Hide Infill in the Shell Drag Mode Only Where the Profile Covers It

A profile with no top or bottom shell layers, or no walls, leaves the infill on the surface,
so hiding it changed the picture. The viewer now reads the print's default region settings
when it loads a print and leaves the infill in place for such a print; per-object overrides
are not consulted.

docs/HLSD/gcode-preview-dragging.md

@@ -0,0 +1,80 @@
+# G-code preview while dragging
+
+The sliced preview draws every toolpath segment of the plate as an instanced box. On a large
+plate that is tens of millions of segments, and the frame is GPU-bound: the cost is the number of
+instances drawn, not anything the CPU does per frame. Dragging the camera over such a plate cannot
+keep up. The `preview_reduced_detail_mode` preference (*Graphics > G-code Preview*, off by
+default) lets the preview draw less while the user drags and put the full toolpaths back when they
+let go.
+
+| Preference | Values | Effect |
+|---|---|---|
+| `preview_reduced_detail_mode` | `off`, `solid`, `shell` | what is drawn while dragging |
+
+libvgcode (`src/libvgcode`) builds and binds the reduced toolpath set, `GCodeViewer` maps the
+preferences onto it and draws the solid model, and `GLCanvas3D` decides when the user is dragging.
+The OpenGL ES path keeps a single set and ignores the preference.
+
+## Two sets, one walk
+
+`ViewerImpl::update_enabled_entities()` walks the visible vertex range once and fills two segment
+index buffers side by side: the **full** set and the **reduced** set (segments and options).
+Building them together is what makes switching free: starting or ending a drag is a buffer
+binding, never a rebuild. A change of mode does rebuild. Nothing is built while the mode
+is off, and the reduced buffers are then uploaded empty so that the last set does not stay
+allocated.
+
+Whatever the mode leaves out, the bottom and top layers of the visible range are kept whole: they
+are the faces the range cuts open, and the top is what the user is looking at.
+
+### Modes
+
+- `EndLayersOnly` (`solid` in the preference) keeps only the two end layers. `GCodeViewer` then
+  draws the sliced objects and the prime tower as opaque solids, see below.
+- `ShellOnly` (`shell`) keeps every role but the sparse infill, internal solid infill, internal
+  bridge infill and gap fill, which lie under the skins or between the walls. Walls, top and
+  bottom surfaces, bridges, supports and the prime tower are drawn whole, so from outside the
+  print looks as it does at rest. The roles come from
+  the slicer, so the classification is exact wherever the slicer's is; what it cannot express is
+  the inside of the prime tower and of a support, which share one role with their outside and are
+  kept whole. A profile with no top or bottom shell layers, or no walls, leaves the infill on the
+  surface, so `GCodeViewer` turns the hiding off for such a print when it loads it, from the
+  print's default region settings; per-object overrides are not consulted.
+
+## The solid model
+
+The preview already loads the sliced objects as shells for its translucent ghost.
+`GCodeViewer::render_solid_model()` draws those shells opaque, in their filament colors, with the
+`gouraud` shader, whose z range cuts them to the visible layer range. The two toolpath layers of
+the reduced set are drawn afterwards and cap the cut with what was really printed there. The
+shells hold only the objects, so while this mode is on the prime tower is added from its sliced
+mesh, positioned as the print placed it. It is added or removed on its own when the mode changes,
+without reloading the objects, keeps its opaque color so that it never appears among the
+translucent shells, and stays out of their bounding box. Supports have no mesh and are not shown,
+and `load_shells()` drops every non-model-part volume, so a negative volume is not cut out.
+
+A plate whose shells are not loaded keeps drawing toolpaths, since the solid model would leave
+only the end layers.
+
+## Deciding that the user is dragging
+
+`GLCanvas3D::_update_preview_interaction()` runs at the top of every preview frame, before the
+canvas decides whether to reuse its cached scene, so that the switch lands in that frame. Dragging
+is `GLCanvas3D::is_user_interacting()`, the same answer the scene cache reads: the camera, the
+navigator, a gizmo, the rectangle selection or either slider being held. A slider reports this from
+ImGui's active id rather than its dirty flag, which is raised and consumed inside one frame. A
+wheel step has no duration, so it holds the reduced set for a 150 ms settle time instead, and the
+frame that restores the toolpaths is scheduled for when that time runs out, since the render timer
+only wakes the idle loop. A drag cut short by focus or capture loss is ended explicitly, and a
+button release wakes the idle loop, because on some platforms nothing else would until the next
+input.
+
+## Reused scene frames
+
+`GLCanvas3D` keeps its last scene pass for frames that only rebuild the overlay (`SceneCache`). Its
+key covers the canvas size, the camera and hover state, not what the toolpath sets draw, so a frame
+that reuses the scene must never be one on which the set is switched.
+`_update_preview_interaction()` therefore reports whether the bound set changed, and a frame on
+which it did redraws the scene. The canvas neither captures nor reuses the scene while the user
+drags, so no reduced frame outlives a drag, and the frame that ends a wheel's settle time is
+requested as a full frame.

src/libslic3r/AppConfig.cpp

@@ -205,6 +205,13 @@ void AppConfig::set_defaults()
     if (get("seq_top_layer_only").empty())
         set("seq_top_layer_only", "1");
 
+    // what the preview draws while the user drags it
+    {
+        const std::string mode = get("preview_reduced_detail_mode");
+        if (mode != "off" && mode != "solid" && mode != "shell")
+            set("preview_reduced_detail_mode", "off");
+    }
+
     // ORCA: darken the layers the preview layer slider is not scrubbed to
     if (get("preview_dim_previous_layers").empty())
         set_bool("preview_dim_previous_layers", false);

src/libvgcode/include/Types.hpp

@@ -158,6 +158,21 @@ enum class EGCodeExtrusionRole : uint8_t
 
 static constexpr std::size_t GCODE_EXTRUSION_ROLES_COUNT = static_cast<std::size_t>(EGCodeExtrusionRole::COUNT);
 
