Merge pull request #524 from como2099/cwm/adapt_mixed_2d_fix_develop

Add check by computing the validity (using positive area) of a quadrilateral element : Fix adapt crash for mixed-element 2D meshes with frozen quad layers

Author: joshia5

ma/maQuality.cc

@@ -569,6 +569,32 @@ bool isLayerElementOk(Mesh* m, Entity* e)
     return isPyramidOk(m, e);
   if (type == apf::Mesh::PRISM)
     return isPrismOk(m, e);
+  if (type == apf::Mesh::QUAD) {
+    // Check if quad is non-degenerate without assuming a specific embedding.
+    // The quad is considered valid if its two sub-triangles have non-zero area
+    // and consistent orientation.
+
+    Entity* v[4];
+    m->getDownward(e, 0, v);
+    Vector p[4];
+    for (int i = 0; i < 4; ++i)
+      m->getPoint(v[i], 0, p[i]);
+
+    // Form two triangles sharing a diagonal
+    Vector n1 = apf::cross(p[1] - p[0], p[2] - p[0]);
+    Vector n2 = apf::cross(p[2] - p[0], p[3] - p[0]);
+
+    double a1 = n1.getLength();
+    double a2 = n2.getLength();
+
+    // Reject degenerate triangles
+    const double eps = 1e-14;
+    if (a1 < eps || a2 < eps)
+      return false;
+
+    // Check consistent orientation (normals point in the same general direction)
+    return (n1 * n2) > eps * a1 * a2;
+  }
   abort();
   return false;
 }