taubin smoothing

Summary: Taubin Smoothing for filtering meshes and making them smoother. Taubin smoothing is an iterative approach.

Reviewed By: nikhilaravi

Differential Revision: D24751149

fbshipit-source-id: fb779e955f1a1f6750e704f1b4c6dfa37aebac1a

pytorch3d/ops/__init__.py

@@ -23,6 +23,7 @@ from .utils import (
     wmean,
 )
 from .vert_align import vert_align
+from .mesh_filtering import taubin_smoothing
 
 
 __all__ = [k for k in globals().keys() if not k.startswith("_")]

pytorch3d/ops/mesh_filtering.py

@@ -0,0 +1,83 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+import torch
+from pytorch3d.structures import Meshes, utils as struct_utils
+
+# ------------------------ Mesh Smoothing ------------------------ #
+# This file contains differentiable operators to filter meshes
+# The ops include
+# 1) Taubin Smoothing
+# TODO(gkioxari) add more! :)
+# ---------------------------------------------------------------- #
+
+
+# ----------------------- Taubin Smoothing ----------------------- #
+
+
+def norm_laplacian(verts: torch.Tensor, edges: torch.Tensor, eps: float = 1e-12):
+    """
+    Norm laplacian computes a variant of the laplacian matrix which weights each
+    affinity with the normalized distance of the neighboring nodes.
+    More concretely,
+    L[i, j] = 1. / wij where wij = ||vi - vj|| if (vi, vj) are neighboring nodes
+
+    Args:
+        verts: tensor of shape (V, 3) containing the vertices of the graph
+        edges: tensor of shape (E, 2) containing the vertex indices of each edge
+    """
+    edge_verts = verts[edges]  # (E, 2, 3)
+    v0, v1 = edge_verts[:, 0], edge_verts[:, 1]
+
+    # Side lengths of each edge, of shape (E,)
+    w01 = 1.0 / ((v0 - v1).norm(dim=1) + eps)
+
+    # Construct a sparse matrix by basically doing:
+    # L[v0, v1] = w01
+    # L[v1, v0] = w01
+    e01 = edges.t()  # (2, E)
+
+    V = verts.shape[0]
+    L = torch.sparse.FloatTensor(e01, w01, (V, V))
+    L = L + L.t()
+
+    return L
+
+
+def taubin_smoothing(
+    meshes: Meshes, lambd: float = 0.53, mu: float = -0.53, num_iter: int = 10
+) -> Meshes:
+    """
+    Taubin smoothing [1] is an iterative smoothing operator for meshes.
+    At each iteration
+        verts := (1 - λ) * verts + λ * L * verts
+        verts := (1 - μ) * verts + μ * L * verts
+
+    This function returns a new mesh with smoothed vertices.
+    Args:
+        meshes: Meshes input to be smoothed
+        lambd, mu: float parameters for Taubin smoothing,
+            lambd > 0, mu < 0
+        num_iter: number of iterations to execute smoothing
+    Returns:
+        mesh: Smoothed input Meshes
+
+    [1] Curve and Surface Smoothing without Shrinkage,
+        Gabriel Taubin, ICCV 1997
+    """
+    verts = meshes.verts_packed()  # V x 3
+    edges = meshes.edges_packed()  # E x 3
+
+    for _ in range(num_iter):
+        L = norm_laplacian(verts, edges)
+        total_weight = torch.sparse.sum(L, dim=1).to_dense().view(-1, 1)
+        verts = (1 - lambd) * verts + lambd * torch.mm(L, verts) / total_weight
+
+        # pyre-ignore
+        L = norm_laplacian(verts, edges)
+        total_weight = torch.sparse.sum(L, dim=1).to_dense().view(-1, 1)
+        verts = (1 - mu) * verts + mu * torch.mm(L, verts) / total_weight
+
+    verts_list = struct_utils.packed_to_list(
+        verts, meshes.num_verts_per_mesh().tolist()
+    )
+    mesh = Meshes(verts=list(verts_list), faces=meshes.faces_list())
+    return mesh

tests/test_mesh_filtering.py

@@ -0,0 +1,74 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+
+
+import unittest
+
+import torch
+from common_testing import TestCaseMixin, get_random_cuda_device
+from pytorch3d.ops import taubin_smoothing
+from pytorch3d.ops.mesh_filtering import norm_laplacian
+from pytorch3d.structures import Meshes
+from pytorch3d.utils import ico_sphere
+
+
+class TestTaubinSmoothing(TestCaseMixin, unittest.TestCase):
+    def setUp(self) -> None:
+        super().setUp()
+        torch.manual_seed(1)
+
+    def test_taubin(self):
+        N = 3
+        device = get_random_cuda_device()
+
+        mesh = ico_sphere(4, device).extend(N)
+        ico_verts = mesh.verts_padded()
+        ico_faces = mesh.faces_padded()
+
+        rand_noise = torch.rand_like(ico_verts) * 0.2 - 0.1
+        z_mask = (ico_verts[:, :, -1] > 0).view(N, -1, 1)
+        rand_noise = rand_noise * z_mask
+        verts = ico_verts + rand_noise
+        mesh = Meshes(verts=verts, faces=ico_faces)
+
+        smooth_mesh = taubin_smoothing(mesh, num_iter=50)
+        smooth_verts = smooth_mesh.verts_padded()
+
+        smooth_dist = (smooth_verts - ico_verts).norm(dim=-1).mean()
+        dist = (verts - ico_verts).norm(dim=-1).mean()
+        self.assertTrue(smooth_dist < dist)
+
+    def test_norm_laplacian(self):
+        V = 32
+        F = 64
+        device = get_random_cuda_device()
+        # random vertices
+        verts = torch.rand((V, 3), dtype=torch.float32, device=device)
+        # random valid faces (no self circles, e.g. (v0, v0, v1))
+        faces = torch.stack([torch.randperm(V) for f in range(F)], dim=0)[:, :3]
+        faces = faces.to(device=device)
+        mesh = Meshes(verts=[verts], faces=[faces])
+        edges = mesh.edges_packed()
+
+        eps = 1e-12
+
+        L = norm_laplacian(verts, edges, eps=eps)
+
+        Lnaive = torch.zeros((V, V), dtype=torch.float32, device=device)
+        for f in range(F):
+            f0, f1, f2 = faces[f]
+            v0 = verts[f0]
+            v1 = verts[f1]
+            v2 = verts[f2]
+
+            w12 = 1.0 / ((v1 - v2).norm() + eps)
+            w02 = 1.0 / ((v0 - v2).norm() + eps)
+            w01 = 1.0 / ((v0 - v1).norm() + eps)
+
+            Lnaive[f0, f1] = w01
+            Lnaive[f1, f0] = w01
+            Lnaive[f0, f2] = w02
+            Lnaive[f2, f0] = w02
+            Lnaive[f1, f2] = w12
+            Lnaive[f2, f1] = w12
+
+        self.assertClose(L.to_dense(), Lnaive)