+//
+// What the reduced set, drawn while the user is dragging, holds in place of the full toolpaths
+//
+enum class EReducedDetailMode : uint8_t
+{
+    // nothing: no reduced set is built
+    Off,
+    // only the bottom and top layers of the visible range, for a caller that draws the print
+    // itself some other way
+    EndLayersOnly,
+    // every role but the infill under the skins and the gap fill: what can be seen from outside
+    ShellOnly,
+    COUNT
+};
+
 //
 // Option types
 //

src/libvgcode/include/Viewer.hpp

@@ -114,6 +114,19 @@ public:
     //
     bool is_dim_previous_layers() const;
     void set_dim_previous_layers(bool value);
+    //
+    // The reduced set drawn while the user drags: what the mode keeps, and always the bottom and
+    // top layers of the visible range. While a mode is set it is built alongside the full set, so
+    // set_reduced_detail() rebuilds nothing. Ignored on the OpenGL ES path.
+    //
+    EReducedDetailMode get_reduced_detail_mode() const;
+    void set_reduced_detail_mode(EReducedDetailMode mode);
+    // Whether the shell mode hides the infill roles; off for a profile that leaves them on the
+    // surface, with no top or bottom shell or no walls.
+    bool get_reduced_detail_hide_infill() const;
+    void set_reduced_detail_hide_infill(bool value);
+    void set_reduced_detail(bool value);
+    bool is_reduced_detail() const;
     float get_dim_previous_layers_brightness() const;
     void set_dim_previous_layers_brightness(float value);
     //

src/libvgcode/src/Settings.hpp

@@ -25,6 +25,11 @@ struct Settings
 		// ORCA: how bright those darkened layers are rendered, 1.0 = unchanged, 0.0 = black
 		float dim_previous_layers_brightness{ 0.4f };
 		bool spiral_vase_mode{ false };
+		// what the reduced set holds and whether it is drawn. Ignored on the OpenGL ES path.
+		EReducedDetailMode reduced_detail_mode{ EReducedDetailMode::Off };
+		// whether the shell mode may hide the infill roles: false when the profile leaves them on the surface
+		bool reduced_detail_hide_infill{ true };
+		bool reduced_detail{ false };
 		//
 		// Required update flags
 		//

src/libvgcode/src/Shaders.hpp

@@ -34,6 +34,10 @@ static const char* Segments_Vertex_Shader =
 // ORCA: 0 during the shadow caster pass - the bias below shifts eye_position but not
 // world_position, so the caster would write a depth the receiver never looks up.
 "uniform float bias_scale;\n"
+// draw the instances last to first, top layers before the ones they hide, so that early depth
+// rejection discards most of the hidden fragments; set when the camera looks down on the print
+"uniform int reverse_order;\n"
+"uniform int instance_count;\n"
 "in int vertex_id;\n"
 "out vec3 color;\n"
 "// ORCA: realistic view - the light the shadow map is able to block, kept apart from the\n"
@@ -59,7 +63,8 @@ static const char* Segments_Vertex_Shader =
 "  return top_diffuse + front_diffuse + top_specular;\n"
 "}\n"
 "void main() {\n"
-"  int id_a = int(texelFetch(segment_index_tex, gl_InstanceID).r);\n"
+"  int instance = (reverse_order != 0) ? instance_count - 1 - gl_InstanceID : gl_InstanceID;\n"
+"  int id_a = int(texelFetch(segment_index_tex, instance).r);\n"
 "  int id_b = id_a + 1;\n"
 "  vec3 pos_a = texelFetch(position_tex, id_a).xyz;\n"
 "  vec3 pos_b = texelFetch(position_tex, id_b).xyz;\n"

src/libvgcode/src/Viewer.cpp

@@ -92,6 +92,36 @@ bool Viewer::is_dim_previous_layers() const
     return m_impl->is_dim_previous_layers();
 }
 
+void Viewer::set_reduced_detail(bool value)
+{
+    m_impl->set_reduced_detail(value);
+}
+
+bool Viewer::is_reduced_detail() const
+{
+    return m_impl->is_reduced_detail();
+}
+
+EReducedDetailMode Viewer::get_reduced_detail_mode() const
+{
+    return m_impl->get_reduced_detail_mode();
+}
+
+void Viewer::set_reduced_detail_mode(EReducedDetailMode mode)
+{
+    m_impl->set_reduced_detail_mode(mode);
+}
+
+bool Viewer::get_reduced_detail_hide_infill() const
+{
+    return m_impl->get_reduced_detail_hide_infill();
+}
+
+void Viewer::set_reduced_detail_hide_infill(bool value)
+{
+    m_impl->set_reduced_detail_hide_infill(value);
+}
+
 void Viewer::set_dim_previous_layers(bool value)
 {
     m_impl->set_dim_previous_layers(value);

src/libvgcode/src/ViewerImpl.cpp

@@ -17,6 +17,7 @@
 #include <algorithm>
 #include <cmath>
 #include <numeric>
+#include <cfloat>
 
 namespace libvgcode {
 
@@ -763,6 +764,8 @@ void ViewerImpl::init(const std::string& opengl_context_version)
     m_uni_segments_height_width_angle_tex_id = glGetUniformLocation(m_segments_shader_id, "height_width_angle_tex");
     m_uni_segments_colors_tex_id             = glGetUniformLocation(m_segments_shader_id, "color_tex");
     m_uni_segments_segment_index_tex_id      = glGetUniformLocation(m_segments_shader_id, "segment_index_tex");
+    m_uni_segments_reverse_order_id          = glGetUniformLocation(m_segments_shader_id, "reverse_order");
+    m_uni_segments_instance_count_id         = glGetUniformLocation(m_segments_shader_id, "instance_count");
     // ORCA: realistic view
     m_uni_segments_shadow_map_id             = glGetUniformLocation(m_segments_shader_id, "shadow_map");
     m_uni_segments_shadow_light_vp_id        = glGetUniformLocation(m_segments_shader_id, "shadow_light_vp");
@@ -899,9 +902,17 @@ void ViewerImpl::reset()
 #else
     m_enabled_segments_count = 0;
     m_enabled_options_count = 0;
+    m_enabled_segments_reduced_count = 0;
+    m_enabled_options_reduced_count = 0;
+    m_enabled_segments_reduced_tex_size = 0;
+    m_enabled_options_reduced_tex_size = 0;
 
     m_settings_used_for_ranges = std::nullopt;
 
