CODEWITHSUNDEEP
pythonimageperformancematplotlibplot
MaxPowers
MaxPowers

Reputation: 5486

Why does plt.savefig() performance decrease when calling in a loop?

I wrote the following code to test the performance of matplotlib's savefig()function:

import matplotlib.pyplot as plt
import numpy as np
from numpy.random import rand
import time

for i in xrange(10):

    init = time.time()
    x = rand(100)
    y = rand(100)
    t_init=time.time()-init

    init = time.time()
    ax = plt.axes()
    t_axes=time.time()-init

    init = time.time()
    num, _, _ = np.histogram2d(x,y)
    t_hist = time.time()-init

    init = time.time()
    ax.imshow(num, extent=[-.5,.5,-.5,.5], interpolation="bicubic")
    t_imshow = time.time()-init

    init = time.time()
    t = ax.text(i*.1,.1, "Value ="+str(i))
    plt.savefig("test/"+str(i)+".png")
    t.remove()
    t_savefig = time.time()-init

    print t_init, t_axes, t_hist, t_imshow, t_savefig

Unexpectedly, the performance of savefig() decreases with every iteration, as shown in the last column of the following table:

t_inint           t_axes            t_hist            t_imshow         t_savefig
4.10079956055e-05 0.114418029785    0.000813007354736 0.00125503540039 0.668319940567
2.28881835938e-05 0.000143051147461 0.00158405303955  0.00119304656982 0.297608137131
1.90734863281e-05 0.000148057937622 0.000726938247681 0.0012149810791  0.356621026993
2.31266021729e-05 0.0001380443573   0.000706911087036 0.0011830329895  0.37288403511
2.28881835938e-05 0.000149011611938 0.000706195831299 0.00119686126709 0.416905879974
2.00271606445e-05 0.000148057937622 0.000704050064087 0.00118589401245 0.505565881729
2.19345092773e-05 0.000140905380249 0.000710010528564 0.00121307373047 0.494667053223
2.09808349609e-05 0.000147819519043 0.000703096389771 0.00119400024414 0.5519759655
2.09808349609e-05 0.000139951705933 0.000716209411621 0.0011990070343  0.624140977859
3.2901763916e-05  0.000142097473145 0.000709056854248 0.00120401382446 0.634006023407

What causes savefig() to slow down? How can I avoid this behavior? Thank you.

Upvotes: 3

Views: 6745

Answers (2)

Greg
Greg

Reputation: 12234

You need to clear your axis between plots, adding plt.cla() will do the trick, there is a great stackoverflow post about clearing figures that is worth a read.

Upvotes: 10

sega_sai
sega_sai

Reputation: 8548

If you do plt.clf() between iterations, then the timing doesn't really increase. My guess is that before you were overplotting on top of previous axes (accumulating them), which lead to longer savefig.

Upvotes: 3

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