your needs (and remove PIL as a dependency). They are documented here. A helpful example using shapes is here.
To add an ellipse to a plot, you first create a Ellipse patch and then add that patch to the axes you're currently working on. Beware that Circle's (or Ellipse's with equal minor radii) will appear elliptical if your aspect ratio is not equal.
In your snippet you call plt.axes() several times. This is unnecessary, as it is just returning the current axes object. I think it is clearer to keep the axes object and directly operate on it rather than repeatedly getting the same object via plt.axes(). As far as axes() is used in your snippet, gca() does the same thing. The end of my script demonstrates this.
I've also replaced your add_collection() line by a plotting a single line. These essentially do the same thing and allows my snippet to be executed as a standalone script.
import matplotlib.pyplot as plt
import matplotlib as mpl
# set up your axes object
ax = plt.axes()
ax.set_aspect('equal', 'datalim')
ax.autoscale_view(True, True, True)
# adding a LineCollection is equivalent to plotting a line
# this will run as a stand alone script
x = range(10)
plt.plot( x, x, 'x-')
# add and ellipse to the axes
c = mpl.patches.Ellipse( (5, 5), 1, 6, angle=45)
ax.add_patch(c)
# you can get the current axes a few ways
ax2 = plt.axes()
c2 = mpl.patches.Ellipse( (7, 7), 1, 6, angle=-45, color='green')
ax2.add_patch(c2)
ax3 = plt.gca()
c3 = mpl.patches.Ellipse( (0, 2), 3, 3, color='black')
ax3.add_patch(c3)
print id(ax), id(ax2), id(ax3)
plt.show()