add another example for create_isosurfaces() (#7068)

cpp/open3d/t/geometry/TriangleMesh.h

@@ -598,7 +598,7 @@ class TriangleMesh : public Geometry, public DrawableGeometry {
             const core::Device &device = core::Device("CPU:0"));
 
     /// Create a mesh from a 3D scalar field (volume) by computing the
-    /// isosurface.  This method uses the Flying Edges dual contouring method
+    /// isosurface. This method uses the Flying Edges dual contouring method
     /// that computes the isosurface similar to Marching Cubes.  The center of
     /// the first voxel of the volume is at the origin (0,0,0).  The center of
     /// the voxel at index [z,y,x] will be at (x,y,z).

cpp/pybind/t/geometry/trianglemesh.cpp

@@ -586,11 +586,30 @@ This example shows how to create a sphere from a volume::
     import open3d as o3d
     import numpy as np
 
-    coords = np.stack(np.meshgrid(*3*[np.linspace(-1,1,num=64)], indexing='ij'), axis=-1)
-    vol = np.linalg.norm(coords, axis=-1) - 0.5
+    grid_coords = np.stack(np.meshgrid(*3*[np.linspace(-1,1,num=64)], indexing='ij'), axis=-1)
+    vol = 0.5 - np.linalg.norm(grid_coords, axis=-1)
     mesh = o3d.t.geometry.TriangleMesh.create_isosurfaces(vol)
     o3d.visualization.draw(mesh)
 
+
+This example shows how to convert a mesh to a signed distance field (SDF) and back to a mesh::
+
+    import open3d as o3d
+    import numpy as np
+
+    mesh1 = o3d.t.geometry.TriangleMesh.create_torus()
+    grid_coords = np.stack(np.meshgrid(*3*[np.linspace(-2,2,num=64, dtype=np.float32)], indexing='ij'), axis=-1)
+
+    scene = o3d.t.geometry.RaycastingScene()
+    scene.add_triangles(mesh1)
+    sdf = scene.compute_signed_distance(grid_coords)
+    mesh2 = o3d.t.geometry.TriangleMesh.create_isosurfaces(sdf)
+
+    # Flip the triangle orientation for SDFs with negative values as "inside" and positive values as "outside"
+    mesh2.triangle.indices = mesh2.triangle.indices[:,[2,1,0]]
+
+    o3d.visualization.draw(mesh2)
+
 )");
 
     triangle_mesh.def(