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matplotlib3dtransparencyalpha

Matplotlib 3d alpha transparency bug?

Using matplotlib, I am trying to create a 3d plot that has three semi-transparent planes along the xy, yz, and xz planes. I am basing my code off of this post, which has a partial workaround for a transparency bug reported three years ago.

If you try out the below code and rotate the graph, you'll see that there are sudden color shifts in the areas where the planes overlap. For example below you see the center area suddenly change from green to blue. Is there a further workaround to prevent this?

Color shift upon rotation

Here is my code:

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
import mpl_toolkits.mplot3d as mp3d

xy = [ (-1, -1, 0),
       ( 1, -1, 0),
       ( 1,  1, 0),
       (-1,  1, 0),
    ]

yz = [ (0, -1, -1),
       (0,  1, -1),
       (0,  1,  1),
       (0, -1,  1),
    ]

xz = [ (-1, 0, -1),
       ( 1, 0, -1),
       ( 1, 0,  1),
       (-1, 0,  1),
    ]

fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
ax.scatter([-1, -1, -1, -1, 1, 1, 1, 1], [-1, -1, 1, 1, -1, -1, 1, 1], [-1, 1, -1, 1, -1, 1, -1, 1])
face1 = mp3d.art3d.Poly3DCollection([xy], alpha=0.5, linewidth=1)
face2 = mp3d.art3d.Poly3DCollection([yz], alpha=0.5, linewidth=1)
face3 = mp3d.art3d.Poly3DCollection([xz], alpha=0.5, linewidth=1)

# This is the key step to get transparency working
alpha = 0.5
face1.set_facecolor((0, 0, 1, alpha))
face2.set_facecolor((0, 1, 0, alpha))
face3.set_facecolor((1, 0, 0, alpha))

ax.add_collection3d(face1)
ax.add_collection3d(face2)
ax.add_collection3d(face3)

plt.show()
Solution

  • As suggested in the comments, you can divide every plane into its four quadrant planes and draw those individually. This way matplotlib is able to determine which of them should be in front and the planes obey transparency.

    enter image description here

    A minimal example:

    import matplotlib.pyplot as plt
import numpy as np
from mpl_toolkits.mplot3d import Axes3D
import mpl_toolkits.mplot3d as mp3d

a = np.array([( 0, 0, 0),( 1, 0, 0),( 1, 1, 0),( 0, 1, 0)])
R1 = np.array([[0,-1,0],[1,0,0],[0,0,1]])
R2 = (R1[::-1].T)[:,[1,0,2]]
R3 = (R1[::-1])[:,[1,0,2]]
f = lambda a,r: np.matmul(r, a.T).T
g = lambda a,r: [a, f(a,r), f(f(a,r),r), f(f(f(a,r),r),r)]


fig = plt.figure()
ax = fig.add_subplot(111, projection=Axes3D.name)
ax.scatter([-1,1], [-1,1], [-1,1], alpha=0.0)

for i, ind , r in zip(range(3),[[0,1,2],[2,0,1],[1,2,0]], [R1,R2,R3]):
    xy = g(a[:,ind], r )
    for x in xy:
        face1 = mp3d.art3d.Poly3DCollection([x] , alpha=0.5, linewidth=1)
        face1.set_facecolor((i//2, i%2, i==0,  0.5))
        ax.add_collection3d(face1)

plt.show()