BUG: plot_surface ignores lightsource when cmap/facecolors are not specified

[eric-wieser]

Three different, undocumented, cases that I think should be the same:

1. plot_surface(..., shade=True, lightsource=..., cmap=..., facecolors="garbage") - uses LightSource.shade, which does some messy and unnecessary trig in LightSource.hillshade, using the orientation of the lightsource.

2. plot_surface(..., shade=True, lightsource=..., cmap=...) - uses _shade_colors, which does a more sensible vector-based approach, but hard-codes a [-1, -1, 0.5] normal vector, rather than respecting the lightsource direction.

3. plot_surface(..., shade=True, lightsource=..., facecolors="garbage") - as above

Regarding (2), I think that there is a typo in axes3d.py, and this change should be made:

         # Shade the data
-        if shade and cmap is not None and fcolors is not None:
+        if shade and cmap is not None and fcolors is None:
             fcolors = self._shade_colors_lightsource(Z, cmap, lightsource)

It seems dumb to only calculate face colors if the user already asked for different ones.

Regarding (3) - I think it would be useful to unify the shading into a normal-vector based approach, which I might try in a later patch

[eric-wieser]

A demo:

import numpy as np
from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

x, y = np.mgrid[-2:2:5j,-2:2:5j]
z = x * y

1. Simple case, showing ignored argument:

   fig, ax = plt.subplots(subplot_kw={"projection": "3d"})
   ax.plot_surface(x, y, z, cmap='viridis', shade=True, lightsource='Literally anything')

   

2. Showing different shading passing in a unrelated argument. Also, cmap can't always be a string

   fig, ax = plt.subplots(subplot_kw={"projection": "3d"})
   ax.plot_surface(x, y, z, cmap='viridis', shade=True, facecolors="do it differently, ok?")
   # TypeError: String object not callable
   ax.plot_surface(x, y, z, cmap=plt.get_cmap('viridis'), shade=True,
       facecolors="do it differently, ok?")

   

3. Showing lightsource calculations being wrong - overhead light should result in symmetric shading:

   from matplotlib.colors import LightSource
   fig, ax = plt.subplots(subplot_kw={"projection": "3d"})
   ax.plot_surface(x, y, z, cmap=plt.get_cmap('viridis'), shade=True,
      facecolors="do it differently, ok?", lightsource=LightSource(altdeg=90))

   

[eric-wieser]

This should probably remain open until some image-comparison tests are added to confirm it is fixed. I suspect the last issue still persists

[github-actions]

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