+    delete_textures(m_enabled_options_reduced_tex_id);
+    delete_buffers(m_enabled_options_reduced_buf_id);
+    delete_textures(m_enabled_segments_reduced_tex_id);
+    delete_buffers(m_enabled_segments_reduced_buf_id);
     delete_textures(m_enabled_options_tex_id);
     delete_buffers(m_enabled_options_buf_id);
     delete_textures(m_enabled_segments_tex_id);
@@ -1161,6 +1172,17 @@ void ViewerImpl::load(GCodeInputData&& gcode_data)
         glsafe(glGenTextures(1, &m_enabled_options_tex_id));
         glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_tex_id));
 
+        // create (but do not fill) the reduced counterparts of the two buffers above
+        glsafe(glGenBuffers(1, &m_enabled_segments_reduced_buf_id));
+        glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_buf_id));
+        glsafe(glGenTextures(1, &m_enabled_segments_reduced_tex_id));
+        glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_tex_id));
+
+        glsafe(glGenBuffers(1, &m_enabled_options_reduced_buf_id));
+        glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_options_reduced_buf_id));
+        glsafe(glGenTextures(1, &m_enabled_options_reduced_tex_id));
+        glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_reduced_tex_id));
+
         glsafe(glBindBuffer(GL_TEXTURE_BUFFER, 0));
         glsafe(glBindTexture(GL_TEXTURE_BUFFER, old_bound_texture));
 #endif // ENABLE_OPENGL_ES
@@ -1172,6 +1194,18 @@ void ViewerImpl::load(GCodeInputData&& gcode_data)
     update_colors();
 }
 
+#ifndef ENABLE_OPENGL_ES
+// the roles that lie under a skin or between walls, never seen from outside the print
+static bool is_hidden_in_shell(EGCodeExtrusionRole role)
+{
+    return role == EGCodeExtrusionRole::InternalInfill ||
+           role == EGCodeExtrusionRole::SolidInfill ||
+           role == EGCodeExtrusionRole::InternalBridgeInfill ||
+           role == EGCodeExtrusionRole::GapFill;
+}
+#endif // ENABLE_OPENGL_ES
+
+
 void ViewerImpl::update_enabled_entities()
 {
     if (m_vertices.empty())
@@ -1179,6 +1213,17 @@ void ViewerImpl::update_enabled_entities()
 
     std::vector<uint32_t> enabled_segments;
     std::vector<uint32_t> enabled_options;
+#ifndef ENABLE_OPENGL_ES
+    // the reduced set is filled by the same walk, so switching to it costs no rebuild. Whatever the
+    // mode leaves out, the bottom and top layers of the visible range are kept whole: they are the
+    // surfaces the range cuts open
+    const EReducedDetailMode reduced_mode = m_settings.reduced_detail_mode;
+    const bool build_reduced = reduced_mode != EReducedDetailMode::Off;
+    const bool hide_infill = m_settings.reduced_detail_hide_infill;
+    std::vector<uint32_t> enabled_segments_reduced;
+    std::vector<uint32_t> enabled_options_reduced;
+    const Interval& layers_range = m_layers.get_view_range();
+#endif // ENABLE_OPENGL_ES
     Interval range = m_view_range.get_visible();
 
     // when top layer only visualization is enabled, we need to render
@@ -1226,6 +1271,20 @@ void ViewerImpl::update_enabled_entities()
             enabled_options.push_back(static_cast<uint32_t>(i));
         else
             enabled_segments.push_back(static_cast<uint32_t>(i));
+
+#ifndef ENABLE_OPENGL_ES
+        if (build_reduced) {
+            const bool end_layer = v.layer_id == layers_range[0] || v.layer_id == layers_range[1];
+            if (end_layer)
+                (v.is_option() ? enabled_options_reduced : enabled_segments_reduced).push_back(static_cast<uint32_t>(i));
+            else if (reduced_mode == EReducedDetailMode::ShellOnly) {
+                if (v.is_option())
+                    enabled_options_reduced.push_back(static_cast<uint32_t>(i));
+                else if (!v.is_extrusion() || !hide_infill || !is_hidden_in_shell(v.role))
+                    enabled_segments_reduced.push_back(static_cast<uint32_t>(i));
+            }
+        }
+#endif // ENABLE_OPENGL_ES
     }
 
 #ifdef ENABLE_OPENGL_ES
@@ -1254,6 +1313,23 @@ void ViewerImpl::update_enabled_entities()
     else
         glsafe(glBufferData(GL_TEXTURE_BUFFER, 0, nullptr, GL_STATIC_DRAW));
 
+    m_enabled_segments_reduced_count = enabled_segments_reduced.size();
+    m_enabled_options_reduced_count = enabled_options_reduced.size();
+    m_enabled_segments_reduced_tex_size = enabled_segments_reduced.size() * sizeof(uint32_t);
+    m_enabled_options_reduced_tex_size = enabled_options_reduced.size() * sizeof(uint32_t);
+
+    // uploaded even when nothing was built, so that the last reduced set is released as soon as
+    // the preference is switched off
+    assert(m_enabled_segments_reduced_buf_id > 0);
+    glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_buf_id));
+    glsafe(glBufferData(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_tex_size,
+                        enabled_segments_reduced.empty() ? nullptr : enabled_segments_reduced.data(), GL_STATIC_DRAW));
+
+    assert(m_enabled_options_reduced_buf_id > 0);
+    glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_options_reduced_buf_id));
+    glsafe(glBufferData(GL_TEXTURE_BUFFER, m_enabled_options_reduced_tex_size,
+                        enabled_options_reduced.empty() ? nullptr : enabled_options_reduced.data(), GL_STATIC_DRAW));
+
     glsafe(glBindBuffer(GL_TEXTURE_BUFFER, 0));
 #endif // ENABLE_OPENGL_ES
 
