Exporter: if AI_SCENE_FLAGS_NON_VERBOSE_FORMAT is not set in input scenes, still check if the data is really in verbose format. This is a pure question of API usability and user surprise.

code/Exporter.cpp

@@ -227,11 +227,47 @@ const aiExportDataBlob* Exporter :: ExportToBlob(  const aiScene* pScene, const
 }
 
 
+// ------------------------------------------------------------------------------------------------
+bool IsVerboseFormat(const aiMesh* mesh) 
+{
+	// avoid slow vector<bool> specialization
+	std::vector<unsigned int> seen(mesh->mNumVertices,0);
+	for(unsigned int i = 0; i < mesh->mNumFaces; ++i) {
+		const aiFace& f = mesh->mFaces[i];
+		for(unsigned int j = 0; j < f.mNumIndices; ++j) {
+			if(++seen[f.mIndices[j]] == 2) {
+				// found a duplicate index
+				return false;
+			}
+		}
+	}
+	return true;
+}
+
+
+// ------------------------------------------------------------------------------------------------
+bool IsVerboseFormat(const aiScene* pScene) 
+{
+	for(unsigned int i = 0; i < pScene->mNumMeshes; ++i) {
+		if(!IsVerboseFormat(pScene->mMeshes[i])) {
+			return false;
+		}
+	}
+	return true;
+}
+
+
 // ------------------------------------------------------------------------------------------------
 aiReturn Exporter :: Export( const aiScene* pScene, const char* pFormatId, const char* pPath, unsigned int pPreprocessing )
 {
 	ASSIMP_BEGIN_EXCEPTION_REGION();
 
+	// when they create scenes from scratch, users will likely create them not in verbose
+	// format. They will likely not be aware that there is a flag in the scene to indicate
+	// this, however. To avoid surprises and bug reports, we check for duplicates in
+	// meshes upfront.
+	const bool is_verbose_format = !(pScene->mFlags & AI_SCENE_FLAGS_NON_VERBOSE_FORMAT) || IsVerboseFormat(pScene);
+
 	pimpl->mError = "";
 	for (size_t i = 0; i < pimpl->mExporters.size(); ++i) {
 		const Exporter::ExportFormatEntry& exp = pimpl->mExporters[i];
@@ -266,7 +302,7 @@ aiReturn Exporter :: Export( const aiScene* pScene, const char* pFormatId, const
 				// If the input scene is not in verbose format, but there is at least postprocessing step that relies on it,
 				// we need to run the MakeVerboseFormat step first.
 				bool must_join_again = false;
-				if (scenecopy->mFlags & AI_SCENE_FLAGS_NON_VERBOSE_FORMAT) {
+				if (!is_verbose_format) {
 					
 					bool verbosify = false;
 					for( unsigned int a = 0; a < pimpl->mPostProcessingSteps.size(); a++) {