How to retrieve the raw figure data from matplotlib?

Question

I am using matplotlib to generate matrices I can train on. I need to get to the raw figure data. Saving and reading the .png works fine, but my code runs 10x longer. Another stack overflow asked a similar question and the solution was to grab the canvas, but that related logic generated a numpy error. Here is my mwe.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.transforms import IdentityTransform


px = 1/plt.rcParams['figure.dpi']  # pixel in inches
fig, ax = plt.subplots(figsize=(384*px, 128*px))
i = 756
plt.text(70, 95, "value {:04d}".format(i), color="black", fontsize=30, transform=IdentityTransform())
plt.axis('off')
plt.savefig("xrtv.png")     # I dont want to do this ...
rtv = plt.imread("xrtv.png")  # or this, but I want access to what imread returns.

gray = lambda rgb: np.dot(rgb[..., :3], [0.299, 0.587, 0.114])
gray = gray(rtv)

This answer provide a solution using fig.canvas.tostring_rgb(). In addition you could use plt.ioff() so nothing will be plotted until plt.show() has been called which can speed up your code. — obchardon
– obchardon, Commented Feb 6, 2023 at 16:00

Maciej Skorski · Accepted Answer · 2023-02-06 18:57:17Z

0

Disabling rendering was a good hint. Consider using a memory buffer to I/O rather than playing with strings. Here is a full example:

import numpy as np
import io
import matplotlib.pyplot as plt
from PIL import Image

# disable rendering and dump to buffer
plt.ioff()
fig,ax = plt.subplots()
ax.plot(np.sin(np.arange(100)))
buf = io.BytesIO()
fig.savefig(buf,format='RGBA')

# plot from buffer 
shape = (int(fig.bbox.bounds[-1]),int(fig.bbox.bounds[-2]),-1)
img_array = np.frombuffer(buf.getvalue(),dtype=np.uint8).reshape(shape)
Image.fromarray(img_array)

edited Feb 6, 2023 at 18:57

answered Feb 6, 2023 at 18:10

Maciej Skorski

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