collapse small triangles

Former-commit-id: b782e55f3ac5707fd5bd0010db07ca953204515b

include/igl/collapse_small_triangles.cpp

@@ -0,0 +1,132 @@
+#include "collapse_small_triangles.h"
+
+#include "bounding_box_diagonal.h"
+#include "doublearea.h"
+#include "edge_lengths.h"
+#include "colon.h"
+#include "faces_first.h"
+
+#include <limits>
+
+#include <iostream>
+
+void igl::collapse_small_triangles(
+  const Eigen::MatrixXd & V,
+  const Eigen::MatrixXi & F,
+  const double eps,
+  Eigen::MatrixXi & FF)
+{
+  using namespace Eigen;
+  using namespace std;
+
+  // Compute bounding box diagonal length
+  double bbd = bounding_box_diagonal(V);
+  MatrixXd l;
+  edge_lengths(V,F,l);
+  VectorXd dblA;
+  doublearea(l,dblA);
+
+  // Minimum area tolerance
+  const double min_dblarea = 2.0*eps*bbd*bbd;
+
+  Eigen::VectorXi FIM = colon<int>(0,V.rows()-1);
+  int num_edge_collapses = 0;
+  // Loop over triangles
+  for(int f = 0;f<F.rows();f++)
+  {
+    if(dblA(f) < min_dblarea)
+    {
+      double minl = 0;
+      int minli = -1;
+      // Find shortest edge
+      for(int e = 0;e<3;e++)
+      {
+        if(minli==-1 || l(f,e)<minl)
+        {
+          minli = e;
+          minl = l(f,e);
+        }
+      }
+      double maxl = 0;
+      int maxli = -1;
+      // Find longest edge
+      for(int e = 0;e<3;e++)
+      {
+        if(maxli==-1 || l(f,e)>maxl)
+        {
+          maxli = e;
+          maxl = l(f,e);
+        }
+      }
+      // Be sure that min and max aren't the same
+      maxli = (minli==maxli?(minli+1)%3:maxli);
+
+      // Collapse min edge maintaining max edge: i-->j
+      // Q: Why this direction?
+      int i = maxli;
+      int j = ((minli+1)%3 == maxli ? (minli+2)%3: (minli+1)%3);
+      assert(i != minli);
+      assert(j != minli);
+      assert(i != j);
+      FIM(F(f,i)) = FIM(F(f,j));
+      num_edge_collapses++;
+    }
+  }
+
+  // Reindex faces
+  MatrixXi rF = F;
+  // Loop over triangles
+  for(int f = 0;f<rF.rows();f++)
+  {
+    for(int i = 0;i<rF.cols();i++)
+    {
+      rF(f,i) = FIM(rF(f,i));
+    }
+  }
+
+  FF.resize(rF.rows(),rF.cols());
+  int num_face_collapses=0;
+  // Only keep uncollapsed faces
+  {
+    int ff = 0;
+    // Loop over triangles
+    for(int f = 0;f<rF.rows();f++)
+    {
+      bool collapsed = false;
+      // Check if any indices are the same
+      for(int i = 0;i<rF.cols();i++)
+      {
+        for(int j = i+1;j<rF.cols();j++)
+        {
+          if(rF(f,i)==rF(f,j))
+          {
+            collapsed = true;
+            num_face_collapses++;
+            break;
+          }
+        }
+      }
+      if(!collapsed)
+      {
+        FF.row(ff++) = rF.row(f);
+      }
+    }
+    // Use conservative resize
+    FF.conservativeResize(ff,FF.cols());
+  }
+  //cout<<"num_edge_collapses: "<<num_edge_collapses<<endl;
+  //cout<<"num_face_collapses: "<<num_face_collapses<<endl;
+  if(num_edge_collapses == 0)
+  {
+    // There must have been a "collapsed edge" in the input
+    assert(num_face_collapses==0);
+    // Base case
+    return;
+  }
+
+  //// force base case
+  //return;
+
+  MatrixXi recFF = FF;
+  return collapse_small_triangles(V,recFF,eps,FF);
+}

include/igl/collapse_small_triangles.h

@@ -0,0 +1,40 @@
+// This file is part of libigl, a simple c++ geometry processing library.
+// 
+// Copyright (C) 2014 Alec Jacobson <alecjacobson@gmail.com>
+// 
+// This Source Code Form is subject to the terms of the Mozilla Public License 
+// v. 2.0. If a copy of the MPL was not distributed with this file, You can 
+// obtain one at http://mozilla.org/MPL/2.0/.
+#ifndef IGL_COLLAPSE_SMALL_TRIANGLES_H
+#define IGL_COLLAPSE_SMALL_TRIANGLES_H
+#include <Eigen/Dense>
+namespace igl
+{
+  // Given a triangle mesh (V,F) compute a new mesh (VV,FF) which contains the
+  // original faces and vertices of (V,F) except any small triangles have been
+  // removed via collapse.
+  //
+  // We are *not* following the rules in "Mesh Optimization" [Hoppe et al]
+  // Section 4.2. But for our purposes we don't care about this criteria.
+  //
+  // Inputs:
+  //   V  #V by 3 list of vertex positions
+  //   F  #F by 3 list of triangle indices into V
+  //   eps  epsilon for smallest allowed area treated as fraction of squared bounding box
+  //     diagonal
+  // Outputs:
+  //   FF  #FF by 3 list of triangle indices into V
+  //
+  //
+  void collapse_small_triangles(
+    const Eigen::MatrixXd & V,
+    const Eigen::MatrixXi & F,
+    const double eps,
+    Eigen::MatrixXi & FF);
+}
+
+#ifndef IGL_STATIC_LIBRARY
+#  include "collapse_small_triangles.cpp"
+#endif
+  
+#endif