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[Bug]: setting norm for imshow and not using a colorbar produces error #28555

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rcomer opened this issue Jul 12, 2024 · 6 comments
Open

[Bug]: setting norm for imshow and not using a colorbar produces error #28555

rcomer opened this issue Jul 12, 2024 · 6 comments

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@rcomer
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rcomer commented Jul 12, 2024 •
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Bug summary

I tried removing the colorbar call from @timhoffm's new example at #28546, and I got an error

Code for reproduction

import matplotlib.pyplot as plt
import numpy as np

from matplotlib import colors

np.random.seed(19680801)

data = np.random.rand(10, 20)

fig, ax = plt.subplots()

norm = colors.Normalize(vmin=np.max(data), vmax=np.min(data))
im = ax.imshow(data, norm=norm)

plt.show()

Actual outcome

With QtAgg:

Traceback (most recent call last):
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/backends/backend_qt.py", line 498, in _draw_idle
    self.draw()
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/backends/backend_agg.py", line 387, in draw
    self.figure.draw(self.renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 95, in draw_wrapper
    result = draw(artist, renderer, *args, **kwargs)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/figure.py", line 3224, in draw
    mimage._draw_list_compositing_images(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 132, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/axes/_base.py", line 3152, in draw
    mimage._draw_list_compositing_images(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 132, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 651, in draw
    im, l, b, trans = self.make_image(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 950, in make_image
    return self._make_image(self._A, bbox, transformed_bbox, clip,
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 551, in _make_image
    output = self.norm(resampled_masked)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/colors.py", line 1395, in __call__
    raise ValueError("minvalue must be less than or equal to maxvalue")
ValueError: minvalue must be less than or equal to maxvalue

With TkAgg:

Exception in Tkinter callback
Traceback (most recent call last):
  File "/home/ruth/miniforge3/envs/mpl-dev/lib/python3.9/tkinter/__init__.py", line 1892, in __call__
    return self.func(*args)
  File "/home/ruth/miniforge3/envs/mpl-dev/lib/python3.9/tkinter/__init__.py", line 814, in callit
    func(*args)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/backends/_backend_tk.py", line 271, in idle_draw
    self.draw()
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/backends/backend_tkagg.py", line 10, in draw
    super().draw()
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/backends/backend_agg.py", line 387, in draw
    self.figure.draw(self.renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 95, in draw_wrapper
    result = draw(artist, renderer, *args, **kwargs)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/figure.py", line 3224, in draw
    mimage._draw_list_compositing_images(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 132, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/axes/_base.py", line 3152, in draw
    mimage._draw_list_compositing_images(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 132, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/artist.py", line 72, in draw_wrapper
    return draw(artist, renderer)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 651, in draw
    im, l, b, trans = self.make_image(
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 950, in make_image
    return self._make_image(self._A, bbox, transformed_bbox, clip,
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/image.py", line 551, in _make_image
    output = self.norm(resampled_masked)
  File "/home/ruth/git_stuff/matplotlib/lib/matplotlib/colors.py", line 1395, in __call__
    raise ValueError("minvalue must be less than or equal to maxvalue")
ValueError: minvalue must be less than or equal to maxvalue

Expected outcome

No error

Additional information

Everything is fine if I add a colorbar.

Operating system

Ubuntu

Matplotlib Version

main, 3.8.4, 3.6.3

Matplotlib Backend

QtAgg and TkAgg

Python version

3.9.19, 3.12.3, 3.11.6

Jupyter version

N/A

Installation

conda
@rcomer rcomer mentioned this issue Jul 12, 2024
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@jklymak
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jklymak commented Jul 12, 2024

There was some logic added to colorbar to have the colorbar limits affect the Norm (@greglucas). I suspect that allows the example to work with a colorbar, but break otherwise?

@tacaswell
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Yeah, I also strongly suspect we are getting saved by the "make-non-singular" logic in the color bar.

Looking at the code setting the min to the max of the data and vica versa looks like a typo?

@greglucas
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There is a typo in the vmin that should be min(data) in the example. Although it is unfortunate that this errors with/without a colorbar.

The colorbar swaps the vmin and vmax for a user through the nonsingular call expansion I think (on mobile and can't verify) which saves this case for the user.

@timhoffm
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Ops, indeed a typo in min/max. -> Fixed for the example.

Should the norm itself check that vmin < vmax (or <=)?

@jklymak
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jklymak commented Jul 12, 2024 •
edited
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We allow other limits to be flipped (eg set_ylim(100, 0) will invert the yaxis). Flipping the limits for the norm is not an unreasonable thing to do, so instead of doing cmap='jet_r' one could do imshow(A, vmin=100, vmax=0, cmap='jet')

@timhoffm
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timhoffm commented Jul 12, 2024 •
edited
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Yes we do this for axis limits. The point is that, we currently do not flip the colorbar, but silently exchange min/max in the background. Changing to vmin=100, vmax=0 with a flipping behavior, is a "silent" behavoir change, which is more problematic compared to not allowing that behavior. But maybe you could sell it as a bug fix.

Note that vmin=100, vmax=0, cmap='jet' is not the same as vmin=0, vmax=100, cmap='jet_r'. Additionally to _r-ing the colormap ``vmin=100, vmax=0should invert the axis direction, i.e. be similar tocolorbar._long_axis().set_inverted(True)`.

@timhoffm timhoffm mentioned this issue Aug 13, 2024
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5 participants
@tacaswell @jklymak @timhoffm @rcomer @greglucas