在 plt.imshow() 中将 alpha 设置为数组会使绘图崩溃

Question

当尝试运行受 matplotlib 文档启发的这段代码时 Blend transparency with color in 2-D images:

import numpy as np
import matplotlib.pyplot as plt

test = np.random.random((300, 300))

plt.figure()
plt.imshow(test, alpha = test/test.max())
plt.show()

我最终得到：

<matplotlib.image.AxesImage at 0x7f8bfe8cc610>Traceback (most recent call last):
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/backends/backend_qt5.py", line 508, in _draw_idle
    self.draw()
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py", line 388, in draw
    self.figure.draw(self.renderer)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/figure.py", line 1709, in draw
    renderer, self, artists, self.suppressComposite)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/image.py", line 135, in _draw_list_compositing_images
    a.draw(renderer)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/axes/_base.py", line 2647, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/image.py", line 135, in _draw_list_compositing_images
    a.draw(renderer)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/image.py", line 619, in draw
    renderer, renderer.get_image_magnification())
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/image.py", line 881, in make_image
    unsampled=unsampled)
  File "/Users/darkvador/opt/anaconda3/lib/python3.7/site-packages/matplotlib/image.py", line 530, in _make_image
    np.asarray(alpha_channel, np.float32) * out_alpha * alpha,
ValueError: operands could not be broadcast together with shapes (740,740) (300,300)

我在 macos-catalina 下，python:

Python 3.7.7 (default, Mar 26 2020, 10:32:53) 
[Clang 4.0.1 (tags/RELEASE_401/final)] :: Anaconda, Inc. on darwin

请注意，在以下情况下不会发生错误：

我评论plt.show()
我使用常量alpha-值
我运行这段代码是经典python3.6下的linux分布

Answer 1

您的 alpha 数组 alpha/alpha.max() 与实际图像数组 test 的形状不匹配。根据您提供的测试代码，我们可以创建一个 alpha 数组并使用下面的代码将其成功显示为 alpha-varying 图像。因此，您必须确保 alpha 数组的形状与 test 数组的形状完全相同。

import numpy as np
import matplotlib.pyplot as plt

test = np.random.random((300, 300))
alpha = test / test.max()
print(alpha)

plt.figure()
plt.imshow(test, alpha = alpha)
plt.show()

生成下图。

Answer 2

从 matplotlib 3.1.3 documentation 开始，alpha 关键字参数必须是标量值。

你应该更新到 Matplotlib 3.3.1，因为给 alpha 参数的数组必须在这两个版本之间实现。

在 plt.imshow() 中将 alpha 设置为数组会使绘图崩溃

Setting alpha as an array in plt.imshow() makes the plot crashe

python

matplotlib

python-3.x

anaconda

conda