4 files changed, 23 insertions, 7 deletions
diff --git a/game/geoData.cpp b/game/geoData.cpp
index 8e662b3..eef5dc7 100644
--- a/game/geoData.cpp
+++ b/game/geoData.cpp
@@ -601,3 +601,13 @@ GeoData::setHeights(const std::span<const GlobalPosition3D> triangleStrip, const
 
 	updateAllVertexNormals(newOrChangedVerts);
 }
+
+void
+GeoData::sanityCheck() const
+{
+	if (!std::ranges::all_of(faces(), [this](const auto face) {
+			return triangle<2>(face).isUp();
+		})) {
+		throw std::logic_error("Upside down faces detected");
+	}
+}
diff --git a/game/geoData.h b/game/geoData.h
index 79924d3..01582a6 100644
--- a/game/geoData.h
+++ b/game/geoData.h
@@ -98,6 +98,8 @@ public:
 		return property(surface, h);
 	}
 
+	void sanityCheck() const;
+
 protected:
 	template<glm::length_t Dim>
 	[[nodiscard]] Triangle<Dim>
diff --git a/lib/triangle.h b/lib/triangle.h
index 888481d..d5547ab 100644
--- a/lib/triangle.h
+++ b/lib/triangle.h
@@ -84,6 +84,14 @@ struct Triangle : public glm::vec<3, glm::vec<Dim, T, Q>> {
 	}
 
 	[[nodiscard]] constexpr auto
+	isUp() const
+	{
+		const auto edgeAB = sideDifference(1);
+		const auto edgeAC = sideDifference(2);
+		return edgeAB.x * edgeAC.y >= edgeAB.y * edgeAC.x;
+	}
+
+	[[nodiscard]] constexpr auto
 	p(const glm::length_t idx) const
 	{
 		return Base::operator[](idx);
diff --git a/test/test-geoData.cpp b/test/test-geoData.cpp
index 5998789..9ec4656 100644
--- a/test/test-geoData.cpp
+++ b/test/test-geoData.cpp
@@ -29,11 +29,9 @@ BOOST_AUTO_TEST_CASE(loadSuccess)
 	BOOST_CHECK_EQUAL(upper, GlobalPosition3D(319950000, 499950000, 571600));
 }
 
-BOOST_AUTO_TEST_CASE(normalsAllPointUp)
+BOOST_AUTO_TEST_CASE(sanityCheck)
 {
-	BOOST_CHECK(std::ranges::all_of(vertices(), [this](auto && vertex) {
-		return normal(vertex).z > 0;
-	}));
+	BOOST_CHECK_NO_THROW(sanityCheck());
 }
 
 BOOST_AUTO_TEST_CASE(trianglesContainsPoints)
@@ -214,9 +212,7 @@ BOOST_DATA_TEST_CASE(deform, loadFixtureJson<DeformTerrainData>("geoData/deform/
 	surface.colorBias = RGB {0, 0, 1};
 	auto gd = std::make_shared<GeoData>(GeoData::createFlat({0, 0}, {1000000, 1000000}, 100));
 	BOOST_CHECK_NO_THROW(gd->setHeights(points, {.surface = surface}));
-	BOOST_CHECK(std::ranges::all_of(gd->vertices(), [&gd](auto && vertex) {
-		return gd->normal(vertex).z > 0;
-	}));
+	BOOST_CHECK_NO_THROW(gd->sanityCheck());
 
 	ApplicationBase ab;
 	TestMainWindow tmw;