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4

In Python and Matplotlib, it is easy to either display the plot as a popup window or save the plot as a PNG file. How can I instead save the plot to a numpy array in RGB format?

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up vote 22 down vote

This is a handy trick for unit tests and the like, when you need to do a pixel-to-pixel comparison with a saved plot.

One way is to use fig.canvas.tostring_rgb and then numpy.fromstring with the approriate dtype. There are other ways as well, but this is the one I tend to use.

E.g.

import matplotlib.pyplot as plt
import numpy as np

# Make a random plot...
fig = plt.figure()
fig.add_subplot(111)

# If we haven't already shown or saved the plot, then we need to
# draw the figure first...
fig.canvas.draw()

# Now we can save it to a numpy array.
data = np.fromstring(fig.canvas.tostring_rgb(), dtype=np.uint8, sep='')
data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))
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Excellent! 12345 – Petter Oct 19 '11 at 15:39
    
Is this only supported on certain backend? Does not seem to be working with macosx backend (tostring_rgb) not found. – mirosval Mar 5 '14 at 15:07
    
Works on Agg, add matplotlib.use('agg') before import matplotlib.pyplot as plt to use it. – mirosval Mar 5 '14 at 15:38
4  
With images, the canvas adds a big margin, so I found it useful to insert fig.tight_layout(pad=0) before drawing. – Dan Allan Oct 14 '14 at 16:01
1  
For figures with lines and text, it can also be important to turn antialiasing off. For lines plt.setp([ax.get_xticklines() + ax.get_yticklines() + ax.get_xgridlines() + ax.get_ygridlines()],antialiased=False) and for text mpl.rcParams['text.antialiased']=False – kmader Nov 3 '16 at 11:13

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