@@ -1461,6 +1537,23 @@ void ViewerImpl::toggle_top_layer_only_view_range()
     update_colors_texture();
 }
 
+// Both decide which vertices land in the reduced set, so the sets are rebuilt.
+void ViewerImpl::set_reduced_detail_mode(EReducedDetailMode mode)
+{
+    if (m_settings.reduced_detail_mode == mode)
+        return;
+    m_settings.reduced_detail_mode = mode;
+    m_settings.update_enabled_entities = true;
+}
+
+void ViewerImpl::set_reduced_detail_hide_infill(bool value)
+{
+    if (m_settings.reduced_detail_hide_infill == value)
+        return;
+    m_settings.reduced_detail_hide_infill = value;
+    m_settings.update_enabled_entities = true;
+}
+
 // ORCA: enable/disable darkening of the layers the layer slider is not scrubbed to
 void ViewerImpl::set_dim_previous_layers(bool value)
 {
@@ -1854,6 +1947,8 @@ size_t ViewerImpl::get_used_gpu_memory() const
     ret += m_colors_tex_size;
     ret += m_enabled_segments_tex_size;
     ret += m_enabled_options_tex_size;
+    ret += m_enabled_segments_reduced_tex_size;
+    ret += m_enabled_options_reduced_tex_size;
 #endif // ENABLE_OPENGL_ES
     return ret;
 }
@@ -2070,7 +2165,8 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
 #ifdef ENABLE_OPENGL_ES
     if (m_texture_data.get_enabled_segments_count() == 0)
 #else
-    if (m_enabled_segments_count == 0)
+    const ActiveSet segments = active_segments();
+    if (segments.count == 0)
 #endif // ENABLE_OPENGL_ES
         return;
 
@@ -2090,6 +2186,15 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
     glsafe(glUniformMatrix4fv(m_uni_segments_view_matrix_id, 1, GL_FALSE, view_matrix.data()));
     glsafe(glUniformMatrix4fv(m_uni_segments_projection_matrix_id, 1, GL_FALSE, projection_matrix.data()));
     glsafe(glUniform3fv(m_uni_segments_camera_position_id, 1, camera_position.data()));
+    // The segments come in print order, bottom layer first. Seen from above, that is back to front,
+    // and every hidden fragment is shaded before the one that covers it. Drawing them last to first
+    // lets the depth test reject the hidden ones instead. The camera looks down when the world's
+    // up axis points towards it, which is the view matrix's (2, 2) entry being positive.
+    const bool top_down = !m_rendering_shadow_casters && view_matrix[10] > 0.0f;
+    glsafe(glUniform1i(m_uni_segments_reverse_order_id, top_down ? 1 : 0));
+#ifndef ENABLE_OPENGL_ES
+    glsafe(glUniform1i(m_uni_segments_instance_count_id, static_cast<int>(segments.count)));
+#endif // ENABLE_OPENGL_ES
     // ORCA: realistic view. The depth pass writes the map it would otherwise read, so it shades
     // with the lookup off.
     glsafe(glUniform1i(m_uni_segments_shadow_map_id, m_shadow_map_texture_unit));
@@ -2138,10 +2243,10 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
     glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_colors_tex_id));
     glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32F, m_colors_buf_id));
     glsafe(glActiveTexture(GL_TEXTURE3));
-    glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_segments_tex_id));
+    glsafe(glBindTexture(GL_TEXTURE_BUFFER, segments.tex_id));
-    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, m_enabled_segments_buf_id));
+    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, segments.buf_id));
 
-    m_segment_template.render(m_enabled_segments_count);
+    m_segment_template.render(segments.count);
 #endif // ENABLE_OPENGL_ES
 
     if (curr_cull_face)
@@ -2167,7 +2272,8 @@ void ViewerImpl::render_options(const Mat4x4& view_matrix, const Mat4x4& project
 #ifdef ENABLE_OPENGL_ES
     if (m_texture_data.get_enabled_options_count() == 0)
 #else
-    if (m_enabled_options_count == 0)
+    const ActiveSet options = active_options();
+    if (options.count == 0)
 #endif // ENABLE_OPENGL_ES
         return;
 
@@ -2225,10 +2331,10 @@ void ViewerImpl::render_options(const Mat4x4& view_matrix, const Mat4x4& project
     glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_colors_tex_id));
     glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32F, m_colors_buf_id));
     glsafe(glActiveTexture(GL_TEXTURE3));
-    glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_tex_id));
+    glsafe(glBindTexture(GL_TEXTURE_BUFFER, options.tex_id));
-    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, m_enabled_options_buf_id));
+    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, options.buf_id));
 
-    m_option_template.render(m_enabled_options_count);
+    m_option_template.render(options.count);
 #endif // ENABLE_OPENGL_ES
 
     if (!curr_cull_face)

src/libvgcode/src/ViewerImpl.hpp

@@ -109,6 +109,21 @@ public:
     // 0.0 = black
     bool is_dim_previous_layers() const { return m_settings.dim_previous_layers; }
     void set_dim_previous_layers(bool value);
+    //
+    // Draw from the reduced set; it is already built, so this is just a buffer binding.
+    //
+    void set_reduced_detail(bool value) {
+#ifdef ENABLE_OPENGL_ES
+        // no reduced set is built on OpenGL ES
+        value = false;
+#endif // ENABLE_OPENGL_ES
+        m_settings.reduced_detail = value;
+    }
+    bool is_reduced_detail() const { return m_settings.reduced_detail; }
+    EReducedDetailMode get_reduced_detail_mode() const { return m_settings.reduced_detail_mode; }
+    void set_reduced_detail_mode(EReducedDetailMode mode);
+    bool get_reduced_detail_hide_infill() const { return m_settings.reduced_detail_hide_infill; }
+    void set_reduced_detail_hide_infill(bool value);
     float get_dim_previous_layers_brightness() const { return m_settings.dim_previous_layers_brightness; }
     void set_dim_previous_layers_brightness(float value);
 
