Part 2.5: Add global shear transform, support clipping, and belt UI improvements

- Implement per-object global shear transform in PrintObject with
  layer Z-offset calculation, config invalidation, and fix for
  shared-object layer optimization breaking copied objects
- Clip support layers to the transformed belt floor plane and begin
  work on tree support adaptation for sheared coordinate space
- Improve belt UI: gray out inactive sub-options, add B keyboard
  shortcut for G-code viewer design-view toggle, fix mesh clipping
  through build plate after shear/scale transform

y' = y + z·cot(α),
  while x' = x and z' = z

getting closer to customizable variant

getting closer

X/Y/Z shear initial

clean up UI

add 1/sin(a) transform, idea taken from blackbelt cura plugin

Things work now (turns out I've been using the wrong set of  transforms)

src/slic3r/GUI/GCodeViewer.cpp

@@ -1271,7 +1271,7 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
 
     m_max_print_height = gcode_result.printable_height;
     m_z_offset = gcode_result.z_offset;
-    m_belt_z_shift = gcode_result.belt_z_shift;
+
 
     // load_toolpaths(gcode_result, build_volume, exclude_bounding_box);
     
@@ -2210,15 +2210,17 @@ void GCodeViewer::render_toolpaths()
 {
     const Camera& camera = wxGetApp().plater()->get_camera();
     Matrix4f view = camera.get_view_matrix().matrix().cast<float>();
-    // Belt "raw" view: apply slicing rotation to view matrix so toolpaths appear
-    // in the slicing frame (rotated part with horizontal layers).
-    if (m_belt_show_raw && m_belt_view_enabled && m_belt_angle_deg > 0.f) {
+    // Belt "designed" view: apply inverse shear to view matrix so toolpaths appear
+    // upright (as originally designed) instead of sheared on the belt.
+    if (m_belt_show_designed && m_belt_view_enabled && m_belt_angle_deg > 0.f) {
         double angle_rad = Geometry::deg2rad(static_cast<double>(m_belt_angle_deg));
-        // Apply R(-alpha, X) * T(0,0,-z_shift) to bring machine coords back to slicing frame.
-        Transform3d slicing_trafo = Transform3d::Identity();
-        slicing_trafo.translate(Vec3d(0., 0., -static_cast<double>(m_belt_z_shift)));
-        slicing_trafo = Eigen::AngleAxisd(-angle_rad, Vec3d::UnitX()) * slicing_trafo;
-        view = (camera.get_view_matrix() * slicing_trafo).matrix().cast<float>();
+        double sin_a = std::sin(angle_rad);
+        if (sin_a > 1e-6) {
+            double cot_alpha = std::cos(angle_rad) / sin_a;
+            Transform3d inverse_shear = Transform3d::Identity();
+            inverse_shear.matrix()(1, 2) = -cot_alpha;  // Y -= Z * cot(α)
+            view = (camera.get_view_matrix() * inverse_shear).matrix().cast<float>();
+        }
     }
     const libvgcode::Mat4x4 converted_view_matrix = libvgcode::convert(view);
     const libvgcode::Mat4x4 converted_projetion_matrix = libvgcode::convert(static_cast<Matrix4f>(camera.get_projection_matrix().matrix().cast<float>()));
@@ -4421,7 +4423,7 @@ void GCodeViewer::render_legend(float &legend_height, int canvas_width, int canv
         ImGui::Spacing();
         ImGui::Dummy({ window_padding, 0 });
         ImGui::SameLine();
-        ImGui::Checkbox("Show raw G-code (slicing frame)", &m_belt_show_raw);
+        ImGui::Checkbox("Show designed view (upright)", &m_belt_show_designed);
     }
 
     legend_height = ImGui::GetCurrentWindow()->Size.y;

src/slic3r/GUI/GCodeViewer.hpp

@@ -239,8 +239,7 @@ mutable bool m_no_render_path { false };
 
     bool  m_belt_view_enabled = false;
     float m_belt_angle_deg = 0.f;
-    float m_belt_z_shift = 0.f;
-    bool  m_belt_show_raw = false;  // Toggle: show raw slicing-frame G-code (rotated part)
+    bool  m_belt_show_designed = false;  // Toggle: show designed (upright) view via inverse shear
 
     libvgcode::Viewer m_viewer;
     bool m_loaded_as_preview{ false };

src/slic3r/GUI/Tab.cpp

@@ -4370,6 +4370,53 @@ void TabPrinter::build_fff()
         optgroup->append_single_option_line("belt_printer");
         optgroup->append_single_option_line("belt_printer_angle");
         optgroup->append_single_option_line("belt_printer_infinite_y");
+        // Per-axis shear: group mode + angle + source on one row per axis
+        {
+            Line line = { L("Shear X"), L("Shear applied to the X axis before slicing") };
+            line.append_option(optgroup->get_option("belt_shear_x"));
+            line.append_option(optgroup->get_option("belt_shear_x_angle"));
+            line.append_option(optgroup->get_option("belt_shear_x_from"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("Shear Y"), L("Shear applied to the Y axis before slicing") };
+            line.append_option(optgroup->get_option("belt_shear_y"));
+            line.append_option(optgroup->get_option("belt_shear_y_angle"));
+            line.append_option(optgroup->get_option("belt_shear_y_from"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("Shear Z"), L("Shear applied to the Z axis before slicing") };
+            line.append_option(optgroup->get_option("belt_shear_z"));
+            line.append_option(optgroup->get_option("belt_shear_z_angle"));
+            line.append_option(optgroup->get_option("belt_shear_z_from"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("Scale X"), L("Scale applied to the X axis before slicing") };
+            line.append_option(optgroup->get_option("belt_scale_x"));
+            line.append_option(optgroup->get_option("belt_scale_x_angle"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("Scale Y"), L("Scale applied to the Y axis before slicing") };
+            line.append_option(optgroup->get_option("belt_scale_y"));
+            line.append_option(optgroup->get_option("belt_scale_y_angle"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("Scale Z"), L("Scale applied to the Z axis before slicing") };
+            line.append_option(optgroup->get_option("belt_scale_z"));
+            line.append_option(optgroup->get_option("belt_scale_z_angle"));
+            optgroup->append_line(line);
+        }
+        {
+            Line line = { L("G-code axis remap"), L("Remap slicing-frame axes to machine axes in G-code output") };
+            line.append_option(optgroup->get_option("belt_gcode_remap_x"));
+            line.append_option(optgroup->get_option("belt_gcode_remap_y"));
+            line.append_option(optgroup->get_option("belt_gcode_remap_z"));
+            optgroup->append_line(line);
+        }
         optgroup->append_single_option_line("support_multi_bed_types","printer_basic_information_printable_space#support-multi-bed-types");
         optgroup->append_single_option_line("best_object_pos", "printer_basic_information_printable_space#best-object-position");
         // todo: for multi_extruder test
@@ -5233,6 +5280,10 @@ void TabPrinter::toggle_options()
         bool is_belt = m_config->opt_bool("belt_printer");
         toggle_line("belt_printer_angle", is_belt);
         toggle_line("belt_printer_infinite_y", is_belt);
+        for (auto el : {"belt_shear_x", "belt_shear_y", "belt_shear_z",
+                        "belt_scale_x", "belt_scale_y", "belt_scale_z",
+                        "belt_gcode_remap_x"})
+            toggle_line(el, is_belt);
     }