tweaks to primitive merging logic; comments + formatting

code/glTF2Exporter.cpp

@@ -744,6 +744,7 @@ void glTF2Exporter::ExportMeshes()
     }
 }
 
+//merges a node's multiple meshes (with one primitive each) into one mesh with multiple primitives
 void glTF2Exporter::MergeMeshes()
 {
     for (unsigned int n = 0; n < mAsset->nodes.Size(); ++n) {
@@ -751,16 +752,34 @@ void glTF2Exporter::MergeMeshes()
 
         unsigned int nMeshes = node->meshes.size();
 
-        if (nMeshes) {
+        //skip if it's 1 or less meshes per node
+        if (nMeshes > 1) {
             Ref<Mesh> firstMesh = node->meshes.at(0);
+            Mesh::Primitive firstPrimitive = firstMesh->primitives.at(0);
 
-            for (unsigned int m = 1; m < nMeshes; ++m) {
+            //loop backwards to allow easy removal of a mesh from a node once it's merged
+            for (unsigned int m = nMeshes - 1; m >= 1; --m) {
                 Ref<Mesh> mesh = node->meshes.at(m);
-                Mesh::Primitive primitive = mesh->primitives.at(0);
-                firstMesh->primitives.push_back(primitive);
+                bool primitivesPushed = false;
+
+                for (unsigned int p = 0; p < mesh->primitives.size(); ++p) {
+                    Mesh::Primitive primitive = mesh->primitives.at(p);
+
+                    if (firstPrimitive.mode == primitive.mode) {
+                        firstMesh->primitives.push_back(primitive);
+                        primitivesPushed = true;
+                    }
+                }
+
+                if (primitivesPushed) {
+                    //remove the merged meshes from the node
+                    node->meshes.erase(node->meshes.begin() + m);
+                }
             }
 
-            node->meshes.erase(node->meshes.begin() + 1, node->meshes.end());
+            //since we were looping backwards, reverse the order of merged primitives to their original order
+            std::reverse(firstMesh->primitives.begin() + 1, firstMesh->primitives.end());
+
         }
     }
 }