Add 3D sphere mesh example with PyVista visualization

examples/sphere.py

@@ -0,0 +1,93 @@
+"""
+Example of creating and visualizing a 3D sphere mesh using Gmsh and PyVista.
+
+This example demonstrates how to create a 3D sphere mesh using Gmsh and visualize it
+using PyVista. The mesh is saved in both .msh format and as PNG images from different
+viewing angles.
+"""
+
+from __future__ import annotations
+
+from typing import Literal
+
+import gmsh
+import numpy as np
+import pyvista as pv
+
+# Set PyVista plotting backend
+pv.set_plot_theme("document")
+pv.global_theme.window_size = [800, 600]
+pv.global_theme.background = "white"
+pv.OFF_SCREEN = True  # Enable off-screen rendering
+
+# Initialize Gmsh
+gmsh.initialize()
+
+# Create a new model
+model = gmsh.model
+model.add("sphere")
+
+# Create a sphere
+sphere = model.occ.addSphere(0, 0, 0, 1.0)
+
+# Synchronize the CAD kernel with the Gmsh model
+model.occ.synchronize()
+
+# Generate the mesh
+model.mesh.generate(3)
+
+# Get the mesh data
+nodes = model.mesh.getNodes()
+elements = model.mesh.getElements()
+
+# Print some basic information
+print(f"Number of nodes: {len(nodes[1]) // 3}")
+print(f"Number of elements: {len(elements[2])}")
+
+# Write the mesh to a file
+gmsh.write("sphere.msh")
+
+# Finalize Gmsh
+gmsh.finalize()
+
+# Create PyVista mesh for visualization
+# Convert nodes to numpy array and reshape
+node_coords = np.array(nodes[1]).reshape(-1, 3)
+print(f"Node coordinates shape: {node_coords.shape}")
+
+# Get tetrahedral elements and convert to 0-based indexing
+tet_elements = np.array(elements[2][1]).reshape(-1, 4) - 1  # Use elements[2][1] for connectivity
+print(f"Tetrahedra connectivity shape: {tet_elements.shape}")
+
+# Create PyVista UnstructuredGrid
+grid = pv.UnstructuredGrid({pv.CellType.TETRA: tet_elements}, node_coords)
+print("Grid information:")
+print(grid)
+
+# Extract surface
+surface = grid.extract_surface()
+print("Surface information:")
+print(surface)
+
+
+def save_view(view_name: str, camera_pos: Literal["xy", "xz", "yz"]) -> None:
+    """
+    Save a view of the mesh from a specified camera position.
+
+    Args:
+        view_name: Name of the view (used in the output filename)
+        camera_pos: Camera position ("xy", "xz", or "yz")
+
+    """
+    plotter = pv.Plotter(off_screen=True)
+    plotter.add_mesh(surface, show_edges=True, color="lightblue", edge_color="black", opacity=0.7)
+    plotter.camera_position = camera_pos
+    plotter.camera.zoom(1.5)
+    plotter.show_axes()
+    plotter.show(screenshot=f"sphere_{view_name}.png")
+
+
+# Save views from different angles
+save_view("xy", "xy")  # Top view
+save_view("xz", "xz")  # Front view
+save_view("yz", "yz")  # Side view