diff --git a/src/libslic3r/Technologies.hpp b/src/libslic3r/Technologies.hpp
index 03b2b1f07..45217c959 100644
--- a/src/libslic3r/Technologies.hpp
+++ b/src/libslic3r/Technologies.hpp
@@ -47,6 +47,8 @@
 #define ENABLE_DELAYED_TRANSPARENT_VOLUMES_RENDERING (1 && ENABLE_2_4_0_ALPHA0)
 // Enable the fix of importing color print view from gcode files into GCodeViewer
 #define ENABLE_FIX_IMPORTING_COLOR_PRINT_VIEW_INTO_GCODEVIEWER (1 && ENABLE_2_4_0_ALPHA0)
+// Enable drawing contours, at cut level, for sinking volumes
+#define ENABLE_SINKING_CONTOURS (1 && ENABLE_2_4_0_ALPHA0)
 
 
 #endif // _prusaslicer_technologies_h_
diff --git a/src/libslic3r/TriangleMesh.cpp b/src/libslic3r/TriangleMesh.cpp
index d709a67cd..360a8b14e 100644
--- a/src/libslic3r/TriangleMesh.cpp
+++ b/src/libslic3r/TriangleMesh.cpp
@@ -698,27 +698,29 @@ struct EdgeToFace {
     bool operator<(const EdgeToFace &other) const { return vertex_low < other.vertex_low || (vertex_low == other.vertex_low && vertex_high < other.vertex_high); }
 };
 
-template<typename ThrowOnCancelCallback>
+template<typename FaceFilter, typename ThrowOnCancelCallback>
 static std::vector<EdgeToFace> create_edge_map(
-    const indexed_triangle_set &its, ThrowOnCancelCallback throw_on_cancel)
+    const indexed_triangle_set &its, FaceFilter face_filter, ThrowOnCancelCallback throw_on_cancel)
 {
     std::vector<EdgeToFace> edges_map;
-    edges_map.assign(its.indices.size() * 3, EdgeToFace());
+    edges_map.reserve(its.indices.size() * 3);
     for (uint32_t facet_idx = 0; facet_idx < its.indices.size(); ++ facet_idx)
-        for (int i = 0; i < 3; ++ i) {
-            EdgeToFace &e2f = edges_map[facet_idx * 3 + i];
-            e2f.vertex_low  = its.indices[facet_idx][i];
-            e2f.vertex_high = its.indices[facet_idx][(i + 1) % 3];
-            e2f.face        = facet_idx;
-            // 1 based indexing, to be always strictly positive.
-            e2f.face_edge   = i + 1;
-            if (e2f.vertex_low > e2f.vertex_high) {
-                // Sort the vertices
-                std::swap(e2f.vertex_low, e2f.vertex_high);
-                // and make the face_edge negative to indicate a flipped edge.
-                e2f.face_edge = - e2f.face_edge;
+        if (face_filter(facet_idx))
+            for (int i = 0; i < 3; ++ i) {
+                edges_map.push_back({});
+                EdgeToFace &e2f = edges_map.back();
+                e2f.vertex_low  = its.indices[facet_idx][i];
+                e2f.vertex_high = its.indices[facet_idx][(i + 1) % 3];
+                e2f.face        = facet_idx;
+                // 1 based indexing, to be always strictly positive.
+                e2f.face_edge   = i + 1;
+                if (e2f.vertex_low > e2f.vertex_high) {
+                    // Sort the vertices
+                    std::swap(e2f.vertex_low, e2f.vertex_high);
+                    // and make the face_edge negative to indicate a flipped edge.
+                    e2f.face_edge = - e2f.face_edge;
+                }
             }
-        }
     throw_on_cancel();
     std::sort(edges_map.begin(), edges_map.end());
 
@@ -727,12 +729,12 @@ static std::vector<EdgeToFace> create_edge_map(
 
 // Map from a face edge to a unique edge identifier or -1 if no neighbor exists.
 // Two neighbor faces share a unique edge identifier even if they are flipped.
-template<typename ThrowOnCancelCallback>
-static inline std::vector<Vec3i> its_face_edge_ids_impl(const indexed_triangle_set &its, ThrowOnCancelCallback throw_on_cancel)
+template<typename FaceFilter, typename ThrowOnCancelCallback>
+static inline std::vector<Vec3i> its_face_edge_ids_impl(const indexed_triangle_set &its, FaceFilter face_filter, ThrowOnCancelCallback throw_on_cancel)
 {
     std::vector<Vec3i> out(its.indices.size(), Vec3i(-1, -1, -1));
 
-    std::vector<EdgeToFace> edges_map = create_edge_map(its, throw_on_cancel);
+    std::vector<EdgeToFace> edges_map = create_edge_map(its, face_filter, throw_on_cancel);
 
     // Assign a unique common edge id to touching triangle edges.
     int num_edges = 0;
@@ -780,12 +782,17 @@ static inline std::vector<Vec3i> its_face_edge_ids_impl(const indexed_triangle_s
 
 std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its)
 {
-    return its_face_edge_ids_impl(its, [](){});
+    return its_face_edge_ids_impl(its, [](const uint32_t){ return true; }, [](){});
 }
 
 std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, std::function<void()> throw_on_cancel_callback)
 {
-    return its_face_edge_ids_impl(its, throw_on_cancel_callback);
+    return its_face_edge_ids_impl(its, [](const uint32_t){ return true; }, throw_on_cancel_callback);
+}
+
+std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, const std::vector<bool> &face_mask)
+{
+    return its_face_edge_ids_impl(its, [&face_mask](const uint32_t idx){ return face_mask[idx]; }, [](){});
 }
 
 // Having the face neighbors available, assign unique edge IDs to face edges for chaining of polygons over slices.
diff --git a/src/libslic3r/TriangleMesh.hpp b/src/libslic3r/TriangleMesh.hpp
index 7aef3055a..c8c7e0dd7 100644
--- a/src/libslic3r/TriangleMesh.hpp
+++ b/src/libslic3r/TriangleMesh.hpp
@@ -118,6 +118,7 @@ private:
 // Used for chaining slice lines into polygons.
 std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its);
 std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, std::function<void()> throw_on_cancel_callback);
+std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, const std::vector<bool> &face_mask);
 // Having the face neighbors available, assign unique edge IDs to face edges for chaining of polygons over slices.
 std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, std::vector<Vec3i> &face_neighbors, bool assign_unbound_edges = false, int *num_edges = nullptr);
 
diff --git a/src/libslic3r/TriangleMeshSlicer.cpp b/src/libslic3r/TriangleMeshSlicer.cpp
index 681ec32f0..ef90402a9 100644
--- a/src/libslic3r/TriangleMeshSlicer.cpp
+++ b/src/libslic3r/TriangleMeshSlicer.cpp
@@ -362,6 +362,35 @@ static inline std::vector<IntersectionLines> slice_make_lines(
     return lines;
 }
 