@@ -362,6 +377,8 @@ private:
     int m_uni_segments_height_width_angle_tex_id{ -1 };
     int m_uni_segments_colors_tex_id{ -1 };
     int m_uni_segments_segment_index_tex_id{ -1 };
+    int m_uni_segments_reverse_order_id{ -1 };
+    int m_uni_segments_instance_count_id{ -1 };
     int m_uni_segments_shadow_map_id{ -1 };
     int m_uni_segments_shadow_light_vp_id{ -1 };
     int m_uni_segments_shadow_intensity_id{ -1 };
@@ -499,6 +516,15 @@ private:
     unsigned int m_enabled_options_tex_id{ 0 };
     size_t m_enabled_options_count{ 0 };
     //
+    // OpenGL buffers to store the reduced set drawn while Settings::reduced_detail is set
+    //
+    unsigned int m_enabled_segments_reduced_buf_id{ 0 };
+    unsigned int m_enabled_segments_reduced_tex_id{ 0 };
+    size_t m_enabled_segments_reduced_count{ 0 };
+    unsigned int m_enabled_options_reduced_buf_id{ 0 };
+    unsigned int m_enabled_options_reduced_tex_id{ 0 };
+    size_t m_enabled_options_reduced_count{ 0 };
+    //
     // Caches for size of data sent to gpu, in bytes
     //
     size_t m_positions_tex_size{ 0 };
@@ -506,6 +532,27 @@ private:
     size_t m_colors_tex_size{ 0 };
     size_t m_enabled_segments_tex_size{ 0 };
     size_t m_enabled_options_tex_size{ 0 };
+    size_t m_enabled_segments_reduced_tex_size{ 0 };
+    size_t m_enabled_options_reduced_tex_size{ 0 };
+
+    // The set the next draw reads from: the reduced one while dragging, if one is built.
+    bool use_reduced_set() const { return m_settings.reduced_detail && m_settings.reduced_detail_mode != EReducedDetailMode::Off; }
+    struct ActiveSet
+    {
+        size_t count{ 0 };
+        unsigned int buf_id{ 0 };
+        unsigned int tex_id{ 0 };
+    };
+    ActiveSet active_segments() const {
+        if (use_reduced_set())
+            return { m_enabled_segments_reduced_count, m_enabled_segments_reduced_buf_id, m_enabled_segments_reduced_tex_id };
+        return { m_enabled_segments_count, m_enabled_segments_buf_id, m_enabled_segments_tex_id };
+    }
+    ActiveSet active_options() const {
+        if (use_reduced_set())
+            return { m_enabled_options_reduced_count, m_enabled_options_reduced_buf_id, m_enabled_options_reduced_tex_id };
+        return { m_enabled_options_count, m_enabled_options_buf_id, m_enabled_options_tex_id };
+    }
 #endif // ENABLE_OPENGL_ES
 
     //

src/slic3r/GUI/GCodeViewer.cpp

@@ -1267,6 +1267,8 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
     if (current_top_layer_only != required_top_layer_only)
         m_viewer.toggle_top_layer_only_view_range();
 
+    read_reduced_detail_preferences();
+
     // ORCA: darken the layers the preview layer slider is not scrubbed to
     m_viewer.set_dim_previous_layers(get_app_config()->get_bool("preview_dim_previous_layers"));
     m_viewer.set_dim_previous_layers_brightness(0.01f * std::stoi(get_app_config()->get("preview_dim_previous_layers_brightness")));
@@ -1490,6 +1492,10 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
 
     // BBS: data for rendering color arrangement recommendation
     m_nozzle_nums = print.config().option<ConfigOptionFloats>("nozzle_diameter")->values.size();
+    // the shell drag mode hides the infill only where the profile covers it: with no top or bottom
+    // shell, or no walls, the infill is the surface. Per-object settings are not consulted.
+    const PrintRegionConfig& region = print.default_region_config();
+    m_viewer.set_reduced_detail_hide_infill(region.top_shell_layers.value > 0 && region.bottom_shell_layers.value > 0 && region.wall_loops.value > 0);
     // Orca hack: Hide filament group for non-bbl printers
     if (!print.is_BBL_printer()) m_nozzle_nums = 1;
     std::vector<int>         filament_maps = print.get_filament_maps();
@@ -1660,6 +1666,7 @@ void GCodeViewer::reset_shell()
 {
     m_shells.volumes.clear();
     m_shells.print_id = -1;
+    m_shells.with_wipe_tower = false;
     m_shell_bounding_box = BoundingBoxf3();
 }
 
@@ -1696,6 +1703,11 @@ void GCodeViewer::reset()
 void GCodeViewer::render_scene(int canvas_width, int canvas_height)
 {
     glsafe(::glEnable(GL_DEPTH_TEST));
+    // while dragging in the solid model mode, the objects stand in for their toolpaths, cut to the
+    // visible layer range; the toolpath set then holds only the range's bottom and top layers
+    if (m_viewer.is_reduced_detail() && solid_model_enabled())
+        render_solid_model(canvas_width, canvas_height);
+    else
         render_shells(canvas_width, canvas_height);
 
     if (m_viewer.get_extrusion_roles_count() == 0)
@@ -2019,6 +2031,56 @@ void GCodeViewer::update_layers_slider_mode()
     // TODO m_layers_slider->SetModeAndOnlyExtruder(one_extruder_printed_model, only_extruder);
 }
 
