Set the metallicFactor to 0 if source file doesn’t have metallicFactor

Otherwise, the models have full metallic materials and appear very dim

code/glTF2Exporter.cpp

@@ -418,7 +418,12 @@ void glTF2Exporter::ExportMaterials()
         GetMatTex(mat, m->pbrMetallicRoughness.baseColorTexture, aiTextureType_DIFFUSE);
         GetMatTex(mat, m->pbrMetallicRoughness.metallicRoughnessTexture, AI_MATKEY_GLTF_PBRMETALLICROUGHNESS_METALLICROUGHNESS_TEXTURE);
         GetMatColor(mat, m->pbrMetallicRoughness.baseColorFactor, AI_MATKEY_COLOR_DIFFUSE);
-        mat->Get(AI_MATKEY_GLTF_PBRMETALLICROUGHNESS_METALLIC_FACTOR, m->pbrMetallicRoughness.metallicFactor);
+
+        if (mat->Get(AI_MATKEY_GLTF_PBRMETALLICROUGHNESS_METALLIC_FACTOR, m->pbrMetallicRoughness.metallicFactor) != AI_SUCCESS) {
+            //if metallicFactor wasn't defined, then the source is likely not a PBR file, and the metallicFactor should be 0
+            m->pbrMetallicRoughness.metallicFactor = 0;
+        }
+
         mat->Get(AI_MATKEY_GLTF_PBRMETALLICROUGHNESS_ROUGHNESS_FACTOR, m->pbrMetallicRoughness.roughnessFactor);
 
         GetMatTex(mat, m->normalTexture, aiTextureType_NORMALS);