add demo to test soft body friction

src/BulletSoftBody/btSoftBodyHelpers.cpp

@@ -1387,6 +1387,76 @@ btSoftBody* btSoftBodyHelpers::CreateFromVtkFile(btSoftBodyWorldInfo& worldInfo,
     return psb;
 }
 
+void btSoftBodyHelpers::generateBoundaryFaces(btSoftBody* psb)
+{
+    int counter = 0;
+    for (int i = 0; i < psb->m_nodes.size(); ++i)
+    {
+        psb->m_nodes[i].index = counter++;
+    }
+    typedef btAlignedObjectArray<int> Index;
+    btAlignedObjectArray<Index> indices;
+    indices.resize(psb->m_tetras.size());
+    for (int i = 0; i < indices.size(); ++i)
+    {
+        Index index;
+        index.push_back(psb->m_tetras[i].m_n[0]->index);
+        index.push_back(psb->m_tetras[i].m_n[1]->index);
+        index.push_back(psb->m_tetras[i].m_n[2]->index);
+        index.push_back(psb->m_tetras[i].m_n[3]->index);
+        indices[i] = index;
+    }
+    
+    std::map<std::vector<int>, std::vector<int> > dict;
+    for (int i = 0; i < indices.size(); ++i)
+    {
+        for (int j = 0; j < 4; ++j)
+        {
+            std::vector<int> f;
+            if (j == 0)
+            {
+                f.push_back(indices[i][1]);
+                f.push_back(indices[i][0]);
+                f.push_back(indices[i][2]);
+            }
+            if (j == 1)
+            {
+                f.push_back(indices[i][3]);
+                f.push_back(indices[i][0]);
+                f.push_back(indices[i][1]);
+            }
+            if (j == 2)
+            {
+                f.push_back(indices[i][3]);
+                f.push_back(indices[i][1]);
+                f.push_back(indices[i][2]);
+            }
+            if (j == 3)
+            {
+                f.push_back(indices[i][2]);
+                f.push_back(indices[i][0]);
+                f.push_back(indices[i][3]);
+            }
+            std::vector<int> f_sorted = f;
+            std::sort(f_sorted.begin(), f_sorted.end());
+            if (dict.find(f_sorted) != dict.end())
+            {
+                dict.erase(f_sorted);
+            }
+            else
+            {
+                dict.insert(std::make_pair(f_sorted, f));
+            }
+        }
+    }
+    
+    for (auto it = dict.begin(); it != dict.end(); ++it)
+    {
+        std::vector<int> f = it->second;
+        psb->appendFace(f[0], f[1], f[2]);
+    }
+}
+
 void btSoftBodyHelpers::writeObj(const char* filename, const btSoftBody* psb)
 {
     std::ofstream fs;