Transparency for Poly3DCollection plot in matplotlib
I made a slight modification to the OP code and got the transparency working. It appears that the facecolors argument of Poly3DCollection overrides the transparency argument, so the solution was to set the color in a separate call to either Poly3DCollection.set_color or Poly3DCollection.set_facecolor:
from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Poly3DCollection
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [0, 2, 1, 1]
y = [0, 0, 1, 0]
z = [0, 0, 0, 1]
vertices = [[0, 1, 2], [0, 1, 3], [0, 2, 3], [1, 2, 3]]
tupleList = zip(x, y, z)
poly3d = [[tupleList[vertices[ix][iy]] for iy in range(len(vertices[0]))] for ix in range(len(vertices))]
ax.scatter(x,y,z)
collection = Poly3DCollection(poly3d, linewidths=1, alpha=0.2)
face_color = [0.5, 0.5, 1] # alternative: matplotlib.colors.rgb2hex([0.5, 0.5, 1])
collection.set_facecolor(face_color)
ax.add_collection3d(collection)
plt.show()
Interestingly, if you explicitly set the edge color with collection.set_edgecolor('k'), the edges will also honor the transparency setting.
I found a nice workaround: After plotting the data, do another plot on top with the same color and lighter line style. Instead of Poly3DCollection I use Line3DCollection, so no faces are plotted. The result looks very much as anticipated.
See below the new plot and the script creating it.
from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Poly3DCollection, Line3DCollection
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [0, 2, 1, 1]
y = [0, 0, 1, 0]
z = [0, 0, 0, 1]
vertices = [[0, 1, 2], [0, 1, 3], [0, 2, 3], [1, 2, 3]]
tupleList = list(zip(x, y, z))
poly3d = [[tupleList[vertices[ix][iy]] for iy in range(len(vertices[0]))] for ix in range(len(vertices))]
ax.scatter(x,y,z)
ax.add_collection3d(Poly3DCollection(poly3d, facecolors='w', linewidths=1, alpha=0.5))
ax.add_collection3d(Line3DCollection(poly3d, colors='k', linewidths=0.2, linestyles=':'))
plt.show()