return a numpy array from a matplotlib plot is too slow, how can i speed it up?

Question

How can I speed up the conversion between matplotlib plots to a numpy arrays? My program creates millions of plots, and for each plot I want to return its numpy array (I do not care about viewing nor saving the plots! I only care about the conversion to numpy arrrays).

I managed to make the conversion with the following code:

data = np.fromstring(fig.canvas.tostring_rgb(), dtype=np.uint8, sep='')

data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))

Unfortunately, since I use: fig = plt.figure(num=1) at the beginning, and plt.clf() at the end, the program shows the images one by one on the figure, which slows everything down (about 1 to 2 frames per second).

I'm searching for a faster solution for the conversion from matplotlib plots to numpy arrays.

Update

I made the aggbackend change, but no improvement, where am I wrong? I'm attaching my code:

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import numpy as np
import imageio
from Game import Init, Draw, Game_step

images = []
Init()
fig = plt.figure(num=1)
Draw()
fig.canvas.draw()

for stp in range(100):
    action_button = np.random.randint(4)
    observation = Game_step(action_button)
    Draw()
    fig.canvas.draw()
    data = np.fromstring(fig.canvas.tostring_rgb(), dtype=np.uint8, sep='')
    data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))
images.append(data)
imageio.mimsave("game.gif", images, duration=1 / 35)

when Init: initialize the game, Draw: plot the current game screenshot in matplotlib, Game_step: taking one action in the game environment The goal is to get the np array of each screen plot

(I used Imagio just for checking, but it redundant)

Try using the `agg` backend without interactive mode being on. — ImportanceOfBeingErnest, Oct 20 '19 at 19:35
Ok after some investigation, I think the problems come from the looped line: fig.canvas.draw(). Unfourtantly, without it, I receive an error while trying to use .canvas.tostring_rgb(). Is any other way to turn a plot to a nparray? Should I use something like FuncAnimate or pyqtgraph? — Elad Prager, Oct 25 '19 at 14:25
No you should use the agg backend and turn interactive mode off. — ImportanceOfBeingErnest, Oct 25 '19 at 21:52
Thanks, I tried it, but still no improvement. I attached above my full code. What I'm doing wrong? — Elad Prager, Oct 26 '19 at 08:10

score 0 · Answer 1 · answered Oct 20 '19 at 18:03

Try using plt.ioff() to turn off interactive mode. This should prevent your figures from being drawn unless you explicitly call draw() or some other function.

SO Post on the semantics of ioff(): Exact semantics of Matplotlib's "interactive mode" (ion(), ioff())?

As well as a link to the documentation: https://matplotlib.org/3.1.0/api/_as_gen/matplotlib.pyplot.ioff.html

return a numpy array from a matplotlib plot is too slow, how can i speed it up?

1 Answers1