[Bug]: PathCollection using alpha ignores 'none' facecolors

[subsurfaceiodev]

Bug summary

As title, using alpha parameter in PathCollection, either as a list or as a scalar value, causes 'none' facecolors to be ignored / filled

Code for reproduction

import matplotlib.pyplot as plt
from matplotlib.collections import PathCollection
from matplotlib.path import Path

# https://matplotlib.org/stable/gallery/shapes_and_collections/compound_path.html

paths = [
    Path(
        [(1, 1), (1, 2), (2, 2), (2, 1), (0, 0)],
        [Path.MOVETO] + [Path.LINETO] * 3 + [Path.CLOSEPOLY]
    ),
    Path(
        [(4, 4), (5, 5), (5, 4), (0, 0)],
        [Path.MOVETO] + [Path.LINETO] * 2 + [Path.CLOSEPOLY]
    ),
]

fig, ax = plt.subplots()
collection = PathCollection(
    paths,
    edgecolors='black',
    facecolors=['blue', 'none'],
    alpha=[1, 1],
    # alpha=1
)
collection.set_transform(ax.transData)
ax.add_artist(collection)
ax.set(xlim=(0, 5), ylim=(0, 5))
plt.show()

Actual outcome

Expected outcome

Additional information

No response

Operating system

Windows

Matplotlib Version

3.8.3

Matplotlib Backend

No response

Python version

3.12

Jupyter version

No response

Installation

pip

[chaoyihu]

When you set alpha = 1, collections::set_alpha() calls _set_facecolor(), which further calls to_rgba_array() in collections.py, Line 766: self._facecolors = mcolors.to_rgba_array(c, self._alpha) in which c is a list of strings: ["blue", "none"], and alpha is [1, 1].

Incolors::to_rgba_array(), Line 509 - 510, the alpha param is used to replace the default alpha values in the rgba sequence:

if alpha is not None:
    rgba[:, 3] = alpha

resulting in the following change:

facecolor rgba before applying alpha 
[[0. 0. 1. 1.]       # this is blue
 [0. 0. 0. 0.]]      # this has no color
facecolor rgba after applying alpha 
[[0. 0. 1. 1.]       # this is still blue
 [0. 0. 0. 1.]]      # this becomes black

To produce the expected outcome, try the following code:

import matplotlib.pyplot as plt
from matplotlib.collections import PathCollection
from matplotlib.path import Path

# https://matplotlib.org/stable/gallery/shapes_and_collections/compound_path.html

paths = [
    Path(
        [(1, 1), (1, 2), (2, 2), (2, 1), (0, 0)],
        [Path.MOVETO] + [Path.LINETO] * 3 + [Path.CLOSEPOLY]
    ),
    Path(
        [(4, 4), (5, 5), (5, 4), (0, 0)],
        [Path.MOVETO] + [Path.LINETO] * 2 + [Path.CLOSEPOLY]
    ),
]

fig, ax = plt.subplots()
collection = PathCollection(
    paths,
    edgecolors='black',
    facecolors=['blue', (0,0,0,0)]    # represent the desired face color using a explicit rgba tuple
)
collection.set_transform(ax.transData)
ax.add_artist(collection)
ax.set(xlim=(0, 5), ylim=(0, 5))
plt.show()