+template<typename TransformVertex, typename FaceFilter>
+static inline IntersectionLines slice_make_lines(
+    const std::vector<stl_vertex>                   &mesh_vertices,
+    const TransformVertex                           &transform_vertex_fn,
+    const std::vector<stl_triangle_vertex_indices>  &mesh_faces,
+    const std::vector<Vec3i>                        &face_edge_ids,
+    const float                                      plane_z, 
+    FaceFilter                                       face_filter)
+{
+    IntersectionLines lines;
+    for (int face_idx = 0; face_idx < mesh_faces.size(); ++ face_idx)
+        if (face_filter(face_idx)) {
+            const Vec3i &indices = mesh_faces[face_idx];
+            stl_vertex vertices[3] { transform_vertex_fn(mesh_vertices[indices(0)]), transform_vertex_fn(mesh_vertices[indices(1)]), transform_vertex_fn(mesh_vertices[indices(2)]) };
+            // find facet extents
+            const float min_z = fminf(vertices[0].z(), fminf(vertices[1].z(), vertices[2].z()));
+            const float max_z = fmaxf(vertices[0].z(), fmaxf(vertices[1].z(), vertices[2].z()));
+            assert(min_z <= plane_z && max_z >= plane_z);
+            int  idx_vertex_lowest = (vertices[1].z() == min_z) ? 1 : ((vertices[2].z() == min_z) ? 2 : 0);            
+            IntersectionLine il;
+            // Ignore horizontal triangles. Any valid horizontal triangle must have a vertical triangle connected, otherwise the part has zero volume.
+            if (min_z != max_z && slice_facet(plane_z, vertices, indices, face_edge_ids[face_idx], idx_vertex_lowest, false, il) == FacetSliceType::Slicing) {
+                assert(il.edge_type != IntersectionLine::FacetEdgeType::Horizontal);
+                lines.emplace_back(il);
+            }
+        }
+    return lines;
+}
+
 // For projecting triangle sets onto slice slabs.
 struct SlabLines {
     // Intersection lines of a slice with a triangle set, CCW oriented.
@@ -1720,6 +1749,69 @@ std::vector<Polygons> slice_mesh(
     return layers;
 }
 
+// Specialized version for a single slicing plane only, running on a single thread.
+Polygons slice_mesh(
+    const indexed_triangle_set       &mesh,
+    // Unscaled Zs
+    const float                       plane_z,
+    const MeshSlicingParams          &params)
+{
+    std::vector<IntersectionLines> lines;
+
+    {
+        bool                trafo_identity = is_identity(params.trafo);
+        Transform3f         tf;
+        std::vector<bool>   face_mask(mesh.indices.size(), false);
+
+        {
+            // 1) Mark vertices as below or above the slicing plane.
+            std::vector<char> vertex_side(mesh.vertices.size(), 0);
+            if (trafo_identity) {
+                for (size_t i = 0; i < mesh.vertices.size(); ++ i) {
+                    float z = mesh.vertices[i].z();
+                    char  s = z < plane_z ? -1 : z == plane_z ? 0 : 1;
+                    vertex_side[i] = s;
+                }
+            } else {
+                tf = make_trafo_for_slicing(params.trafo);
+                for (size_t i = 0; i < mesh.vertices.size(); ++ i) {
+                    //FIXME don't need to transform x & y, just Z.
+                    float z = (tf * mesh.vertices[i]).z();
+                    char  s = z < plane_z ? -1 : z == plane_z ? 0 : 1;
+                    vertex_side[i] = s;
+                }
+            }
+
+            // 2) Mark faces crossing the plane.
+            for (size_t i = 0; i < mesh.indices.size(); ++ i) {
+                const Vec3i &face = mesh.indices[i];
+                int sides[3] = { vertex_side[face(0)], vertex_side[face(1)], vertex_side[face(2)] };
+                face_mask[i] = sides[0] * sides[1] <= 0 || sides[1] * sides[2] <= 0 || sides[0] * sides[2] <= 0;
+            }
+        }
+
+        // 3) Calculate face neighbors for just the faces in face_mask.
+        std::vector<Vec3i> face_edge_ids = its_face_edge_ids(mesh, face_mask);
+
+        // 4) Slice "face_mask" triangles, collect line segments.
+        // It likely is not worthwile to copy the vertices. Apply the transformation in place.
+        if (trafo_identity) {
+            lines.emplace_back(slice_make_lines(
+                mesh.vertices, [](const Vec3f &p) { return Vec3f(scaled<float>(p.x()), scaled<float>(p.y()), p.z()); }, 
+                mesh.indices, face_edge_ids, plane_z, [&face_mask](int face_idx) { return face_mask[face_idx]; }));
+        } else {
+            // Transform the vertices, scale up in XY, not in Z.
+            lines.emplace_back(slice_make_lines(mesh.vertices, [tf](const Vec3f& p) { return tf * p; }, mesh.indices, face_edge_ids, plane_z,
+                [&face_mask](int face_idx) { return face_mask[face_idx]; }));
+        }
+    }
+
+    // 5) Chain the line segments.
+    std::vector<Polygons> layers = make_loops(lines, params, [](){});
+    assert(layers.size() == 1);
+    return layers.front();
+}
+
 std::vector<ExPolygons> slice_mesh_ex(
     const indexed_triangle_set       &mesh,
     const std::vector<float>         &zs,
diff --git a/src/libslic3r/TriangleMeshSlicer.hpp b/src/libslic3r/TriangleMeshSlicer.hpp
index 9b02f5573..7ea7ac3a9 100644
--- a/src/libslic3r/TriangleMeshSlicer.hpp
+++ b/src/libslic3r/TriangleMeshSlicer.hpp
@@ -52,6 +52,12 @@ std::vector<Polygons>           slice_mesh(
     const MeshSlicingParams          &params,
     std::function<void()>             throw_on_cancel = []{});
 
+// Specialized version for a single slicing plane only, running on a single thread.
+Polygons                        slice_mesh(
+    const indexed_triangle_set       &mesh,
+    const float                       plane_z,
+    const MeshSlicingParams          &params);
+
 std::vector<ExPolygons>         slice_mesh_ex(
     const indexed_triangle_set       &mesh,
     const std::vector<float>         &zs,
diff --git a/src/slic3r/GUI/3DScene.cpp b/src/slic3r/GUI/3DScene.cpp
index f84d221bd..5894a750e 100644
--- a/src/slic3r/GUI/3DScene.cpp
+++ b/src/slic3r/GUI/3DScene.cpp
@@ -9,9 +9,9 @@
 #include "3DScene.hpp"
 #include "GLShader.hpp"
 #include "GUI_App.hpp"
-#if ENABLE_ENVIRONMENT_MAP
+#if ENABLE_ENVIRONMENT_MAP || ENABLE_SINKING_CONTOURS
 #include "Plater.hpp"
-#endif // ENABLE_ENVIRONMENT_MAP
+#endif // ENABLE_ENVIRONMENT_MAP || ENABLE_SINKING_CONTOURS
 
 #include "libslic3r/ExtrusionEntity.hpp"
 #include "libslic3r/ExtrusionEntityCollection.hpp"
@@ -24,6 +24,9 @@
 #include "libslic3r/Utils.hpp"
 #include "libslic3r/AppConfig.hpp"
 #include "libslic3r/PresetBundle.hpp"
+#if ENABLE_SINKING_CONTOURS
+#include "libslic3r/Tesselate.hpp"
+#endif // ENABLE_SINKING_CONTOURS
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -284,6 +287,74 @@ void GLIndexedVertexArray::render(
     glsafe(::glBindBuffer(GL_ARRAY_BUFFER, 0));
 }
 
