diff -r 204dc77e5654 -r 7c4a63a02632 src/modeleditcontext.cpp
--- a/src/modeleditcontext.cpp	Sat Mar 07 01:25:37 2020 +0200
+++ b/src/modeleditcontext.cpp	Mon Mar 09 22:12:50 2020 +0200
@@ -31,12 +31,21 @@
 	return id;
 }
 
+void Model::EditContext::remove(int position)
+{
+	this->model().remove(position);
+}
+
 void Model::EditContext::setObjectProperty(
-	ldraw::Object* object,
+	ldraw::id_t id,
 	ldraw::Property property,
 	const QVariant& value)
 {
-	object->setProperty(property, value);
+	ldraw::Object* object = this->model().objectAt(id);
+	if (object != nullptr)
+	{
+		object->setProperty(property, value);
+	}
 }
 
 void Model::EditContext::invertObject(ldraw::id_t id)
@@ -54,17 +63,61 @@
 	return this->storedModel;
 }
 
+namespace
+{
+	using PropertyTriplet = std::tuple<ldraw::Property, ldraw::Property, ldraw::Property>;
+}
+
+static std::array<PropertyTriplet, 2> diagonalToPointProperties(ldraw::Diagonal diagonal)
+{
+	std::array<PropertyTriplet, 2> result;
+	switch (diagonal)
+	{
+	case ldraw::Diagonal::Diagonal_13:
+		result = {
+			std::make_tuple(ldraw::Property::Point1, ldraw::Property::Point2, ldraw::Property::Point3),
+			std::make_tuple(ldraw::Property::Point1, ldraw::Property::Point3, ldraw::Property::Point4)
+		};
+		break;
+	case ldraw::Diagonal::Diagonal_24:
+		result = {
+			std::make_tuple(ldraw::Property::Point1, ldraw::Property::Point2, ldraw::Property::Point4),
+			std::make_tuple(ldraw::Property::Point2, ldraw::Property::Point3, ldraw::Property::Point4)
+		};
+		break;
+	}
+	return result;
+}
+
 auto ldraw::splitQuadrilateral(
 	Model::EditContext& editor,
 	ldraw::quadrilateralid_t quadrilateral_id,
-	ldraw::QuadrilateralSplit splitType
+	ldraw::Diagonal splitType
 ) -> std::optional<std::pair<ldraw::triangleid_t, ldraw::triangleid_t>>
 {
 	std::optional<std::pair<ldraw::triangleid_t, ldraw::triangleid_t>> result;
-	const ldraw::Quadrilateral* quadrilateral = editor.model().get(quadrilateral_id);
+	QModelIndex index;
+	const ldraw::Quadrilateral* quadrilateral = editor.model().get(quadrilateral_id, &index);
 	if (quadrilateral != nullptr)
 	{
-
+		Q_ASSERT(index.isValid());
+		int position = index.row();
+		editor.remove(position);
+		std::vector<ldraw::triangleid_t> triangles;
+		triangles.reserve(2);
+		const std::array<PropertyTriplet, 2> split = diagonalToPointProperties(splitType);
+		for (const PropertyTriplet& properties : split)
+		{
+			const ldraw::triangleid_t triangle = editor.insert<ldraw::Triangle>(
+				position,
+				quadrilateral->getProperty(std::get<0>(properties)).value<glm::vec3>(),
+				quadrilateral->getProperty(std::get<1>(properties)).value<glm::vec3>(),
+				quadrilateral->getProperty(std::get<2>(properties)).value<glm::vec3>(),
+				ldraw::Color{quadrilateral->getProperty(ldraw::Property::Color).toInt()});
+			triangles.push_back(triangle);
+			position += 1;
+		}
+		result = {triangles[0], triangles[1]};
 	}
 	return result;
 }