Re: [matplotlib-devel] Bug in imsave

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

ceil doesn't work for me
I tried
figsize = [np.ceil(x / float(dpi)) for x in (arr.shape[1], arr.shape[0])]
and it fails on my second test when dpi=2 and i=25
result=26x26

Gary


On 9 April 2013 07:03, Michael Droettboom <md...@st...> wrote:

>  On 04/08/2013 02:38 PM, Benjamin Root wrote:
>
>
>
>
> On Sun, Apr 7, 2013 at 4:39 AM, gary ruben <gar...@gm...> wrote:
>
>>   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
>>
>>
>  Good catch on this.  So you are saying that this bug was introduced
> relatively recently with the dpi kwarg?  I would suggest you make a PR so
> that this doesn't get lost (or at the very least file a bug report).  As
> for the fix itself, off the top of my head, wouldn't math.ceil() do what we
> want?  I prefer to be explicit in any intents rather than just simply (x +
> 0.5) / float(dpi).  As for the test, I would wonder if some sort of
> restricted version of that test might not be good to do?  I am thinking
> about 10 different sized plots and we wouldn't even need to have a the
> assert check or file removal test, as the image comparison framework would
> throw an exception when attempting to compare two images of different sizes
> (I think...).
>
>
> I think this bug goes back at least to hash 17192518, by me in May 2010.
> The suggested fix (probably using ceil to be more explicit as you suggest)
> seems right.
>
> As for the test, I think just `imsave` to a buffer, and loading that
> buffer back in with `imread` and asserting the sizes are the same should be
> sufficient.
>
> Mike
>
>
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