Trying to create a semitransparentPoly3DCollection fails as shown in the following.

from mpl_toolkits.mplot3d import Axes3Dfrom mpl_toolkits.mplot3d.art3d import Poly3DCollectionimport matplotlib.pyplot as pltfig = plt.figure()ax = Axes3D(fig)x = [0, 1, 1, 0]y = [0, 0, 1, 1]z = [0, 1, 0, 1]verts = [list(zip(x, y, z))]alpha=0.5fc = "C0"# This line fails to give a semitransparent artistpc = Poly3DCollection(verts, alpha = alpha, facecolors=fc, linewidths=1)ax.add_collection3d(pc)plt.show()

It gives the following output, which still has an alpha of 1.

Non-Working options

Now one may try several things. The following isnot working:

• Setting facecolor and alpha individually, facecolor first,

    pc = Poly3DCollection(verts, linewidths=1)  pc.set_facecolor(fc)   pc.set_alpha(alpha)

• Setting facecolors and alpha at instantiation

    pc = Poly3DCollection(verts, alpha = alpha, facecolors=fc, linewidths=1)

Working options

However, each of the following is correctly giving a semitransparent artist. This would be the expected outcome for any of the above as well.

• Setting facecolor and alpha individually, alpha first,

    pc = Poly3DCollection(verts, linewidths=1)  pc.set_alpha(alpha)   # Order reversed   pc.set_facecolor(fc)

• Setting alpha at instantiation, facecolor afterwards

    pc = Poly3DCollection(verts, alpha = alpha, linewidths=1)  pc.set_facecolor(fc)

• Setting facecolor and alpha at instantiation

    pc = Poly3DCollection(verts, alpha = alpha, facecolor=fc, linewidths=1)

It seems there had been a similar bug in previous versions,https://stackoverflow.com/questions/18897786/transparency-for-poly3dcollection-plot-in-matplotlib which was reported to be fixed in the meantime but might have been reintroduced in a newer version.

In the 2D case the following works as expected and produces a semtransparent artist.

from matplotlib.collections import PolyCollectionimport matplotlib.pyplot as pltfig, ax = plt.subplots()x = [0, 1, 1, 0]y = [0, 0, 1, 1]verts = [list(zip(x, y))]alpha=0.5fc = "C0"pc = PolyCollection(verts, alpha = alpha, facecolors=fc, linewidths=1)ax.add_collection(pc)ax.autoscale()plt.show()

[All of the code in this issue has been tested with matplotlib 2.1]