This only occurs with specific image dimensions (e.g. 481,321). It's seems fine at other sizes nearby:

>>> im= np.zeros((479,321))
>>> im.shape[:2]
(479, 321)
>>> plt.imsave('./test',im)
>>> img = plt.imread('test.png')
>>> img.shape[:2]
(479, 321)

>>> im= np.zeros((482,321))
>>> im.shape[:2]
(482, 321)
>>> plt.imsave('./test',im)
>>> img = plt.imread('test.png')
>>> img.shape[:2]
(482, 321)

>>> im= np.zeros((481,321))
>>> im.shape[:2]
(481, 321)
>>> plt.imsave('./test',im)
>>> img = plt.imread('test.png')
>>> img.shape[:2]
(480, 321)

>>> im= np.zeros((480,321))
>>> im.shape[:2]
(480, 321)
>>> plt.imsave('./test',im)
>>> img = plt.imread('test.png')
>>> img.shape[:2]
(480, 321)

>>> im= np.zeros((479,321))
>>> im.shape[:2]
(479, 321)
>>> plt.imsave('./test',im)
>>> img = plt.imread('test.png')
>>> img.shape[:2]
(479, 321)
>>>

Stepping through the execution of imsave() the following occurs:

[x / float(dpi) for x in (arr.shape[1], arr.shape[0])]  #  [3.21, 4.81]
fig = Figure(figsize=figsize, dpi=dpi, frameon=False)
canvas = FigureCanvas(fig)
im = fig.figimage(arr, cmap=cmap, vmin=vmin, vmax=vmax, origin=origin)
im.get_size()  #  (481, 321)
im.get_extent()  #  (-0.5, 320.5, -0.5, 480.5)

Following all the way into backend_agg.print_png we find the values still correct:

self.figure.get_window_extent()
TransformedBbox(Bbox([[0.0, 0.0], [3.21, 4.81]]), Affine2D(array([[ 100.,    0.,    0.], [   0.,  100.,    0.], [   0.,    0.,    1.]])))

Replacing the output function to use PIL as a backend has no effect:

#_png.write_png(renderer._renderer, filename_or_obj, self.figure.dpi)
buf, size = self.print_to_buffer()
image = Image.frombuffer('RGBA', size, buf, 'raw', 'RGBA', 0, 1)
return image.save(filename_or_obj, format='png')

img.shape[:2]
(480, 321)

As print_to_buffer uses renderer._renderer this suggests the problem is there. The renderer is being passed the correct values:

renderer = self.get_renderer()
renderer, renderer.width, renderer.height   # <matplotlib.backends.backend_agg.RendererAgg object at 0x10b630850> 321.0 481.0

So the fault appears to be in .buffer_rgba() which I think is in the C library _backend_agg.so.

Uh oh!

imsave reduces 1row from the image #4280

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