+#if ENABLE_SINKING_CONTOURS
+const float GLVolume::SinkingContours::HalfWidth = 0.25f;
+
+void GLVolume::SinkingContours::render()
+{
+    update();
+
+    glsafe(::glPushMatrix());
+    glsafe(::glTranslated(m_shift.x(), m_shift.y(), m_shift.z()));
+    m_model.render();
+    glsafe(::glPopMatrix());
+}
+
+void GLVolume::SinkingContours::update()
+{
+    if (m_parent.is_sinking() && !m_parent.is_below_printbed()) {
+        const BoundingBoxf3& box = m_parent.transformed_convex_hull_bounding_box();
+        if (!m_old_box.size().isApprox(box.size()) || m_old_box.min.z() != box.min.z()) {
+            m_old_box = box;
+            m_shift = Vec3d::Zero();
+
+            const TriangleMesh& mesh = GUI::wxGetApp().plater()->model().objects[m_parent.object_idx()]->volumes[m_parent.volume_idx()]->mesh();
+            assert(mesh.has_shared_vertices());
+
+            m_model.reset();
+            GUI::GLModel::InitializationData init_data;
+            MeshSlicingParams slicing_params;
+            slicing_params.trafo = m_parent.world_matrix();
+            Polygons polygons = union_(slice_mesh(mesh.its, 0.0f, slicing_params));
+            for (Polygon& polygon : polygons) {
+                if (polygon.is_clockwise())
+                    polygon.reverse();
+                Polygons outer_polys = offset(polygon, float(scale_(HalfWidth)));
+                assert(outer_polys.size() == 1);
+                if (outer_polys.empty())
+                    // no outer contour, skip
+                    continue;
+
+                ExPolygon expoly(std::move(outer_polys.front()));
+                expoly.holes = offset(polygon, -float(scale_(HalfWidth)));
+                polygons_reverse(expoly.holes);
+
+                GUI::GLModel::InitializationData::Entity entity;
+                entity.type = GUI::GLModel::PrimitiveType::Triangles;
+                const std::vector<Vec3d> triangulation = triangulate_expolygon_3d(expoly);
+                for (const Vec3d& v : triangulation) {
+                    entity.positions.emplace_back(v.cast<float>() + Vec3f(0.0f, 0.0f, 0.015f)); // add a small positive z to avoid z-fighting
+                    entity.normals.emplace_back(Vec3f::UnitZ());
+                    const size_t positions_count = entity.positions.size();
+                    if (positions_count % 3 == 0) {
+                        entity.indices.emplace_back(positions_count - 3);
+                        entity.indices.emplace_back(positions_count - 2);
+                        entity.indices.emplace_back(positions_count - 1);
+                    }
+                }
+                init_data.entities.emplace_back(entity);
+            }
+
+            m_model.init_from(init_data);
+        }
+        else
+            m_shift = box.center() - m_old_box.center();
+    }
+    else
+        m_model.reset();
+}
+#endif // ENABLE_SINKING_CONTOURS
+
 const std::array<float, 4> GLVolume::SELECTED_COLOR = { 0.0f, 1.0f, 0.0f, 1.0f };
 const std::array<float, 4> GLVolume::HOVER_SELECT_COLOR = { 0.4f, 0.9f, 0.1f, 1.0f };
 const std::array<float, 4> GLVolume::HOVER_DESELECT_COLOR = { 1.0f, 0.75f, 0.75f, 1.0f };
@@ -304,6 +375,9 @@ GLVolume::GLVolume(float r, float g, float b, float a)
     : m_transformed_bounding_box_dirty(true)
     , m_sla_shift_z(0.0)
     , m_transformed_convex_hull_bounding_box_dirty(true)
+#if ENABLE_SINKING_CONTOURS
+    , m_sinking_contours(*this)
+#endif // ENABLE_SINKING_CONTOURS
     // geometry_id == 0 -> invalid
     , geometry_id(std::pair<size_t, size_t>(0, 0))
     , extruder_id(0)
@@ -321,6 +395,9 @@ GLVolume::GLVolume(float r, float g, float b, float a)
     , force_transparent(false)
     , force_native_color(false)
     , force_neutral_color(false)
+#if ENABLE_SINKING_CONTOURS
+    , force_sinking_contours(false)
+#endif // ENABLE_SINKING_CONTOURS
     , tverts_range(0, size_t(-1))
     , qverts_range(0, size_t(-1))
 {
@@ -517,6 +594,13 @@ bool GLVolume::is_below_printbed() const
     return transformed_convex_hull_bounding_box().max(2) < 0.0;
 }
 
+#if ENABLE_SINKING_CONTOURS
+void GLVolume::render_sinking_contours()
+{
+    m_sinking_contours.render();
+}
+#endif // ENABLE_SINKING_CONTOURS
+
 std::vector<int> GLVolumeCollection::load_object(
     const ModelObject       *model_object,
     int                      obj_idx,
@@ -752,6 +836,68 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType type, bool disab
     if (disable_cullface)
         glsafe(::glDisable(GL_CULL_FACE));
 
+#if ENABLE_SINKING_CONTOURS
+    GLVolumeWithIdAndZList to_render = volumes_to_render(volumes, type, view_matrix, filter_func);
+    for (GLVolumeWithIdAndZ& volume : to_render) {
+        volume.first->set_render_color();
+
+        // render sinking contours of non-hovered volumes
+        if (volume.first->is_sinking() && !volume.first->is_below_printbed() &&
+            volume.first->hover == GLVolume::HS_None && !volume.first->force_sinking_contours) {
+            shader->stop_using();
+            volume.first->render_sinking_contours();
+            shader->start_using();
+        }
+
+        glsafe(::glEnableClientState(GL_VERTEX_ARRAY));
+        glsafe(::glEnableClientState(GL_NORMAL_ARRAY));
+
+        shader->set_uniform("uniform_color", volume.first->render_color);
+        shader->set_uniform("z_range", m_z_range, 2);
+        shader->set_uniform("clipping_plane", m_clipping_plane, 4);
+        shader->set_uniform("print_box.min", m_print_box_min, 3);
+        shader->set_uniform("print_box.max", m_print_box_max, 3);
+        shader->set_uniform("print_box.actived", volume.first->shader_outside_printer_detection_enabled);
+        shader->set_uniform("print_box.volume_world_matrix", volume.first->world_matrix());
+        shader->set_uniform("slope.actived", m_slope.active && !volume.first->is_modifier && !volume.first->is_wipe_tower);
+        shader->set_uniform("slope.volume_world_normal_matrix", static_cast<Matrix3f>(volume.first->world_matrix().matrix().block(0, 0, 3, 3).inverse().transpose().cast<float>()));
+        shader->set_uniform("slope.normal_z", m_slope.normal_z);
+
+#if ENABLE_ENVIRONMENT_MAP
+        unsigned int environment_texture_id = GUI::wxGetApp().plater()->get_environment_texture_id();
+        bool use_environment_texture = environment_texture_id > 0 && GUI::wxGetApp().app_config->get("use_environment_map") == "1";
+        shader->set_uniform("use_environment_tex", use_environment_texture);
+        if (use_environment_texture)
+            glsafe(::glBindTexture(GL_TEXTURE_2D, environment_texture_id));
+#endif // ENABLE_ENVIRONMENT_MAP
+        glcheck();
+
+        volume.first->render();
+
+#if ENABLE_ENVIRONMENT_MAP
+        if (use_environment_texture)
+            glsafe(::glBindTexture(GL_TEXTURE_2D, 0));
+#endif // ENABLE_ENVIRONMENT_MAP
+
+        glsafe(::glBindBuffer(GL_ARRAY_BUFFER, 0));
+        glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0));
+
+        glsafe(::glDisableClientState(GL_VERTEX_ARRAY));
+        glsafe(::glDisableClientState(GL_NORMAL_ARRAY));
+    }
+
+    for (GLVolumeWithIdAndZ& volume : to_render) {
+        // render sinking contours of hovered/displaced volumes
+        if (volume.first->is_sinking() && !volume.first->is_below_printbed() &&
+            (volume.first->hover != GLVolume::HS_None || volume.first->force_sinking_contours)) {
+            shader->stop_using();
+            glsafe(::glDepthFunc(GL_ALWAYS));
+            volume.first->render_sinking_contours();
+            glsafe(::glDepthFunc(GL_LESS));
+            shader->start_using();
+        }
+    }
+#else
     glsafe(::glEnableClientState(GL_VERTEX_ARRAY));
     glsafe(::glEnableClientState(GL_NORMAL_ARRAY));
  
@@ -791,6 +937,7 @@ void GLVolumeCollection::render(GLVolumeCollection::ERenderType type, bool disab
 
     glsafe(::glDisableClientState(GL_VERTEX_ARRAY));
     glsafe(::glDisableClientState(GL_NORMAL_ARRAY));
+#endif // ENABLE_SINKING_CONTOURS
 
     if (disable_cullface)
         glsafe(::glEnable(GL_CULL_FACE));
diff --git a/src/slic3r/GUI/3DScene.hpp b/src/slic3r/GUI/3DScene.hpp
index fad4427e8..a257db56a 100644
--- a/src/slic3r/GUI/3DScene.hpp
+++ b/src/slic3r/GUI/3DScene.hpp
@@ -8,6 +8,10 @@
 #include "libslic3r/Utils.hpp"
 #include "libslic3r/Geometry.hpp"
 
