Matplotlib 3.2.1 savefig empty image when fig size matches data size exactly

Bug report

Bug summary

Saving a figure to an image file with the same pixel dimensions as data loaded with imshow does not work in 3.2.1, producing an empty image. If the dimensions are changed by even one pixel, normal behavior returns.

This bug appeared in 3.2.1 and I have confirmed that it does not occur with 3.1.1.

I noticed in the release notes for 3.2.0 that some changes were made to the default image interpolation; perhaps an unhandled condition was introduced here when no interpolation is required. However, I note that changing the interpolation mode had no effect on this bug.

Code for reproduction

import numpy as np
from matplotlib import pylab as plt

x_size, y_size = (100, 100)
fig = plt.figure(frameon=False, dpi=100, figsize=(y_size/100, x_size/100))
ax = plt.Axes(fig, [0., 0., 1., 1.])
fig.add_axes(ax)
ax.set_axis_off()
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)

data = np.random.rand(100, 100)
ax.imshow(data)
fig.savefig('/tmp/image.png', bbox_inches='tight', pad_inches=0)

# In matplotlib 3.2.1, empty file with size 100x100 pixels output
# if random data shape is changed to something other than 100x100, output
# works correctly.

# In matplotlib 3.1.1, file output is 100x100 with data shown correctly.

Actual outcome

Expected outcome

Produced with matplotlib 3.1.1

Matplotlib version

Operating system: Linux version 5.6.13-arch1-1
Matplotlib version: 3.2.1
Matplotlib backend (print(matplotlib.get_backend())): TkAgg
Python version: 3.8.3
Other libraries: numpy 1.18.4

Default system install on Arch linux package python-matplotlib-3.2.1-1-x86_64

If you remove pad_inches=0 this works fine.

This stopping working seems to bisect to a9b8b6e which I think has more to do with extents than the image work. #16734

Furthermore, if you do pad_inches=0.0000000001 it works fine, or the axes size to 0.999999 etc it works fine. So its definitely that you are hitting a precise integer somehow. But if you don't do the tight_bbox, or pad_inches, you get exactly what you want.

Thanks for the quick response and work-around. Hopefully it will be possible to get to the underlying issue.

I don't recall the origin of the pad and bbox options in this code, though I would note that it is surprising that not specifying the pad_inches and bbox_inches options results in no padding in this case. From the documentation, the default value for pad_inches is 0.1 inch.

pad_inches only matters if you use bbox_inches='tight'

This was broken by #16734 which disables the draw methods on the renderer (because to compute the bounding boxes we don't need to actually draw turn the crank on the renderer methods, only the artist methods) but we never re-set the methods to their original values.

This is not normally a problem as the tight bounding box is a different size that original size so the caching in

matplotlib/lib/matplotlib/backends/backend_agg.py

Lines 411 to 421 in 4356b67

    
           def get_renderer(self, cleared=False): 
        
               w, h = self.figure.bbox.size 
        
               key = w, h, self.figure.dpi 
        
               reuse_renderer = (hasattr(self, "renderer") 
        
                                 and getattr(self, "_lastKey", None) == key) 
        
               if not reuse_renderer: 
        
                   self.renderer = RendererAgg(w, h, self.figure.dpi) 
        
                   self._lastKey = key 
        
               elif cleared: 
        
                   self.renderer.clear() 
        
               return self.renderer

misses and we get a new, un-patched renderer. However, in this case the figure is already the correct size so we hit the cache and get back the renderer with the draw methods stripped off which then results in an empty output.

I can see a bunch of hacky ways to fix this, but not sure which one is the best.

It seems to me that draw_disabled=True should just make a new un-cached renderer?

The problem is that where we know if we have flagged it as draw_disabled (in backend bases) is not where we know if there is a cache or not (deep in the print_XYZ functions in the backends).

FIX: do not let no-op monkey patches to renderer leak out

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Matplotlib 3.2.1 savefig empty image when fig size matches data size exactly #17542

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	def get_renderer(self, cleared=False):
	w, h = self.figure.bbox.size
	key = w, h, self.figure.dpi
	reuse_renderer = (hasattr(self, "renderer")
	and getattr(self, "_lastKey", None) == key)
	if not reuse_renderer:
	self.renderer = RendererAgg(w, h, self.figure.dpi)
	self._lastKey = key
	elif cleared:
	self.renderer.clear()
	return self.renderer

Matplotlib 3.2.1 savefig empty image when fig size matches data size exactly #17542

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jklymak commented on May 31, 2020

jklymak commented on May 31, 2020

rdgraham commented on May 31, 2020

jklymak commented on May 31, 2020

tacaswell commented on Jun 3, 2020

jklymak commented on Jun 3, 2020

tacaswell commented on Jun 3, 2020

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