From 1175f48d05888fba705363729ecf5ef2c75f0c5d Mon Sep 17 00:00:00 2001
From: Dmitry Kostenko <codeforsmile@gmail.com>
Date: Mon, 8 Nov 2021 23:13:50 +0100
Subject: [PATCH] Detect 'insane' normals in checkMeshNormals.

Detect non-zero normals which point in the opposite direction from the
face plane normal.
---
 src/client/mesh.cpp | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/src/client/mesh.cpp b/src/client/mesh.cpp
index c56eba2e2..070200889 100644
--- a/src/client/mesh.cpp
+++ b/src/client/mesh.cpp
@@ -331,6 +331,9 @@ void recalculateBoundingBox(scene::IMesh *src_mesh)
 
 bool checkMeshNormals(scene::IMesh *mesh)
 {
+	// Assume correct normals if this many first faces get it right.
+	static const u16 MAX_FACES_TO_CHECK = 9;
+
 	u32 buffer_count = mesh->getMeshBufferCount();
 
 	for (u32 i = 0; i < buffer_count; i++) {
@@ -344,6 +347,19 @@ bool checkMeshNormals(scene::IMesh *mesh)
 
 		if (!std::isfinite(length) || length < 1e-10f)
 			return false;
+
+		const u16 count = MYMIN(MAX_FACES_TO_CHECK * 3, buffer->getIndexCount());
+		for (u16 i = 0; i < count; i += 3) {
+
+			core::plane3df plane(buffer->getPosition(buffer->getIndices()[i]),
+					buffer->getPosition(buffer->getIndices()[i+1]),
+					buffer->getPosition(buffer->getIndices()[i+2]));
+
+			for (u16 j = 0; j < 3; j++)
+				if (plane.Normal.dotProduct(buffer->getNormal(buffer->getIndices()[j])) < 0)
+					return false;
+		}
+
 	}
 
 	return true;