Transparency for Poly3DCollection plot in Matplotlib

A Poly3DCollection in Matplotlib allows you to create 3D polygonal surfaces. To make these surfaces transparent, you use the alpha parameter to control opacity levels between 0 (fully transparent) and 1 (fully opaque).

Basic Setup

First, let's create a simple 3D tetrahedron with transparent faces ?

from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Poly3DCollection, Line3DCollection
import numpy as np

# Set up the figure
fig = plt.figure(figsize=(8, 6))
ax = fig.add_subplot(111, projection='3d')

# Define vertices of a tetrahedron
x = [0, 2, 1, 1]
y = [0, 0, 1, 0]
z = [0, 0, 0, 1]

# Define which vertices form each face
vertices = [[0, 1, 2], [0, 1, 3], [0, 2, 3], [1, 2, 3]]

# Convert to coordinate tuples
tupleList = list(zip(x, y, z))
poly3d = [[tupleList[vertices[ix][iy]] 
           for iy in range(len(vertices[0]))] 
          for ix in range(len(vertices))]

# Add transparent polygons
ax.add_collection3d(Poly3DCollection(poly3d, facecolors='cyan', alpha=0.3))

# Add edge lines for clarity
ax.add_collection3d(Line3DCollection(poly3d, colors='black', linewidths=1))

# Add vertex points
ax.scatter(x, y, z, color='red', s=50)

plt.show()

Controlling Transparency Levels

Different alpha values create different transparency effects ?

from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Poly3DCollection
import numpy as np

fig, axes = plt.subplots(1, 3, figsize=(15, 5), subplot_kw={'projection': '3d'})

# Create a simple cube
vertices = np.array([
    [0, 0, 0], [1, 0, 0], [1, 1, 0], [0, 1, 0],  # bottom face
    [0, 0, 1], [1, 0, 1], [1, 1, 1], [0, 1, 1]   # top face
])

# Define the 6 faces of the cube
faces = [
    [vertices[j] for j in [0, 1, 2, 3]],  # bottom
    [vertices[j] for j in [4, 5, 6, 7]],  # top
    [vertices[j] for j in [0, 1, 5, 4]],  # front
    [vertices[j] for j in [2, 3, 7, 6]],  # back
    [vertices[j] for j in [0, 3, 7, 4]],  # left
    [vertices[j] for j in [1, 2, 6, 5]]   # right
]

alpha_values = [0.2, 0.5, 0.8]
titles = ['Low Transparency (?=0.2)', 'Medium Transparency (?=0.5)', 'High Transparency (?=0.8)']

for i, (ax, alpha, title) in enumerate(zip(axes, alpha_values, titles)):
    # Add the cube with different alpha values
    ax.add_collection3d(Poly3DCollection(faces, facecolors='blue', alpha=alpha, edgecolors='black'))
    
    ax.set_xlim(0, 1)
    ax.set_ylim(0, 1)
    ax.set_zlim(0, 1)
    ax.set_title(title)

plt.tight_layout()
plt.show()

Multiple Overlapping Transparent Objects

Transparency is especially useful when displaying multiple overlapping 3D objects ?

from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Poly3DCollection
import numpy as np

fig = plt.figure(figsize=(10, 8))
ax = fig.add_subplot(111, projection='3d')

# Create two overlapping pyramids
def create_pyramid(center, size):
    cx, cy, cz = center
    vertices = [
        [cx-size, cy-size, cz],      # base corners
        [cx+size, cy-size, cz],
        [cx+size, cy+size, cz],
        [cx-size, cy+size, cz],
        [cx, cy, cz+size*2]          # apex
    ]
    
    faces = [
        [vertices[j] for j in [0, 1, 2, 3]],  # base
        [vertices[j] for j in [0, 1, 4]],     # side faces
        [vertices[j] for j in [1, 2, 4]],
        [vertices[j] for j in [2, 3, 4]],
        [vertices[j] for j in [3, 0, 4]]
    ]
    return faces

# Create two pyramids at different positions
pyramid1 = create_pyramid((0, 0, 0), 1)
pyramid2 = create_pyramid((0.5, 0.5, 0.5), 1)

# Add first pyramid (red, semi-transparent)
ax.add_collection3d(Poly3DCollection(pyramid1, facecolors='red', alpha=0.4, edgecolors='darkred'))

# Add second pyramid (blue, semi-transparent)  
ax.add_collection3d(Poly3DCollection(pyramid2, facecolors='blue', alpha=0.4, edgecolors='darkblue'))

ax.set_xlim(-2, 3)
ax.set_ylim(-2, 3)  
ax.set_zlim(0, 4)

ax.set_xlabel('X axis')
ax.set_ylabel('Y axis')
ax.set_zlabel('Z axis')
ax.set_title('Overlapping Transparent Pyramids')

plt.show()

Key Parameters

Conclusion

Use the alpha parameter in Poly3DCollection to create transparent 3D surfaces. Lower alpha values increase transparency, making it easier to visualize overlapping or internal structures in 3D plots.