Making figure transparent with colored background

Question

I have a bit of a situation. What I need is a plot with a black background with several white circles drawn on top of that black background.

I managed to do this usi

Answer 1

Circles masked with colorbar

Colormaps can have an alpha channel, so if your data is on a mesh with high vs low values showing circle vs not-circle, one set of those values can be transparent.

This only works for me when saving the figure programmatically, with the transparent keyword; not from the Python image window.

Starting from one of the matplotlib gallery examples (in a gimp-alike, I can cut & paste segments and the transparency is right):

# plot transparent circles with a black background
import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap
from matplotlib.cm import Greys


dark_low = ((0., 1., 1.),
         (.3, 1., 0.),
         (1., 0., 0.))
         
cdict = {'red':  dark_low,

     'green': dark_low,

     'blue': dark_low}

cdict3 = {'red':  dark_low,

     'green': dark_low,

     'blue': dark_low,
               
     'alpha': ((0.0, 0.0, 0.0),
               (0.3, 0.0, 1.0),
               (1.0, 1.0, 1.0))
    }

greys = LinearSegmentedColormap('Greys', cdict)
plt.register_cmap(cmap=greys)
    
dropout_high = LinearSegmentedColormap('Dropout', cdict3)
plt.register_cmap(cmap = dropout_high)

# Make some illustrative fake data:

x = np.arange(0, np.pi, 0.1)
y = np.arange(0, 2*np.pi, 0.1)
X, Y = np.meshgrid(x,y)
Z = np.cos(X) * np.sin(Y) * 10

# Make the figure:

plt.figure()
plt.subplot(1,3,1)
plt.imshow(Z, cmap=Greys)
plt.title('Smooth\ncolorbar')
plt.colorbar()
plt.subplot(1,3,2)
plt.imshow(Z, cmap=greys)
plt.title('Linear\ncolorbar')
plt.colorbar()
plt.subplot(1,3,3)
plt.imshow(Z, cmap = dropout_high)
plt.title('Alpha crops\n colorbar')
plt.colorbar()
plt.savefig('dropout_cmap', transparent=True)

And as a layer over another image. Interesting, the colorbar with alpha channel doesn't have transparency. That seems like a bug.