+void GCodeViewer::set_interacting(bool interacting)
+{
+    // with no shells to stand in for the toolpaths, the solid model would leave only the end layers
+    const bool usable = !solid_model_enabled() || !m_shells.volumes.empty();
+    m_viewer.set_reduced_detail(interacting && usable);
+}
+
+void GCodeViewer::read_reduced_detail_preferences()
+{
+    m_reduced_detail_mode = reduced_detail_mode_from_string(get_app_config()->get("preview_reduced_detail_mode"));
+    apply_reduced_detail_settings();
+}
+
+void GCodeViewer::apply_reduced_detail_settings()
+{
+    m_viewer.set_reduced_detail_mode(m_reduced_detail_mode);
+}
+
+void GCodeViewer::set_reduced_detail_mode(const std::string& mode)
+{
+    const bool was_solid = solid_model_enabled();
+    m_reduced_detail_mode = reduced_detail_mode_from_string(mode);
+    apply_reduced_detail_settings();
+    reload_shells_if_solid_model_changed(was_solid);
+}
+
+libvgcode::EReducedDetailMode GCodeViewer::reduced_detail_mode_from_string(const std::string& mode)
+{
+    if (mode == "solid")
+        return libvgcode::EReducedDetailMode::EndLayersOnly;
+    if (mode == "shell")
+        return libvgcode::EReducedDetailMode::ShellOnly;
+    return libvgcode::EReducedDetailMode::Off;
+}
+
+void GCodeViewer::reload_shells_if_solid_model_changed(bool was_enabled)
+{
+    if (was_enabled == solid_model_enabled() || m_shells.print_id == -1)
+        return;
+    // only the prime tower comes and goes with the mode: a full reload would drop the shells
+    // whenever the print has moved on since they were loaded, leaving the solid model nothing to draw
+    if (wxGetApp().plater() == nullptr)
+        return;
+    // the shells are loaded from the current plate's print, which is not the plater's own
+    const Print& print = wxGetApp().plater()->get_partplate_list().get_current_fff_print();
+    if (static_cast<int>(print.id().id) != m_shells.print_id)
+        return;
+    update_shell_wipe_tower(print, m_gl_data_initialized);
+}
+
 void GCodeViewer::set_layers_z_range(const std::array<unsigned int, 2>& layers_z_range)
 {
     m_viewer.set_layers_view_range(static_cast<uint32_t>(layers_z_range[0]), static_cast<uint32_t>(layers_z_range[1]));
@@ -2343,7 +2405,11 @@ void GCodeViewer::export_toolpaths_to_obj(const char* filename) const
 void GCodeViewer::load_shells(const Print& print, bool initialized, bool force_previewing)
 {
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << boost::format(": initialized=%1%, force_previewing=%2%")%initialized %force_previewing;
+    // the shells can load before the first G-code does, so the preferences are read here as well
+    read_reduced_detail_preferences();
     if ((print.id().id == m_shells.print_id)&&(print.get_modified_count() == m_shells.print_modify_count)) {
+        // the prime tower comes and goes on its own, without reloading the objects
+        update_shell_wipe_tower(print, initialized);
         //BBS: update force previewing logic
         if (force_previewing)
             m_shells.previewing = force_previewing;
@@ -2452,10 +2518,45 @@ void GCodeViewer::load_shells(const Print& print, bool initialized, bool force_p
     m_shells.print_id = print.id().id;
     m_shells.print_modify_count = print.get_modified_count();
     m_shells.previewing = true;
+    update_shell_wipe_tower(print, initialized);
     BOOST_LOG_TRIVIAL(debug) << __FUNCTION__ << boost::format(": shell loaded, id change to %1%, modify_count %2%, object count %3%, glvolume count %4%")
         % m_shells.print_id % m_shells.print_modify_count % object_count %m_shells.volumes.volumes.size();
 }
 
+// The prime tower as it was sliced, so that the solid model shows what the print shows. It keeps its
+// opaque colour, so it never appears among the translucent shells, and stays out of their bounding box.
+void GCodeViewer::update_shell_wipe_tower(const Print& print, bool initialized)
+{
+    const bool with_wipe_tower = solid_model_enabled() && print.is_step_done(psWipeTower) && print.wipe_tower_data().wipe_tower_mesh_data;
+    if (with_wipe_tower == m_shells.with_wipe_tower)
+        return;
+    m_shells.with_wipe_tower = with_wipe_tower;
+    GLVolumePtrs& volumes = m_shells.volumes.volumes;
+    if (!with_wipe_tower) {
+        volumes.erase(std::remove_if(volumes.begin(), volumes.end(), [](GLVolume* volume) {
+            if (!volume->is_wipe_tower)
+                return false;
+            delete volume;
+            return true;
+        }), volumes.end());
+        return;
+    }
+    const PrintConfig& config = print.config();
+    const int plate_idx = print.get_plate_index();
+    const Vec3d plate_origin = print.get_plate_origin();
+    const float x = static_cast<float>(config.wipe_tower_x.get_at(plate_idx) + plate_origin.x());
+    const float y = static_cast<float>(config.wipe_tower_y.get_at(plate_idx) + plate_origin.y());
+    const size_t first_new = volumes.size();
+    m_shells.volumes.load_real_wipe_tower_preview(1000 + plate_idx, x, y, print.wipe_tower_data().wipe_tower_mesh_data->real_wipe_tower_mesh,
+                                                  print.wipe_tower_data().wipe_tower_mesh_data->real_brim_mesh, true,
+                                                  static_cast<float>(config.wipe_tower_rotation_angle), false, initialized);
+    for (size_t i = first_new; i < volumes.size(); ++i) {
+        volumes[i]->zoom_to_volumes = false;
+        volumes[i]->force_native_color = true;
+        volumes[i]->set_render_color();
+    }
+}
+
 void GCodeViewer::render_toolpaths()
 {
     const Camera& camera = wxGetApp().plater()->get_camera();
@@ -2656,6 +2757,50 @@ void GCodeViewer::render_shells(int canvas_width, int canvas_height)
     glsafe(::glDepthMask(GL_TRUE));
 }
 
