test: replace the disabled convex_hull_2d test (#14892)

test(libslic3r): replace the disabled convex_hull_2d test, closing #11269

The last "failing libslic3r test" from #11269 was the disabled
SCENARIO("2D convex hull of sinking object", "[3mf][.]") in test_3mf.cpp.
It checked ModelObject::convex_hull_2d for a sinking object against
PrusaSlicer's reference hull, but Orca's convex_hull_2d does not clip
geometry below the bed the way PrusaSlicer's its_convex_hull_2d_above does,
so the reference never matched. The test also wrote a debug mesh to a
hardcoded /tmp path and its comparison loop was inverted.

Remove it and add tests/libslic3r/test_model.cpp characterizing
convex_hull_2d on non-sinking transforms (identity and scale+offset),
where the projected footprint is unambiguous. Homed in a Model test file
since it exercises ModelObject, not 3MF.

tests/libslic3r/CMakeLists.txt

@@ -29,6 +29,7 @@ add_executable(${_TEST_NAME}_tests
     test_stl.cpp
     test_meshboolean.cpp
     test_marchingsquares.cpp
+    test_model.cpp
     test_utils.cpp
     test_timeutils.cpp
     test_voronoi.cpp

tests/libslic3r/test_3mf.cpp

@@ -509,64 +509,3 @@ SCENARIO("Nozzle-group metadata .3mf round-trip", "[3mf][MultiNozzle]") {
         boost::filesystem::remove_all(backup_dir);
     }
 }
-
-SCENARIO("2D convex hull of sinking object", "[3mf][.]") {
-    GIVEN("model") {
-        // load a model
-        Model model;
-        std::string src_file = std::string(TEST_DATA_DIR) + "/test_3mf/Prusa.stl";
-        REQUIRE(load_stl(src_file.c_str(), &model));
-        model.add_default_instances();
-
-        WHEN("model is rotated, scaled and set as sinking") {
-            ModelObject* object = model.objects[0];
-            object->center_around_origin(false);
-
-	    // This outputs the same exact data as the Prusaslicer test
-	    write_debug_stl("3mf/orca.ascii", object->volumes[0]->mesh());
-
-            // set instance's attitude so that it is rotated, scaled (and sinking? how is it sinking? the rotation? does it matter if it's sinking?)
-            ModelInstance* instance = object->instances[0];
-            instance->set_rotation(X, -M_PI / 4.0);
-            instance->set_offset(Vec3d::Zero());
-            instance->set_scaling_factor({ 2.0, 2.0, 2.0 });
-
-            // calculate 2D convex hull
-	    auto trafo = instance->get_transformation().get_matrix();
-
-	    // This matrix is the same exact matrix as the Prusaslicer test
-	    CAPTURE(trafo);
-            Polygon hull_2d = object->convex_hull_2d(trafo);
-
-	    // But we get different hull_2d.points here (and somehow decimal numbers despite being int64_t values, but that's probabaly printing configuration somewhere -- Prusaslicer's prints out with newlines between the X&Y and not one between coordinates, which is about the worse possible output).
-	    // I think it's something to do with PrusaSlicer ignoring everything under the Z plane, which makes sense from the results.
-	    // See the comments added to ModelObject::convex_hull_2d for more information.
-
-            // verify result
-            Points result = {
-                { -91501496, -15914144 },
-                { 91501496, -15914144 },
-                { 91501496, 4243 },
-                { 78229680, 4246883 },
-                { 56898100, 4246883 },
-                { -85501496, 4242641 },
-                { -91501496, 4243 }
-            };
-
-            THEN("2D convex hull should match with reference") {
-                // Allow 1um error due to floating point rounding.
-                bool res = hull_2d.points.size() == result.size();
-                if (res) {
-                    for (size_t i = 0; i < result.size(); ++ i) {
-                        const Point &p1 = result[i];
-                        const Point &p2 = hull_2d.points[i];
-                        CHECK((std::abs(p1.x() - p2.x()) > 1 || std::abs(p1.y() - p2.y()) > 1));
-                    }
-                }
-
-                CAPTURE(hull_2d.points);
-                REQUIRE(res);
-            }
-        }
-    }
-}

tests/libslic3r/test_model.cpp

@@ -0,0 +1,40 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/Model.hpp"
+
+using namespace Slic3r;
+
+// convex_hull_2d does not clip geometry below the bed, so these cases avoid
+// sinking transforms.
+TEST_CASE("A part's 2D convex hull is its footprint projected onto the bed", "[Model]")
+{
+    Model model;
+    ModelObject* object = model.add_object();
+    // Keep the cube's raw coordinates ([0,20] on every axis): the default
+    // add_volume re-centers the geometry, which would move the footprint.
+    object->add_volume(make_cube(20, 20, 20), ModelVolumeType::MODEL_PART, false);
+
+    SECTION("identity transform yields the 20 mm square") {
+        const Polygon hull   = object->convex_hull_2d(Geometry::Transformation{}.get_matrix());
+        const BoundingBox bb = hull.bounding_box();
+        CHECK(hull.size() == 4);
+        CHECK(bb.min.x() == scaled(0.));
+        CHECK(bb.min.y() == scaled(0.));
+        CHECK(bb.max.x() == scaled(20.));
+        CHECK(bb.max.y() == scaled(20.));
+    }
+
+    SECTION("scaling and offset move and grow the footprint") {
+        Geometry::Transformation t;
+        t.set_scaling_factor({2, 2, 2}); // cube now spans [0,40]
+        t.set_offset({10, 5, 0});        // then shift +10 in X, +5 in Y
+
+        const Polygon hull   = object->convex_hull_2d(t.get_matrix());
+        const BoundingBox bb = hull.bounding_box();
+        CHECK(hull.size() == 4);
+        CHECK(bb.min.x() == scaled(10.));
+        CHECK(bb.min.y() == scaled(5.));
+        CHECK(bb.max.x() == scaled(50.));
+        CHECK(bb.max.y() == scaled(45.));
+    }
+}