Hi, I haven't looked at this list for a long time, so hi to all the active
devs.
I just came across a problem in the image.py imsave() function.
Problem:
At an ipython --pylab prompt, typing
a = rand(29,29)
imsave('test.png', a)
incorrectly generates a 28x28 pixel image.
This happens for several array sizes (I initially saw it for 226x226).
Line 1272 of image.py is
figsize = [x / float(dpi) for x in (arr.shape[1], arr.shape[0])]
figsize interacts with the bounding box code somehow to create off-by-one
results for certain values of array shape.
Possible solution:
To fix this properly, I think the float cast should be removed and
integer-based math should be used, but I thought that since it was rounding
down in the cases I saw, a change to the following should fix this:
figsize = [(x + 0.5) / float(dpi) for x in (arr.shape[1], arr.shape[0])]
and indeed this seems to work.
To verify this, I ran the following brute-force test of all image sizes up
to 1024x1024 at some common dpi values:
import matplotlib.pyplot as plt
import numpy as np
import os
for dpi in [75, 100, 150, 300, 600, 1200]:
for i in range(1,1025):
print dpi, i,
im = np.random.random((i,i))
fname = 'test{:03d}.png'.format(i)
plt.imsave(fname, im, dpi=dpi)
im = plt.imread(fname)[:,:,0]
print im.shape
assert im.shape == (i, i)
os.remove(fname)
and everything passed.
For completeness I also explored the dpi-space with these loop ranges and
the above loop body:
for dpi in range(1, 101):
for i in range(25, 36):
...
and all was still well.
Workaround:
For the moment I've set dpi=1 in my call to imsave which effectively
reverts its behaviour to the original imsave code prior to the introduction
of the dpi parameter.
I think this testing is rigorous enough to make this change.
If you agree, has anyone got time to change this, or shall I do a pull
request when I have time?
thanks,
Gary
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