+#if ENABLE_SINKING_CONTOURS
+#include "GLModel.hpp"
+#endif // ENABLE_SINKING_CONTOURS
+
 #include <functional>
 
 #define HAS_GLSAFE
@@ -250,6 +254,9 @@ public:
     enum EHoverState : unsigned char
     {
         HS_None,
+#if ENABLE_SINKING_CONTOURS
+        HS_Hover,
+#endif // ENABLE_SINKING_CONTOURS
         HS_Select,
         HS_Deselect
     };
@@ -262,7 +269,7 @@ private:
     Geometry::Transformation m_volume_transformation;
 
     // Shift in z required by sla supports+pad
-    double        		  m_sla_shift_z;
+    double        m_sla_shift_z;
     // Bounding box of this volume, in unscaled coordinates.
     BoundingBoxf3 m_transformed_bounding_box;
     // Whether or not is needed to recalculate the transformed bounding box.
@@ -274,6 +281,26 @@ private:
     // Whether or not is needed to recalculate the transformed convex hull bounding box.
     bool          m_transformed_convex_hull_bounding_box_dirty;
 
+#if ENABLE_SINKING_CONTOURS
+    class SinkingContours
+    {
+        static const float HalfWidth;
+        GLVolume& m_parent;
+        GUI::GLModel m_model;
+        BoundingBoxf3 m_old_box;
+        Vec3d m_shift{ Vec3d::Zero() };
+
+    public:
+        SinkingContours(GLVolume& volume) : m_parent(volume) {}
+        void render();
+
+    private:
+        void update();
+    };
+
+    SinkingContours m_sinking_contours;
+#endif // ENABLE_SINKING_CONTOURS
+
 public:
     // Color of the triangles / quads held by this volume.
     std::array<float, 4> color;
@@ -334,7 +361,11 @@ public:
 	    bool                force_native_color : 1;
         // Whether or not render this volume in neutral
         bool                force_neutral_color : 1;
-	};
+#if ENABLE_SINKING_CONTOURS
+        // Whether or not to force rendering of sinking contours
+        bool                force_sinking_contours : 1;
+#endif // ENABLE_SINKING_CONTOURS
+    };
 
     // Is mouse or rectangle selection over this object to select/deselect it ?
     EHoverState         	hover;
@@ -461,6 +492,9 @@ public:
 
     bool                is_sinking() const;
     bool                is_below_printbed() const;
+#if ENABLE_SINKING_CONTOURS
+    void                render_sinking_contours();
+#endif // ENABLE_SINKING_CONTOURS
 
     // Return an estimate of the memory consumed by this class.
     size_t 				cpu_memory_used() const { 
diff --git a/src/slic3r/GUI/GLCanvas3D.cpp b/src/slic3r/GUI/GLCanvas3D.cpp
index 4b1a5c338..6f34f4051 100644
--- a/src/slic3r/GUI/GLCanvas3D.cpp
+++ b/src/slic3r/GUI/GLCanvas3D.cpp
@@ -792,7 +792,7 @@ void GLCanvas3D::SequentialPrintClearance::set_polygons(const Polygons& polygons
     for (const Polygon& poly : polygons) {
         triangles_count += poly.points.size() - 2;
     }
-    size_t vertices_count = 3 * triangles_count;
+    const size_t vertices_count = 3 * triangles_count;
 
     if (m_render_fill) {
         GLModel::InitializationData fill_data;
@@ -803,13 +803,13 @@ void GLCanvas3D::SequentialPrintClearance::set_polygons(const Polygons& polygons
         entity.normals.reserve(vertices_count);
         entity.indices.reserve(vertices_count);
 
-        ExPolygons polygons_union = union_ex(polygons);
+        const ExPolygons polygons_union = union_ex(polygons);
         for (const ExPolygon& poly : polygons_union) {
-            std::vector<Vec3d> triangulation = triangulate_expolygon_3d(poly, false);
+            const std::vector<Vec3d> triangulation = triangulate_expolygon_3d(poly);
             for (const Vec3d& v : triangulation) {
                 entity.positions.emplace_back(v.cast<float>() + Vec3f(0.0f, 0.0f, 0.0125f)); // add a small positive z to avoid z-fighting
                 entity.normals.emplace_back(Vec3f::UnitZ());
-                size_t positions_count = entity.positions.size();
+                const size_t positions_count = entity.positions.size();
                 if (positions_count % 3 == 0) {
                     entity.indices.emplace_back(positions_count - 3);
                     entity.indices.emplace_back(positions_count - 2);
@@ -2938,6 +2938,20 @@ void GLCanvas3D::on_mouse(wxMouseEvent& evt)
         return;
     }
 
+#if ENABLE_SINKING_CONTOURS
+    for (GLVolume* volume : m_volumes.volumes) {
+        volume->force_sinking_contours = false;
+    }
+
+    auto show_sinking_contours = [this]() {
+        const Selection::IndicesList& idxs = m_selection.get_volume_idxs();
+        for (unsigned int idx : idxs) {
+            m_volumes.volumes[idx]->force_sinking_contours = true;
+        }
+        m_dirty = true;
+    };
+#endif // ENABLE_SINKING_CONTOURS
+
     if (m_gizmos.on_mouse(evt)) {
         if (wxWindow::FindFocus() != m_canvas)
             // Grab keyboard focus for input in gizmo dialogs.
@@ -2962,6 +2976,21 @@ void GLCanvas3D::on_mouse(wxMouseEvent& evt)
             default: { break; }
             }
         }
+#if ENABLE_SINKING_CONTOURS
+        else if (evt.Dragging()) {
+            switch (m_gizmos.get_current_type())
+            {
+            case GLGizmosManager::EType::Move:
+            case GLGizmosManager::EType::Scale:
+            case GLGizmosManager::EType::Rotate:
+            {
+                show_sinking_contours();
+                break;
+            }
+            default: { break; }
+            }
+        }
+#endif // ENABLE_SINKING_CONTOURS
 
         return;
     }
@@ -3271,6 +3300,11 @@ void GLCanvas3D::on_mouse(wxMouseEvent& evt)
     else
         evt.Skip();
 
+#if ENABLE_SINKING_CONTOURS
+    if (m_moving)
+        show_sinking_contours();
+#endif // ENABLE_SINKING_CONTOURS
+
 #ifdef __WXMSW__
 	if (on_enter_workaround)
 		m_mouse.position = Vec2d(-1., -1.);
@@ -5579,6 +5613,11 @@ void GLCanvas3D::_update_volumes_hover_state()
                 }
             }
         }
+#if ENABLE_SINKING_CONTOURS
+        else if (volume.selected)
+            volume.hover = GLVolume::HS_Hover;
+#endif // ENABLE_SINKING_CONTOURS
+
     }
 }
 
diff --git a/src/slic3r/GUI/GLModel.cpp b/src/slic3r/GUI/GLModel.cpp
index 6cc09c7ba..6d54ec20e 100644
--- a/src/slic3r/GUI/GLModel.cpp
+++ b/src/slic3r/GUI/GLModel.cpp
@@ -7,6 +7,9 @@
 
 #include "libslic3r/TriangleMesh.hpp"
 #include "libslic3r/Model.hpp"
+#if ENABLE_SINKING_CONTOURS
+#include "libslic3r/Polygon.hpp"
+#endif // ENABLE_SINKING_CONTOURS
 
 #include <boost/filesystem/operations.hpp>
 #include <boost/algorithm/string/predicate.hpp>
@@ -87,6 +90,35 @@ void GLModel::init_from(const TriangleMesh& mesh)
     m_render_data.emplace_back(data);
 }
 
