[matplotlib-devel] Bug in imsave

[gary r. <gar...@gm...>]

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