+// The sliced objects and the prime tower drawn opaque, in their filament colours, cut to the
+// visible layer range by the shader's z range. The toolpaths of the range's bottom and top layers
+// are drawn afterwards and cap the cut.
+void GCodeViewer::render_solid_model(int canvas_width, int canvas_height)
+{
+    if (m_shells.volumes.empty())
+        return;
+    // gouraud_light has no z range, so it could not cut the model
+    GLShaderProgram* shader = wxGetApp().get_shader("gouraud");
+    if (shader == nullptr)
+        return;
+
+    const libvgcode::Interval& layers = m_viewer.get_layers_view_range();
+    const float z_top = m_viewer.get_layer_z(layers[1]) - m_z_offset + 0.001f;
+    const float z_bottom = (layers[0] > 0) ? m_viewer.get_layer_z(layers[0] - 1) - m_z_offset - 0.001f : -FLT_MAX;
+
+    std::vector<float> alphas;
+    alphas.reserve(m_shells.volumes.volumes.size());
+    for (GLVolume* volume : m_shells.volumes.volumes) {
+        alphas.push_back(volume->color.a());
+        volume->color.a(1.0f);
+        volume->set_render_color();
+    }
+    m_shells.volumes.set_z_range(z_bottom, z_top);
+    // gouraud also clips by this plane, which nothing else sets on the shells
+    m_shells.volumes.set_clipping_plane(ClippingPlane::ClipsNothing().get_data());
+
+    shader->start_using();
+    // the 3D view leaves its shadow settings on the shared program
+    shader->set_uniform("shadow_intensity", 0.0f);
+    const Camera& camera = wxGetApp().plater()->get_camera();
+    shader->set_uniform("z_far", camera.get_far_z());
+    shader->set_uniform("z_near", camera.get_near_z());
+    m_shells.volumes.render(GLVolumeCollection::ERenderType::Opaque, false, camera.get_view_matrix(), camera.get_projection_matrix(), {canvas_width, canvas_height});
+    shader->stop_using();
+
+    m_shells.volumes.set_z_range(-FLT_MAX, FLT_MAX);
+    size_t k = 0;
+    for (GLVolume* volume : m_shells.volumes.volumes) {
+        volume->color.a(alphas[k++]);
+        volume->set_render_color();
+    }
+}
+
 //BBS
 void GCodeViewer::render_all_plates_stats(const std::vector<const GCodeProcessorResult*>& gcode_result_list, bool show /*= true*/) const {
     if (!show)

src/slic3r/GUI/GCodeViewer.hpp

@@ -174,6 +174,8 @@ public:
         int  print_id{-1};
         int  print_modify_count{-1};
         bool previewing{false};
+        // the prime tower was loaded with the objects, for the solid model
+        bool with_wipe_tower{false};
     };
     //BBS
     ConflictResultOpt m_conflict_result;
@@ -234,6 +236,17 @@ private:
 
     bool m_legend_visible{ true };
     bool m_legend_enabled{ true };
+    // the reduced-detail preferences, pushed to libvgcode by apply_reduced_detail_settings()
+    libvgcode::EReducedDetailMode m_reduced_detail_mode{ libvgcode::EReducedDetailMode::Off };
+    void read_reduced_detail_preferences();
+    void apply_reduced_detail_settings();
+    static libvgcode::EReducedDetailMode reduced_detail_mode_from_string(const std::string& mode);
+    // in the solid model mode, the sliced objects are drawn as solid shapes instead of toolpaths
+    bool solid_model_enabled() const { return m_reduced_detail_mode == libvgcode::EReducedDetailMode::EndLayersOnly; }
+    void render_solid_model(int canvas_width, int canvas_height);
+    // the prime tower is only among the shells for the solid model, so it is added or removed when that changes
+    void reload_shells_if_solid_model_changed(bool was_enabled);
+    void update_shell_wipe_tower(const Print& print, bool initialized);
 
     float m_legend_height;
     PrintEstimatedStatistics m_print_statistics;
@@ -291,7 +304,7 @@ public:
     // void _render_calibration_thumbnail_internal(ThumbnailData& thumbnail_data, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
     // void _render_calibration_thumbnail_framebuffer(ThumbnailData& thumbnail_data, unsigned int w, unsigned int h, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
     // void render_calibration_thumbnail(ThumbnailData& thumbnail_data, unsigned int w, unsigned int h, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
-    bool has_data() const { return !m_viewer.get_extrusion_roles().empty(); }
+    bool has_data() const { return m_viewer.get_extrusion_roles_count() != 0; }
 
     bool can_export_toolpaths() const;
     std::vector<int> get_plater_extruder();
@@ -363,6 +376,14 @@ public:
     void set_dim_previous_layers_brightness(float value) { m_viewer.set_dim_previous_layers_brightness(value); }
     float get_dim_previous_layers_brightness() const { return m_viewer.get_dim_previous_layers_brightness(); }
 
+    // whether the mouse is holding either slider's handle
+    bool is_slider_dragging() const { return m_layers_slider->is_dragging() || m_moves_slider->is_dragging(); }
+    // while the user drags the camera or a slider, draw the reduced set, if the preference asks for one
+    void set_interacting(bool interacting);
+    bool is_reduced_detail() const { return m_viewer.is_reduced_detail(); }
+    // the preference's string value: "off", "solid" or "shell"
+    void set_reduced_detail_mode(const std::string& mode);
+
     void set_layers_z_range(const std::array<unsigned int, 2>& layers_z_range);
 
     bool is_legend_shown() const { return m_legend_visible && m_legend_enabled; }

src/slic3r/GUI/GLCanvas3D.cpp

@@ -2084,6 +2084,11 @@ void GLCanvas3D::_render_frame(bool scene_dirty, bool only_init)
     const bool overlay_tick = m_fps_overlay_tick;
     m_fps_overlay_tick = false;
 
+    // Whether the preview draws its reduced set is decided before the cached scene is consulted,
+    // since switching changes what the scene pass draws.
+    if (m_canvas_type == ECanvasType::CanvasPreview && m_render_preview && m_gcode_viewer.has_data() && _update_preview_interaction())
+        scene_dirty = true;
+
     // An overlay-only frame reuses the last scene pass. The overlay is rebuilt either way, and drawn
     // below once it is known whether the frame differs from the one on screen.
     const bool reuse_scene = !scene_dirty && _can_reuse_cached_scene(camera);
@@ -3277,9 +3282,16 @@ void GLCanvas3D::bind_event_handlers()
                 if (m_selection_edit.kind != SelectionEdit::None)
                     finish_selection_edit();
                 ImGui::SetWindowFocus(nullptr);
+                // a drag cut short never sees its button release, which would leave the reduced set drawn
+                if (m_canvas_type == CanvasPreview && m_mouse.dragging && m_gcode_viewer.is_reduced_detail())
+                    mouse_up_cleanup();
                 render();
                 evt.Skip();
             });
+        m_canvas->Bind(wxEVT_MOUSE_CAPTURE_LOST, [this](wxMouseCaptureLostEvent&) {
+                if (m_canvas_type == CanvasPreview && m_mouse.dragging && m_gcode_viewer.is_reduced_detail())
+                    mouse_up_cleanup();
+            });
         m_event_handlers_bound = true;
 