+#if ENABLE_SINKING_CONTOURS
+void GLModel::init_from(const Polygons& polygons, float z)
+{
+    auto append_polygon = [](const Polygon& polygon, float z, GUI::GLModel::InitializationData& data) {
+        if (!polygon.empty()) {
+            GUI::GLModel::InitializationData::Entity entity;
+            entity.type = GUI::GLModel::PrimitiveType::LineLoop;
+            // contour
+            entity.positions.reserve(polygon.size() + 1);
+            entity.indices.reserve(polygon.size() + 1);
+            unsigned int id = 0;
+            for (const Point& p : polygon) {
+                Vec3f position = unscale(p.x(), p.y(), 0.0).cast<float>();
+                position.z() = z;
+                entity.positions.emplace_back(position);
+                entity.indices.emplace_back(id++);
+            }
+            data.entities.emplace_back(entity);
+        }
+    };
+
+    InitializationData init_data;
+    for (const Polygon& polygon : polygons) {
+        append_polygon(polygon, z, init_data);
+    }
+    init_from(init_data);
+}
+#endif // ENABLE_SINKING_CONTOURS
+
 bool GLModel::init_from_file(const std::string& filename)
 {
     if (!boost::filesystem::exists(filename))
diff --git a/src/slic3r/GUI/GLModel.hpp b/src/slic3r/GUI/GLModel.hpp
index ee3d2f8f9..cab2fe220 100644
--- a/src/slic3r/GUI/GLModel.hpp
+++ b/src/slic3r/GUI/GLModel.hpp
@@ -9,6 +9,10 @@
 namespace Slic3r {
 
 class TriangleMesh;
+#if ENABLE_SINKING_CONTOURS
+class Polygon;
+using Polygons = std::vector<Polygon>;
+#endif // ENABLE_SINKING_CONTOURS
 
 namespace GUI {
 
@@ -58,6 +62,9 @@ namespace GUI {
 
         void init_from(const InitializationData& data);
         void init_from(const TriangleMesh& mesh);
+#if ENABLE_SINKING_CONTOURS
+        void init_from(const Polygons& polygons, float z);
+#endif // ENABLE_SINKING_CONTOURS
         bool init_from_file(const std::string& filename);
 
         // if entity_id == -1 set the color of all entities
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoBase.hpp b/src/slic3r/GUI/Gizmos/GLGizmoBase.hpp
index ac83b794b..2fb9c296a 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoBase.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoBase.hpp
@@ -154,8 +154,8 @@ public:
 
     void update(const UpdateData& data);
 
-    void render() const { m_tooltip.clear(); on_render(); }
-    void render_for_picking() const { on_render_for_picking(); }
+    void render() { m_tooltip.clear(); on_render(); }
+    void render_for_picking() { on_render_for_picking(); }
     void render_input_window(float x, float y, float bottom_limit);
 
     virtual std::string get_tooltip() const { return ""; }
@@ -175,8 +175,8 @@ protected:
     virtual void on_start_dragging() {}
     virtual void on_stop_dragging() {}
     virtual void on_update(const UpdateData& data) {}
-    virtual void on_render() const = 0;
-    virtual void on_render_for_picking() const = 0;
+    virtual void on_render() = 0;
+    virtual void on_render_for_picking() = 0;
     virtual void on_render_input_window(float x, float y, float bottom_limit) {}
 
     // Returns the picking color for the given id, based on the BASE_ID constant
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoCut.cpp b/src/slic3r/GUI/Gizmos/GLGizmoCut.cpp
index 6fc382cbc..707726e08 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoCut.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoCut.cpp
@@ -16,7 +16,9 @@
 #include "slic3r/GUI/GUI_ObjectManipulation.hpp"
 #include "libslic3r/AppConfig.hpp"
 #include "libslic3r/Model.hpp"
-
+#if ENABLE_SINKING_CONTOURS
+#include "libslic3r/TriangleMeshSlicer.hpp"
+#endif // ENABLE_SINKING_CONTOURS
 
 namespace Slic3r {
 namespace GUI {
@@ -82,14 +84,18 @@ void GLGizmoCut::on_update(const UpdateData& data)
         set_cut_z(m_start_z + calc_projection(data.mouse_ray));
 }
 
-void GLGizmoCut::on_render() const
+void GLGizmoCut::on_render()
 {
-    BoundingBoxf3 box = bounding_box();
+    const BoundingBoxf3 box = bounding_box();
     Vec3d plane_center = box.center();
     plane_center.z() = m_cut_z;
     m_max_z = box.max.z();
     set_cut_z(m_cut_z);
 
+#if ENABLE_SINKING_CONTOURS
+    update_contours();
+#endif // ENABLE_SINKING_CONTOURS
+
     const float min_x = box.min.x() - Margin;
     const float max_x = box.max.x() + Margin;
     const float min_y = box.min.y() - Margin;
@@ -136,9 +142,17 @@ void GLGizmoCut::on_render() const
     m_grabbers[0].render(m_hover_id == 0, (float)((box.size().x() + box.size().y() + box.size().z()) / 3.0));
 
     shader->stop_using();
+
+#if ENABLE_SINKING_CONTOURS
+    glsafe(::glPushMatrix());
+    glsafe(::glTranslated(m_cut_contours.shift.x(), m_cut_contours.shift.y(), m_cut_contours.shift.z()));
+    glsafe(::glLineWidth(2.0f));
+    m_cut_contours.contours.render();
+    glsafe(::glPopMatrix());
+#endif // ENABLE_SINKING_CONTOURS
 }
 
-void GLGizmoCut::on_render_for_picking() const
+void GLGizmoCut::on_render_for_picking()
 {
     glsafe(::glDisable(GL_DEPTH_TEST));
     render_grabbers_for_picking(m_parent.get_selection().get_bounding_box());
@@ -199,7 +213,7 @@ void GLGizmoCut::on_render_input_window(float x, float y, float bottom_limit)
         perform_cut(m_parent.get_selection());
 }
 
-void GLGizmoCut::set_cut_z(double cut_z) const
+void GLGizmoCut::set_cut_z(double cut_z)
 {
     // Clamp the plane to the object's bounding box
     m_cut_z = std::clamp(cut_z, 0.0, m_max_z);
@@ -261,5 +275,47 @@ BoundingBoxf3 GLGizmoCut::bounding_box() const
     return ret;
 }
 
+#if ENABLE_SINKING_CONTOURS
+void GLGizmoCut::update_contours()
+{
+    const Selection& selection = m_parent.get_selection();
+    const GLVolume* first_glvolume = selection.get_volume(*selection.get_volume_idxs().begin());
+    const BoundingBoxf3& box = first_glvolume->transformed_convex_hull_bounding_box();
+
+    const int object_idx = selection.get_object_idx();
+    const int instance_idx = selection.get_instance_idx();
+
+    if (0.0 < m_cut_z && m_cut_z < m_max_z) {
+        if (m_cut_contours.cut_z != m_cut_z || m_cut_contours.object_idx != object_idx || m_cut_contours.instance_idx != instance_idx) {
+            m_cut_contours.cut_z = m_cut_z;
+
+            if (m_cut_contours.object_idx != object_idx) {
+                m_cut_contours.mesh = wxGetApp().plater()->model().objects[object_idx]->raw_mesh();
+                m_cut_contours.mesh.repair();
+            }
+
+            m_cut_contours.position = box.center();
+            m_cut_contours.shift = Vec3d::Zero();
+            m_cut_contours.object_idx = object_idx;
+            m_cut_contours.instance_idx = instance_idx;
+            m_cut_contours.contours.reset();
+
+            MeshSlicingParams slicing_params;
+            slicing_params.trafo = first_glvolume->get_instance_transformation().get_matrix();
+            const Polygons polys = slice_mesh(m_cut_contours.mesh.its, m_cut_z, slicing_params);
+            if (!polys.empty()) {
+                m_cut_contours.contours.init_from(polys, static_cast<float>(m_cut_z));
+                m_cut_contours.contours.set_color(-1, { 1.0f, 1.0f, 1.0f, 1.0f });
+            }
+        }
+        else if (box.center() != m_cut_contours.position) {
+            m_cut_contours.shift = box.center() - m_cut_contours.position;
+        }
+    }
+    else
+        m_cut_contours.contours.reset();
+}
+#endif // ENABLE_SINKING_CONTOURS
+
 } // namespace GUI
 } // namespace Slic3r
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoCut.hpp b/src/slic3r/GUI/Gizmos/GLGizmoCut.hpp
index dd80c4d69..abd879350 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoCut.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoCut.hpp
@@ -2,7 +2,10 @@
 #define slic3r_GLGizmoCut_hpp_
 
