From 5922c749937ab1e9c1d92537af8aba95177de42f Mon Sep 17 00:00:00 2001
From: Andre Sailer <andre.philippe.sailer@cern.ch>
Date: Mon, 15 Jan 2024 17:16:27 +0100
Subject: [PATCH] AssimWriter: implement facetIsDegenerated function

---
 DDCAD/include/DDCAD/Utilities.h | 24 ++++++++++++++++++++++++
 DDCAD/src/ASSIMPWriter.cpp      |  6 ++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/DDCAD/include/DDCAD/Utilities.h b/DDCAD/include/DDCAD/Utilities.h
index 325a5c408..6e12f0401 100644
--- a/DDCAD/include/DDCAD/Utilities.h
+++ b/DDCAD/include/DDCAD/Utilities.h
@@ -13,8 +13,11 @@
 #ifndef DDCAD_UTILITIES_H
 #define DDCAD_UTILITIES_H
 
+#include <vector>
+
 #include <TGeoTessellated.h>
 #include <TGeoVector3.h>
+
 /// Namespace for the AIDA detector description toolkit
 namespace dd4hep {
 
@@ -47,6 +50,27 @@ namespace dd4hep {
       str << "{" << v1 << ", " << v2 <<  ", " << v3 << "}";
       return str;
     }
+
+    // Determine if the facet is degenerated by calculating its determinant
+    inline bool facetIsDegenerated(std::vector<ROOT::Geom::Vertex_t> const& vertices){
+      const ROOT::Geom::Vertex_t& v1 = vertices[0];
+      const ROOT::Geom::Vertex_t& v2 = vertices[1];
+      const ROOT::Geom::Vertex_t& v3 = vertices[2];
+      constexpr double epsilon = 1.e-20;
+      // v1.x v2.x v3.x v1.x v2.x
+      //
+      // v1.y v2.y v3.y	v1.y v2.y
+      //
+      // v1.z v2.z v3.z	v1.z v2.z
+      double det =  0.0
+	+ v1.x() * v2.y() * v3.z()
+	+ v2.x() * v3.y() * v1.z()
+	+ v3.x() * v1.y() * v2.z()
+	- v1.z() * v2.y() * v3.x()
+	- v2.z() * v3.y() * v1.x()
+	- v3.z() * v1.y() * v2.x();
+      return det < epsilon;
+    }
   }
 }
 #endif
diff --git a/DDCAD/src/ASSIMPWriter.cpp b/DDCAD/src/ASSIMPWriter.cpp
index 82c692ded..f31219174 100644
--- a/DDCAD/src/ASSIMPWriter.cpp
+++ b/DDCAD/src/ASSIMPWriter.cpp
@@ -158,15 +158,17 @@ namespace  {
             ++nskip;
             continue;
           }
-#if ROOT_VERSION_CODE < ROOT_VERSION(6,31,1)
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6,31,1)
+          bool degenerated = dd4hep::cad::facetIsDegenerated(vertices);
+#else
           bool degenerated = true;
           TGeoFacet f(&vertices, 3, vv0, vv1, vv2);
           f.ComputeNormal(degenerated);
+#endif
           if ( degenerated )    {
             ++nskip;
             continue;
           }
-#endif
           tes->AddFacet(vv0, vv1, vv2);
         }
 #else