Fix some overhang detection (#15694)

tests/fff_print/test_perimeters.cpp

@@ -495,3 +495,138 @@ TEST_CASE("Loops waiting for the infill are extruded after it", "[Perimeters]")
     CHECK(ceiling_roles(true)  == std::vector<std::string>{ "perimeter", "infill", "perimeter" });
     CHECK(ceiling_roles(false) == std::vector<std::string>{ "perimeter", "infill" });
 }
+
+namespace {
+
+// The rib spans z=[0,5] and the slab z=[5,6], so this is the slab's first layer - the only one whose
+// support comes from the rib rather than from the slab below it.
+const double slab_first_layer_z = 5.2;
+
+// Rib widths either side of what the wall generators can print. At a 0.4mm nozzle the classic generator
+// builds nothing thinner than nozzle/3 = 0.133mm and Arachne drops anything below min_feature_size, 25%
+// of the nozzle = 0.1mm. 0.08mm is under both thresholds, 0.3mm over both.
+const double unprintable_rib = 0.08;
+const double printable_rib   = 0.3;
+
+// A 4x5mm anchor tower carrying a 20x5mm slab at z=[5,6], with a rib `rib_width` wide running the whole
+// length of the slab beneath its y=0 edge; a `rib_width` of 0 leaves the rib out. Nothing else is under
+// that edge, so whether the wall along it is an overhang rests entirely on the rib. Overhang detection
+// grows the lower slices by half the nozzle diameter before it asks, which carries either rib past the
+// 0.21mm from the slab edge to that wall - the unprintable one only fails to reach it once it is filtered
+// out for being unprintable.
+Print &slab_over_rib(Print &print, Model &model, double rib_width, const DynamicPrintConfig &config)
+{
+    ModelObject *object = model.add_object();
+    object->name = "slab_over_rib.stl";
+    object->add_volume(make_cube(4., 5., 6.), ModelVolumeType::MODEL_PART, false);
+    if (rib_width > 0.) {
+        TriangleMesh rib = make_cube(20., rib_width, 5.);
+        rib.translate(4.f, 0.f, 0.f);
+        object->add_volume(std::move(rib), ModelVolumeType::MODEL_PART, false);
+    }
+    TriangleMesh slab = make_cube(20., 5., 1.);
+    slab.translate(4.f, 0.f, 5.f);
+    object->add_volume(std::move(slab), ModelVolumeType::MODEL_PART, false);
+    object->add_instance();
+    object->ensure_on_bed();
+
+    print.auto_assign_extruders(object);
+    print.apply(model, config);
+    print.validate();
+    print.set_status_silent();
+    return print;
+}
+
+// Every setting the assertions below depend on, so none of them rests on a default. The wall line widths
+// are pinned because the rib widths above are chosen against the distance from the slab edge to its outer
+// wall, and min_feature_size because it is one of the two thresholds under test.
+DynamicPrintConfig printable_rib_config(const char *wall_generator, bool detect_thin_wall)
+{
+    DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({
+        { "wall_generator",                wall_generator },
+        { "layer_height",                  0.2 },  // puts a layer boundary exactly on the top of the rib
+        { "initial_layer_print_height",    0.2 },
+        { "nozzle_diameter",               "0.4" },
+        { "outer_wall_line_width",         0.42 },
+        { "inner_wall_line_width",         0.45 },
+        { "wall_loops",                    2 },
+        { "detect_overhang_wall",          true },
+        { "detect_thin_wall",              detect_thin_wall },
+        { "min_feature_size",              "25%" },
+        { "raft_layers",                   0 },
+        // Anything that adds, drops or reorders walls would move length between the roles being counted.
+        { "extra_perimeters_on_overhangs", false },
+        { "overhang_reverse",              false },
+        { "only_one_wall_top",             false },
+        { "only_one_wall_first_layer",     false },
+        { "unsupported_wall_last",         false },
+        { "sparse_infill_density",         "15%" },
+    });
+    return config;
+}
+
+// Length of every overhang perimeter path on the layer at `print_z`, loops and open extrusions alike.
+double overhang_length_at(const Print &print, double print_z)
+{
+    double len = 0.;
+    const auto add_entity = [&len](const ExtrusionEntity *entity, auto &&self) -> void {
+        const auto add_paths = [&len](const ExtrusionPaths &paths) {
+            for (const ExtrusionPath &path : paths)
+                if (path.role() == erOverhangPerimeter)
+                    len += path.length();
+        };
+        if (const auto *coll = dynamic_cast<const ExtrusionEntityCollection*>(entity)) {
+            for (const ExtrusionEntity *child : coll->entities)
+                self(child, self);
+        } else if (const auto *loop = dynamic_cast<const ExtrusionLoop*>(entity)) {
+            add_paths(loop->paths);
+        } else if (const auto *multi = dynamic_cast<const ExtrusionMultiPath*>(entity)) {
+            add_paths(multi->paths);
+        } else if (const auto *path = dynamic_cast<const ExtrusionPath*>(entity)) {
+            if (path->role() == erOverhangPerimeter)
+                len += path->length();
+        }
+    };
+
+    for (const Layer *layer : print.objects().front()->layers()) {
+        if (std::abs(layer->print_z - print_z) > EPSILON)
+            continue;
+        for (const LayerRegion *region : layer->regions())
+            add_entity(&region->perimeters, add_entity);
+    }
+    return len;
+}
+
+} // namespace
+
+// A sliver the wall generator prints nothing for holds nothing up, so it cannot be what decides that the
+// wall above it is not an overhang. The rib under the slab is the only thing that edge of the slab could
+// rest on: below the threshold of the active generator the slab has to come out exactly as it does with
+// no rib at all, and the last check is the control - a rib the generator does print anchors that wall,
+// without which the first check would hold for want of any sensitivity to the rib.
+TEST_CASE("A lower layer sliver too thin to print does not support the wall above it", "[Perimeters]")
+{
+    const char *wall_generator   = GENERATE("classic", "arachne");
+    const bool  detect_thin_wall = GENERATE(true, false);
+    CAPTURE(wall_generator, detect_thin_wall);
+
+    auto overhang_for = [wall_generator, detect_thin_wall](double rib_width) {
+        Print print;
+        Model model;
+        slab_over_rib(print, model, rib_width, printable_rib_config(wall_generator, detect_thin_wall));
+        print.process();
+        REQUIRE_FALSE(print.objects().empty());
+        return overhang_length_at(print, slab_first_layer_z);
+    };
+
+    const double no_rib      = overhang_for(0.);
+    const double unprintable = overhang_for(unprintable_rib);
+    const double printable   = overhang_for(printable_rib);
+
+    // Only where the slab meets the tower is it held up from below, so both of its 20mm walls overhang.
+    REQUIRE(no_rib > scale_(30.));
+    CHECK_THAT(unprintable, Catch::Matchers::WithinAbs(no_rib, scale_(1.)));
+    // A rib that does get printed takes the 20mm outer wall running along it out of the overhangs.
+    CHECK(printable < no_rib - scale_(15.));
+}