 #include "GLGizmoBase.hpp"
-
+#if ENABLE_SINKING_CONTOURS
+#include "slic3r/GUI/GLModel.hpp"
+#include "libslic3r/TriangleMesh.hpp"
+#endif // ENABLE_SINKING_CONTOURS
 
 namespace Slic3r {
 namespace GUI {
@@ -13,8 +16,8 @@ class GLGizmoCut : public GLGizmoBase
     static const double Margin;
     static const std::array<float, 4> GrabberColor;
 
-    mutable double m_cut_z{ 0.0 };
-    mutable double m_max_z{ 0.0 };
+    double m_cut_z{ 0.0 };
+    double m_max_z{ 0.0 };
     double m_start_z{ 0.0 };
     Vec3d m_drag_pos;
     Vec3d m_drag_center;
@@ -22,11 +25,26 @@ class GLGizmoCut : public GLGizmoBase
     bool m_keep_lower{ true };
     bool m_rotate_lower{ false };
 
+#if ENABLE_SINKING_CONTOURS
+    struct CutContours
+    {
+        TriangleMesh mesh;
+        GLModel contours;
+        double cut_z{ 0.0 };
+        Vec3d position{ Vec3d::Zero() };
+        Vec3d shift{ Vec3d::Zero() };
+        int object_idx{ -1 };
+        int instance_idx{ -1 };
+    };
+
+    CutContours m_cut_contours;
+#endif // ENABLE_SINKING_CONTOURS
+
 public:
     GLGizmoCut(GLCanvas3D& parent, const std::string& icon_filename, unsigned int sprite_id);
 
     double get_cut_z() const { return m_cut_z; }
-    void set_cut_z(double cut_z) const;
+    void set_cut_z(double cut_z);
 
     std::string get_tooltip() const override;
 
@@ -39,14 +57,17 @@ protected:
     virtual bool on_is_activable() const override;
     virtual void on_start_dragging() override;
     virtual void on_update(const UpdateData& data) override;
-    virtual void on_render() const override;
-    virtual void on_render_for_picking() const override;
+    virtual void on_render() override;
+    virtual void on_render_for_picking() override;
     virtual void on_render_input_window(float x, float y, float bottom_limit) override;
 
 private:
     void perform_cut(const Selection& selection);
     double calc_projection(const Linef3& mouse_ray) const;
     BoundingBoxf3 bounding_box() const;
+#if ENABLE_SINKING_CONTOURS
+    void update_contours();
+#endif // ENABLE_SINKING_CONTOURS
 };
 
 } // namespace GUI
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoFlatten.cpp b/src/slic3r/GUI/Gizmos/GLGizmoFlatten.cpp
index fc5edff6d..094805abc 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoFlatten.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoFlatten.cpp
@@ -54,7 +54,7 @@ void GLGizmoFlatten::on_start_dragging()
     }
 }
 
-void GLGizmoFlatten::on_render() const
+void GLGizmoFlatten::on_render()
 {
     const Selection& selection = m_parent.get_selection();
 
@@ -69,7 +69,7 @@ void GLGizmoFlatten::on_render() const
         glsafe(::glTranslatef(0.f, 0.f, selection.get_volume(*selection.get_volume_idxs().begin())->get_sla_shift_z()));
         glsafe(::glMultMatrixd(m.data()));
         if (this->is_plane_update_necessary())
-			const_cast<GLGizmoFlatten*>(this)->update_planes();
+            update_planes();
         for (int i = 0; i < (int)m_planes.size(); ++i) {
             if (i == m_hover_id)
                 glsafe(::glColor4f(0.9f, 0.9f, 0.9f, 0.75f));
@@ -86,7 +86,7 @@ void GLGizmoFlatten::on_render() const
     glsafe(::glDisable(GL_BLEND));
 }
 
-void GLGizmoFlatten::on_render_for_picking() const
+void GLGizmoFlatten::on_render_for_picking()
 {
     const Selection& selection = m_parent.get_selection();
 
@@ -99,7 +99,7 @@ void GLGizmoFlatten::on_render_for_picking() const
         glsafe(::glTranslatef(0.f, 0.f, selection.get_volume(*selection.get_volume_idxs().begin())->get_sla_shift_z()));
         glsafe(::glMultMatrixd(m.data()));
         if (this->is_plane_update_necessary())
-			const_cast<GLGizmoFlatten*>(this)->update_planes();
+            update_planes();
         for (int i = 0; i < (int)m_planes.size(); ++i) {
             glsafe(::glColor4fv(picking_color_component(i).data()));
             m_planes[i].vbo.render();
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoFlatten.hpp b/src/slic3r/GUI/Gizmos/GLGizmoFlatten.hpp
index f6fe06451..ab3c2c7ba 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoFlatten.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoFlatten.hpp
@@ -53,8 +53,8 @@ protected:
     virtual std::string on_get_name() const override;
     virtual bool on_is_activable() const override;
     virtual void on_start_dragging() override;
-    virtual void on_render() const override;
-    virtual void on_render_for_picking() const override;
+    virtual void on_render() override;
+    virtual void on_render_for_picking() override;
     virtual void on_set_state() override;
     virtual CommonGizmosDataID on_get_requirements() const override;
 };
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp b/src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp
index 9d2de1d03..46608b4c7 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoHollow.cpp
@@ -61,7 +61,7 @@ void GLGizmoHollow::set_sla_support_data(ModelObject*, const Selection&)
 
 
 
-void GLGizmoHollow::on_render() const
+void GLGizmoHollow::on_render()
 {
     const Selection& selection = m_parent.get_selection();
     const CommonGizmosDataObjects::SelectionInfo* sel_info = m_c->selection_info();
@@ -88,7 +88,7 @@ void GLGizmoHollow::on_render() const
 }
 
 
-void GLGizmoHollow::on_render_for_picking() const
+void GLGizmoHollow::on_render_for_picking()
 {
     const Selection& selection = m_parent.get_selection();
 //#if ENABLE_RENDER_PICKING_PASS
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoHollow.hpp b/src/slic3r/GUI/Gizmos/GLGizmoHollow.hpp
index 1bcf1713e..2cf08de2a 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoHollow.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoHollow.hpp
@@ -39,8 +39,8 @@ public:
 private:
     bool on_init() override;
     void on_update(const UpdateData& data) override;
-    void on_render() const override;
-    void on_render_for_picking() const override;
+    void on_render() override;
+    void on_render_for_picking() override;
 
     void render_points(const Selection& selection, bool picking = false) const;
     void hollow_mesh(bool postpone_error_messages = false);
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoMove.cpp b/src/slic3r/GUI/Gizmos/GLGizmoMove.cpp
index 7a9e69fac..1211864eb 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoMove.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoMove.cpp
@@ -87,7 +87,7 @@ void GLGizmoMove3D::on_update(const UpdateData& data)
         m_displacement.z() = calc_projection(data);
 }
 
-void GLGizmoMove3D::on_render() const
+void GLGizmoMove3D::on_render()
 {
     const Selection& selection = m_parent.get_selection();
 
