[gltf2-exporter] Adding FB_ngon_encoding support

code/AssetLib/glTF2/glTF2Asset.h

@@ -1108,6 +1108,7 @@ public:
         bool KHR_materials_clearcoat;
         bool KHR_materials_transmission;
         bool KHR_draco_mesh_compression;
+        bool FB_ngon_encoding;
     } extensionsUsed;
 
     //! Keeps info about the required extensions

code/AssetLib/glTF2/glTF2AssetWriter.inl

@@ -507,6 +507,19 @@ namespace glTF2 {
             Mesh::Primitive& p = m.primitives[i];
             Value prim;
             prim.SetObject();
+
+            // Extensions
+            {
+                Value exts;
+                exts.SetObject();
+
+                Value FB_ngon_encoding;
+                FB_ngon_encoding.SetObject();
+
+                exts.AddMember(StringRef("FB_ngon_encoding"), FB_ngon_encoding, w.mAl);
+                prim.AddMember("extensions", exts, w.mAl);
+            }
+
             {
                 prim.AddMember("mode", Value(int(p.mode)).Move(), w.mAl);
 
@@ -874,6 +887,10 @@ namespace glTF2 {
             if (this->mAsset.extensionsUsed.KHR_materials_transmission) {
                 exts.PushBack(StringRef("KHR_materials_transmission"), mAl);
             }
+
+            if (this->mAsset.extensionsUsed.FB_ngon_encoding) {
+                exts.PushBack(StringRef("FB_ngon_encoding"), mAl);
+            }
         }
 
         if (!exts.Empty())

code/AssetLib/glTF2/glTF2Exporter.cpp

@@ -97,6 +97,9 @@ glTF2Exporter::glTF2Exporter(const char* filename, IOSystem* pIOSystem, const ai
 
     mAsset.reset( new Asset( pIOSystem ) );
 
+    // Always on as our triangulation process is aware of this type of encoding
+    mAsset->extensionsUsed.FB_ngon_encoding = true;
+
     if (isBinary) {
         mAsset->SetAsBinary();
     }

code/PostProcessing/TriangulateProcess.cpp

@@ -217,10 +217,21 @@ bool TriangulateProcess::TriangulateMesh( aiMesh* pMesh)
         // if it's a simple point,line or triangle: just copy it
         if( face.mNumIndices <= 3)
         {
+            // ngon encoding: making sure that triangles are not recognized as false ngons.
+            // To do so, we make sure the first indice is not the same as previous triangle emitted.
+            unsigned int prev_first_indice = (unsigned int)-1;
+            if (curOut != out) prev_first_indice = (curOut - 1)->mIndices[0];
+
             aiFace& nface = *curOut++;
             nface.mNumIndices = face.mNumIndices;
             nface.mIndices    = face.mIndices;
 
+            if (nface.mIndices[0] == prev_first_indice) {
+                // rotate indices to avoid ngon encoding false ngons
+                std::swap(nface.mIndices[0], nface.mIndices[2]);
+                std::swap(nface.mIndices[1], nface.mIndices[2]);
+            }
+
             face.mIndices = nullptr;
             continue;
         }