From 193b02cdac6cf38ad87d57c20e8e19da33157e0f Mon Sep 17 00:00:00 2001
From: RichardTea <31507749+RichardTea@users.noreply.github.com>
Date: Mon, 9 Dec 2019 10:42:50 +0000
Subject: [PATCH] Odd negative scale: OptimizeGraph

OptimizeGraph postprocessing now reverses face order when
node scale is mirroring.
Fixes flip to backfacing in models that mirrored some nodes.

(Odd count of negative scale components, negative determinant)
---
 code/PostProcessing/ConvertToLHProcess.h |  5 ++--
 code/PostProcessing/OptimizeGraph.cpp    | 32 ++++++++++++++++--------
 code/PostProcessing/OptimizeGraph.h      |  6 ++---
 3 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/code/PostProcessing/ConvertToLHProcess.h b/code/PostProcessing/ConvertToLHProcess.h
index f32b91fc3..0c4a3a091 100644
--- a/code/PostProcessing/ConvertToLHProcess.h
+++ b/code/PostProcessing/ConvertToLHProcess.h
@@ -137,8 +137,9 @@ public:
     // -------------------------------------------------------------------
     void Execute( aiScene* pScene);
 
-protected:
-    void ProcessMesh( aiMesh* pMesh);
+public:
+    /** Some other types of post-processing require winding order flips */
+    static void ProcessMesh( aiMesh* pMesh);
 };
 
 // ---------------------------------------------------------------------------
diff --git a/code/PostProcessing/OptimizeGraph.cpp b/code/PostProcessing/OptimizeGraph.cpp
index 709f465e2..43bd7a3ee 100644
--- a/code/PostProcessing/OptimizeGraph.cpp
+++ b/code/PostProcessing/OptimizeGraph.cpp
@@ -47,6 +47,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "OptimizeGraph.h"
 #include "ProcessHelper.h"
+#include "ConvertToLHProcess.h"
 #include <assimp/Exceptional.h>
 #include <assimp/SceneCombiner.h>
 #include <stdio.h>
@@ -135,7 +136,7 @@ void OptimizeGraphProcess::CollectNewChildren(aiNode *nd, std::list<aiNode *> &n
 		nodes.push_back(nd);
 
 		// Now check for possible optimizations in our list of child nodes. join as many as possible
-		aiNode *join_master = NULL;
+		aiNode *join_master = nullptr;
 		aiMatrix4x4 inv;
 
 		const LockedSetType::const_iterator end = locked.end();
@@ -172,7 +173,7 @@ void OptimizeGraphProcess::CollectNewChildren(aiNode *nd, std::list<aiNode *> &n
 			join_master->mName.length = ::ai_snprintf(join_master->mName.data, MAXLEN, "$MergedNode_%i", count_merged++);
 
 			unsigned int out_meshes = 0;
-			for (std::list<aiNode *>::iterator it = join.begin(); it != join.end(); ++it) {
+			for (std::list<aiNode *>::const_iterator it = join.cbegin(); it != join.cend(); ++it) {
 				out_meshes += (*it)->mNumMeshes;
 			}
 
@@ -183,17 +184,26 @@ void OptimizeGraphProcess::CollectNewChildren(aiNode *nd, std::list<aiNode *> &n
 					*tmp++ = join_master->mMeshes[n];
 				}
 
-				for (std::list<aiNode *>::iterator it = join.begin(); it != join.end(); ++it) {
-					for (unsigned int n = 0; n < (*it)->mNumMeshes; ++n) {
+				for (const aiNode *join_node : join) {
+					for (unsigned int n = 0; n < join_node->mNumMeshes; ++n) {
 
-						*tmp = (*it)->mMeshes[n];
+						*tmp = join_node->mMeshes[n];
 						aiMesh *mesh = mScene->mMeshes[*tmp++];
 
+						// Assume the transformation is affine
 						// manually move the mesh into the right coordinate system
-						const aiMatrix3x3 IT = aiMatrix3x3((*it)->mTransformation).Inverse().Transpose();
+
+						// Check for odd negative scale (mirror)
+						if (join_node->mTransformation.Determinant() < 0) {
+							// Reverse the mesh face winding order
+                            FlipWindingOrderProcess::ProcessMesh(mesh);
+						}
+
+                        // Update positions, normals and tangents
+						const aiMatrix3x3 IT = aiMatrix3x3(join_node->mTransformation).Inverse().Transpose();
 						for (unsigned int a = 0; a < mesh->mNumVertices; ++a) {
 
-							mesh->mVertices[a] *= (*it)->mTransformation;
+							mesh->mVertices[a] *= join_node->mTransformation;
 
 							if (mesh->HasNormals())
 								mesh->mNormals[a] *= IT;
@@ -204,7 +214,7 @@ void OptimizeGraphProcess::CollectNewChildren(aiNode *nd, std::list<aiNode *> &n
 							}
 						}
 					}
-					delete *it; // bye, node
+					delete join_node; // bye, node
 				}
 				delete[] join_master->mMeshes;
 				join_master->mMeshes = meshes;
@@ -304,7 +314,7 @@ void OptimizeGraphProcess::Execute(aiScene *pScene) {
 	ai_assert(nodes.size() == 1);
 
 	if (dummy_root->mNumChildren == 0) {
-		pScene->mRootNode = NULL;
+		pScene->mRootNode = nullptr;
 		throw DeadlyImportError("After optimizing the scene graph, no data remains");
 	}
 
@@ -318,11 +328,11 @@ void OptimizeGraphProcess::Execute(aiScene *pScene) {
 		// Remove the dummy root node again.
 		pScene->mRootNode = dummy_root->mChildren[0];
 
-		dummy_root->mChildren[0] = NULL;
+		dummy_root->mChildren[0] = nullptr;
 		delete dummy_root;
 	}
 
-	pScene->mRootNode->mParent = NULL;
+	pScene->mRootNode->mParent = nullptr;
 	if (!DefaultLogger::isNullLogger()) {
 		if (nodes_in != nodes_out) {
 			ASSIMP_LOG_INFO_F("OptimizeGraphProcess finished; Input nodes: ", nodes_in, ", Output nodes: ", nodes_out);
diff --git a/code/PostProcessing/OptimizeGraph.h b/code/PostProcessing/OptimizeGraph.h
index 82cc5db3f..d2a6de9a2 100644
--- a/code/PostProcessing/OptimizeGraph.h
+++ b/code/PostProcessing/OptimizeGraph.h
@@ -75,13 +75,13 @@ public:
     ~OptimizeGraphProcess();
 
     // -------------------------------------------------------------------
-    bool IsActive( unsigned int pFlags) const;
+    bool IsActive( unsigned int pFlags) const override;
 
     // -------------------------------------------------------------------
-    void Execute( aiScene* pScene);
+    void Execute( aiScene* pScene) override;
 
     // -------------------------------------------------------------------
-    void SetupProperties(const Importer* pImp);
+    void SetupProperties(const Importer* pImp) override;
 
     // -------------------------------------------------------------------
     /** @brief Add a list of node names to be locked and not modified.