@@ -151,7 +151,7 @@ void GLGizmoMove3D::on_render() const
     }
 }
 
-void GLGizmoMove3D::on_render_for_picking() const
+void GLGizmoMove3D::on_render_for_picking()
 {
     glsafe(::glDisable(GL_DEPTH_TEST));
 
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoMove.hpp b/src/slic3r/GUI/Gizmos/GLGizmoMove.hpp
index 9a86852d6..baa2df739 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoMove.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoMove.hpp
@@ -39,8 +39,8 @@ protected:
     virtual void on_start_dragging() override;
     virtual void on_stop_dragging() override;
     virtual void on_update(const UpdateData& data) override;
-    virtual void on_render() const override;
-    virtual void on_render_for_picking() const override;
+    virtual void on_render() override;
+    virtual void on_render_for_picking() override;
 
 private:
     double calc_projection(const UpdateData& data) const;
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
index a551759b6..04c74c487 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
@@ -66,8 +66,8 @@ class GLGizmoPainterBase : public GLGizmoBase
 private:
     ObjectID m_old_mo_id;
     size_t m_old_volumes_size = 0;
-    void on_render() const override {}
-    void on_render_for_picking() const override {}
+    void on_render() override {}
+    void on_render_for_picking() override {}
 
 public:
     GLGizmoPainterBase(GLCanvas3D& parent, const std::string& icon_filename, unsigned int sprite_id);
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp b/src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp
index 8b12bdc3d..a495db4f1 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp
@@ -125,7 +125,7 @@ void GLGizmoRotate::on_update(const UpdateData& data)
     m_angle = theta;
 }
 
-void GLGizmoRotate::on_render() const
+void GLGizmoRotate::on_render()
 {
     if (!m_grabbers[0].enabled)
         return;
@@ -169,7 +169,7 @@ void GLGizmoRotate::on_render() const
     glsafe(::glPopMatrix());
 }
 
-void GLGizmoRotate::on_render_for_picking() const
+void GLGizmoRotate::on_render_for_picking()
 {
     const Selection& selection = m_parent.get_selection();
 
@@ -483,17 +483,17 @@ void GLGizmoRotate3D::on_stop_dragging()
         m_gizmos[m_hover_id].stop_dragging();
 }
 
-void GLGizmoRotate3D::on_render() const
+void GLGizmoRotate3D::on_render()
 {
     glsafe(::glClear(GL_DEPTH_BUFFER_BIT));
 
-    if ((m_hover_id == -1) || (m_hover_id == 0))
+    if (m_hover_id == -1 || m_hover_id == 0)
         m_gizmos[X].render();
 
-    if ((m_hover_id == -1) || (m_hover_id == 1))
+    if (m_hover_id == -1 || m_hover_id == 1)
         m_gizmos[Y].render();
 
-    if ((m_hover_id == -1) || (m_hover_id == 2))
+    if (m_hover_id == -1 || m_hover_id == 2)
         m_gizmos[Z].render();
 }
 
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoRotate.hpp b/src/slic3r/GUI/Gizmos/GLGizmoRotate.hpp
index 3fc347222..a51f900bf 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoRotate.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoRotate.hpp
@@ -55,8 +55,8 @@ protected:
     std::string on_get_name() const override { return ""; }
     void on_start_dragging() override;
     void on_update(const UpdateData& data) override;
-    void on_render() const override;
-    void on_render_for_picking() const override;
+    void on_render() override;
+    void on_render_for_picking() override;
 
 private:
     void render_circle() const;
@@ -124,10 +124,10 @@ protected:
             g.update(data);
         }
     }
-    void on_render() const override;
-    void on_render_for_picking() const override
+    void on_render() override;
+    void on_render_for_picking() override
     {
-        for (const GLGizmoRotate& g : m_gizmos) {
+        for (GLGizmoRotate& g : m_gizmos) {
             g.render_for_picking();
         }
     }
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoScale.cpp b/src/slic3r/GUI/Gizmos/GLGizmoScale.cpp
index fb1b148b4..849046962 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoScale.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoScale.cpp
@@ -115,7 +115,7 @@ void GLGizmoScale3D::on_update(const UpdateData& data)
         do_scale_uniform(data);
 }
 
-void GLGizmoScale3D::on_render() const
+void GLGizmoScale3D::on_render()
 {
     const Selection& selection = m_parent.get_selection();
 
@@ -294,7 +294,7 @@ void GLGizmoScale3D::on_render() const
     }
 }
 
-void GLGizmoScale3D::on_render_for_picking() const
+void GLGizmoScale3D::on_render_for_picking()
 {
     glsafe(::glDisable(GL_DEPTH_TEST));
     render_grabbers_for_picking(m_parent.get_selection().get_bounding_box());
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoScale.hpp b/src/slic3r/GUI/Gizmos/GLGizmoScale.hpp
index 39021640a..35615df4a 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoScale.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoScale.hpp
@@ -52,8 +52,8 @@ protected:
     virtual bool on_is_activable() const override;
     virtual void on_start_dragging() override;
     virtual void on_update(const UpdateData& data) override;
-    virtual void on_render() const override;
-    virtual void on_render_for_picking() const override;
+    virtual void on_render() override;
+    virtual void on_render_for_picking() override;
 
 private:
     void render_grabbers_connection(unsigned int id_1, unsigned int id_2) const;
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp b/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp
index f28f060bd..39107e7be 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp
@@ -74,7 +74,7 @@ void GLGizmoSlaSupports::set_sla_support_data(ModelObject* model_object, const S
 
 
 
-void GLGizmoSlaSupports::on_render() const
+void GLGizmoSlaSupports::on_render()
 {
     ModelObject* mo = m_c->selection_info()->model_object();
     const Selection& selection = m_parent.get_selection();
@@ -101,7 +101,7 @@ void GLGizmoSlaSupports::on_render() const
 }
 
 
-void GLGizmoSlaSupports::on_render_for_picking() const
+void GLGizmoSlaSupports::on_render_for_picking()
 {
     const Selection& selection = m_parent.get_selection();
     //glsafe(::glEnable(GL_DEPTH_TEST));
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.hpp b/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.hpp
index 1859dbfd1..6982ecf76 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.hpp
@@ -70,8 +70,8 @@ public:
 private:
     bool on_init() override;
     void on_update(const UpdateData& data) override;
-    void on_render() const override;
-    void on_render_for_picking() const override;
+    void on_render() override;
+    void on_render_for_picking() override;
 
     void render_points(const Selection& selection, bool picking = false) const;
     bool unsaved_changes() const;
diff --git a/src/slic3r/GUI/Plater.cpp b/src/slic3r/GUI/Plater.cpp
index ef6193586..9b59026dc 100644
--- a/src/slic3r/GUI/Plater.cpp
+++ b/src/slic3r/GUI/Plater.cpp
@@ -4689,6 +4689,7 @@ void Plater::render_project_state_debug_window() const { p->render_project_state
 #endif // ENABLE_PROJECT_DIRTY_STATE
 
 Sidebar&        Plater::sidebar()           { return *p->sidebar; }
+const Model&    Plater::model() const       { return p->model; }
 Model&          Plater::model()             { return p->model; }
 const Print&    Plater::fff_print() const   { return p->fff_print; }
 Print&          Plater::fff_print()         { return p->fff_print; }
diff --git a/src/slic3r/GUI/Plater.hpp b/src/slic3r/GUI/Plater.hpp
index 7cd0be0fe..9c37dfe2b 100644
--- a/src/slic3r/GUI/Plater.hpp
+++ b/src/slic3r/GUI/Plater.hpp
@@ -149,6 +149,7 @@ public:
 #endif // ENABLE_PROJECT_DIRTY_STATE
 
