Bug summary

Using a combination of the following:

• pcolormesh with a set alpha parameter
• a colormap that uses the alpha channel for color mapping
  results in the colormap using the alpha parameter from pcolormesh as a constant.

Expected result would be that all resulting values from the colormapping are additionally blended with the alpha parameter from pcolormesh.

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib.colors

cdict ={'red':[(0.0, None, 1.0),
               (1.0, 1.0,  None)],
     'green': [(0.0, None, 0.0),
               (1.0, 0.0,  None)],
     'blue':  [(0.0, None, 0.0),
               (1.0, 0.0,  None)],
     'alpha': [(0.0, None, 0.0),
               (1.0,  1.0, None)]}
cmap = matplotlib.colors.LinearSegmentedColormap('custom',cdict,256)

H = [[1,2,3]]
yedges = [0,1]
xedges = [0,1,2,3]

f, (ax1, ax2) = plt.subplots(2, 1)

# case 1: no alpha, colormap defines alpha of values
ax1.pcolormesh(xedges, yedges, H, cmap=cmap)
ax1.set_title("pcolormesh without alpha parameter")

# case 2: with alpha, alpha is passed to colormap and every value has the same alpha
ax2.pcolormesh(xedges, yedges, H, cmap=cmap, alpha=0.5)
ax2.set_title("pcolormesh with alpha parameter 0.5")

plt.show()

Actual outcome

Expected outcome

The expected outcome is, that the "normal" output of pcolormesh is additionally alpha blended.

Additional information

This happens when an alpha parameter is passed to pcolormesh.
I suspect that the alpha parameter is passed to the colormap call, which effectively sets the colormap result alpha value to a constant.

Operating system

MacOS

Matplotlib Version

3.10.1

Matplotlib Backend

macosx

Python version

Python 3.13.1

Jupyter version

No response

Installation

pip