     Sidebar& sidebar();
+    const Model& model() const;
     Model& model();
     const Print& fff_print() const;
     Print& fff_print();
diff --git a/src/slic3r/GUI/Selection.cpp b/src/slic3r/GUI/Selection.cpp
index 007e9bc0f..ce44e5442 100644
--- a/src/slic3r/GUI/Selection.cpp
+++ b/src/slic3r/GUI/Selection.cpp
@@ -699,14 +699,14 @@ void Selection::rotate(const Vec3d& rotation, TransformationType transformation_
         int rot_axis_max = 0;
         if (rotation.isApprox(Vec3d::Zero())) {
             for (unsigned int i : m_list) {
-                GLVolume &volume = *(*m_volumes)[i];
+                GLVolume &v = *(*m_volumes)[i];
                 if (m_mode == Instance) {
-                    volume.set_instance_rotation(m_cache.volumes_data[i].get_instance_rotation());
-                    volume.set_instance_offset(m_cache.volumes_data[i].get_instance_position());
+                    v.set_instance_rotation(m_cache.volumes_data[i].get_instance_rotation());
+                    v.set_instance_offset(m_cache.volumes_data[i].get_instance_position());
                 }
                 else if (m_mode == Volume) {
-                    volume.set_volume_rotation(m_cache.volumes_data[i].get_volume_rotation());
-                    volume.set_volume_offset(m_cache.volumes_data[i].get_volume_position());
+                    v.set_volume_rotation(m_cache.volumes_data[i].get_volume_rotation());
+                    v.set_volume_offset(m_cache.volumes_data[i].get_volume_position());
                 }
             }
         }
@@ -747,22 +747,22 @@ void Selection::rotate(const Vec3d& rotation, TransformationType transformation_
             };
 
             for (unsigned int i : m_list) {
-                GLVolume &volume = *(*m_volumes)[i];
+                GLVolume &v = *(*m_volumes)[i];
                 if (is_single_full_instance())
-                    rotate_instance(volume, i);
+                    rotate_instance(v, i);
                 else if (is_single_volume() || is_single_modifier()) {
                     if (transformation_type.independent())
-                        volume.set_volume_rotation(volume.get_volume_rotation() + rotation);
+                        v.set_volume_rotation(v.get_volume_rotation() + rotation);
                     else {
                         const Transform3d m = Geometry::assemble_transform(Vec3d::Zero(), rotation);
                         const Vec3d new_rotation = Geometry::extract_euler_angles(m * m_cache.volumes_data[i].get_volume_rotation_matrix());
-                        volume.set_volume_rotation(new_rotation);
+                        v.set_volume_rotation(new_rotation);
                     }
                 }
                 else
                 {
                     if (m_mode == Instance)
-                        rotate_instance(volume, i);
+                        rotate_instance(v, i);
                     else if (m_mode == Volume) {
                         // extracts rotations from the composed transformation
                         Transform3d m = Geometry::assemble_transform(Vec3d::Zero(), rotation);
@@ -770,9 +770,9 @@ void Selection::rotate(const Vec3d& rotation, TransformationType transformation_
                         if (transformation_type.joint()) {
                             const Vec3d local_pivot = m_cache.volumes_data[i].get_instance_full_matrix().inverse() * m_cache.dragging_center;
                             const Vec3d offset = m * (m_cache.volumes_data[i].get_volume_position() - local_pivot);
-                            volume.set_volume_offset(local_pivot + offset);
+                            v.set_volume_offset(local_pivot + offset);
                         }
-                        volume.set_volume_rotation(new_rotation);
+                        v.set_volume_rotation(new_rotation);
                     }
                 }
             }
@@ -840,9 +840,9 @@ void Selection::scale(const Vec3d& scale, TransformationType transformation_type
     bool is_sla = wxGetApp().preset_bundle->printers.get_edited_preset().printer_technology() == ptSLA;
 
     for (unsigned int i : m_list) {
-        GLVolume &volume = *(*m_volumes)[i];
+        GLVolume &v = *(*m_volumes)[i];
         if (!is_sla) {
-            if (volume.is_modifier)
+            if (v.is_modifier)
                 is_any_volume_sinking = true;
             else
                 is_any_volume_sinking |= std::find(m_cache.sinking_volumes.begin(), m_cache.sinking_volumes.end(), i) != m_cache.sinking_volumes.end();
@@ -854,23 +854,23 @@ void Selection::scale(const Vec3d& scale, TransformationType transformation_type
                 // extracts scaling factors from the composed transformation
                 Vec3d new_scale(new_matrix.col(0).norm(), new_matrix.col(1).norm(), new_matrix.col(2).norm());
                 if (transformation_type.joint())
-                    volume.set_instance_offset(m_cache.dragging_center + m * (m_cache.volumes_data[i].get_instance_position() - m_cache.dragging_center));
+                    v.set_instance_offset(m_cache.dragging_center + m * (m_cache.volumes_data[i].get_instance_position() - m_cache.dragging_center));
 
-                volume.set_instance_scaling_factor(new_scale);
+                v.set_instance_scaling_factor(new_scale);
             }
             else {
                 if (transformation_type.world() && (std::abs(scale.x() - scale.y()) > EPSILON || std::abs(scale.x() - scale.z()) > EPSILON)) {
                     // Non-uniform scaling. Transform the scaling factors into the local coordinate system.
                     // This is only possible, if the instance rotation is mulitples of ninety degrees.
-                    assert(Geometry::is_rotation_ninety_degrees(volume.get_instance_rotation()));
-                    volume.set_instance_scaling_factor((volume.get_instance_transformation().get_matrix(true, false, true, true).matrix().block<3, 3>(0, 0).transpose() * scale).cwiseAbs());
+                    assert(Geometry::is_rotation_ninety_degrees(v.get_instance_rotation()));
+                    v.set_instance_scaling_factor((v.get_instance_transformation().get_matrix(true, false, true, true).matrix().block<3, 3>(0, 0).transpose() * scale).cwiseAbs());
                 }
                 else
-                    volume.set_instance_scaling_factor(scale);
+                    v.set_instance_scaling_factor(scale);
             }
         }
         else if (is_single_volume() || is_single_modifier())
-            volume.set_volume_scaling_factor(scale);
+            v.set_volume_scaling_factor(scale);
         else {
             Transform3d m = Geometry::assemble_transform(Vec3d::Zero(), Vec3d::Zero(), scale);
             if (m_mode == Instance) {
@@ -878,9 +878,9 @@ void Selection::scale(const Vec3d& scale, TransformationType transformation_type
                 // extracts scaling factors from the composed transformation
                 Vec3d new_scale(new_matrix.col(0).norm(), new_matrix.col(1).norm(), new_matrix.col(2).norm());
                 if (transformation_type.joint())
-                    volume.set_instance_offset(m_cache.dragging_center + m * (m_cache.volumes_data[i].get_instance_position() - m_cache.dragging_center));
+                    v.set_instance_offset(m_cache.dragging_center + m * (m_cache.volumes_data[i].get_instance_position() - m_cache.dragging_center));
 
-                volume.set_instance_scaling_factor(new_scale);
+                v.set_instance_scaling_factor(new_scale);
             }
             else if (m_mode == Volume) {
                 Eigen::Matrix<double, 3, 3, Eigen::DontAlign> new_matrix = (m * m_cache.volumes_data[i].get_volume_scale_matrix()).matrix().block(0, 0, 3, 3);
@@ -888,9 +888,9 @@ void Selection::scale(const Vec3d& scale, TransformationType transformation_type
                 Vec3d new_scale(new_matrix.col(0).norm(), new_matrix.col(1).norm(), new_matrix.col(2).norm());
                 if (transformation_type.joint()) {
                     Vec3d offset = m * (m_cache.volumes_data[i].get_volume_position() + m_cache.volumes_data[i].get_instance_position() - m_cache.dragging_center);
-                    volume.set_volume_offset(m_cache.dragging_center - m_cache.volumes_data[i].get_instance_position() + offset);
+                    v.set_volume_offset(m_cache.dragging_center - m_cache.volumes_data[i].get_instance_position() + offset);
                 }
-                volume.set_volume_scaling_factor(new_scale);
+                v.set_volume_scaling_factor(new_scale);
             }
         }
     }