         m_canvas->Bind(wxEVT_GESTURE_PAN, &GLCanvas3D::on_gesture, this);
@@ -3355,6 +3367,17 @@ void GLCanvas3D::on_idle(wxIdleEvent& evt)
     m_overlay_dirty |= imgui_requires_extra_frame;
 #endif // ENABLE_ENHANCED_IMGUI_SLIDER_FLOAT
     m_dirty |= GLTexture::Compressor::has_compressed_texture_to_refresh();
+    // the render timer only wakes the idle loop; the frame that puts the preview's toolpaths back
+    // after a wheel burst has to be asked for here, once the settle time is really up
+    if (m_preview_settle_pending) {
+        const auto now = std::chrono::steady_clock::now();
+        if (now >= m_preview_interaction_until) {
+            m_preview_settle_pending = false;
+            m_dirty = true;
+        }
+        else // the timer fired early
+            schedule_extra_frame(static_cast<int>(std::chrono::duration_cast<std::chrono::milliseconds>(m_preview_interaction_until - now).count()) + 1);
+    }
 
     if (!m_dirty && !m_overlay_dirty)
         return;
@@ -3885,6 +3908,10 @@ void GLCanvas3D::on_mouse_wheel(wxMouseEvent& evt)
         return;
     }
 
+    // only a wheel the panels did not take moves the camera
+    if (m_canvas_type == CanvasPreview)
+        note_preview_interaction();
+
 #ifdef __WXMSW__
 	// For some reason the Idle event is not being generated after the mouse scroll event in case of scrolling with the two fingers on the touch pad,
 	// if the event is not allowed to be passed further.
@@ -3985,6 +4012,11 @@ void GLCanvas3D::on_fps_overlay_timer(wxTimerEvent& evt)
     wxWakeUpIdle();
 }
 
+void GLCanvas3D::note_preview_interaction()
+{
+    m_preview_interaction_until = std::chrono::steady_clock::now() + std::chrono::milliseconds(150);
+}
+
 void GLCanvas3D::schedule_extra_frame(int milliseconds)
 {
     // Schedule idle event right now
@@ -5631,6 +5663,9 @@ void GLCanvas3D::mouse_up_cleanup()
     m_mouse.ignore_left_up = false;
     m_mouse.ignore_right_up = false;
     m_dirty = true;
+    // the frame that follows a release puts the preview's toolpaths back, and on some platforms
+    // no idle event follows a button release until the next input
+    wxWakeUpIdle();
 
     if (m_canvas->HasCapture())
         m_canvas->ReleaseMouse();
@@ -7779,13 +7814,20 @@ bool GLCanvas3D::_is_scene_cacheable() const
         return false;
 #endif
 
-    // The scene follows the cursor during a drag, under a gizmo that draws at the cursor, and while
+    // The scene follows the cursor while the user drags, under a gizmo that draws at the cursor, and
-    // the cursor is on the layer height bar, where the object shader draws a band at its height.
+    // while the cursor is on the layer height bar, where the object shader draws a band at its height.
     const GLGizmoBase* gizmo = m_gizmos.get_current();
     const bool cursor_on_layers_bar = is_layers_editing_enabled() &&
         m_layers_editing.bar_rect_contains(*this, (float)m_mouse.position.x(), (float)m_mouse.position.y());
-    return !m_mouse.dragging && !m_gizmos.is_dragging() && !m_rectangle_selection.is_dragging() &&
+    return !is_user_interacting() && (gizmo == nullptr || !gizmo->render_follows_cursor()) && !cursor_on_layers_bar;
-           (gizmo == nullptr || !gizmo->render_follows_cursor()) && !cursor_on_layers_bar;
+}
+
+// Whether the user is holding something that moves the scene: the camera, the navigator, a gizmo,
+// the rectangle selection or a preview slider.
+bool GLCanvas3D::is_user_interacting() const
+{
+    return m_mouse.dragging || m_navigator_dragging || m_gizmos.is_dragging() || m_rectangle_selection.is_dragging() ||
+           m_gcode_viewer.is_slider_dragging();
 }
 
 bool GLCanvas3D::_is_frame_skipping_enabled() const
@@ -8776,6 +8818,24 @@ void GLCanvas3D::_render_wireframe_overlay()
     shader->stop_using();
 }
 
+// The reduced set is drawn while the camera, the navigator or either slider is dragged. A wheel
+// step has no duration, so it holds the reduced set for a settle time instead, and the frame that
+// restores the full toolpaths is scheduled for when that time runs out. Returns whether what the scene
+// pass draws changed, since a frame that reuses the cached scene would hide the change.
+bool GLCanvas3D::_update_preview_interaction()
+{
+    const auto now = std::chrono::steady_clock::now();
+    const bool settling = now < m_preview_interaction_until;
+    const bool dragging = is_user_interacting();
+    const bool was_reduced = m_gcode_viewer.is_reduced_detail();
+    m_gcode_viewer.set_interacting(dragging || settling);
+    if (settling && !dragging && m_gcode_viewer.is_reduced_detail()) {
+        m_preview_settle_pending = true;
+        schedule_extra_frame(static_cast<int>(std::chrono::duration_cast<std::chrono::milliseconds>(m_preview_interaction_until - now).count()) + 1);
+    }
+    return m_gcode_viewer.is_reduced_detail() != was_reduced;
+}
+
 //BBS: GUI refactor: add canvas size as parameters
 void GLCanvas3D::_render_gcode(int canvas_width, int canvas_height)
 {

src/slic3r/GUI/GLCanvas3D.hpp

@@ -656,6 +656,10 @@ private:
     ECursorType m_cursor_type;
     GLSelectionRectangle m_rectangle_selection;
     bool m_navigator_dragging{ false };
+    // until when a wheel step keeps the preview's reduced set drawn
+    std::chrono::time_point<std::chrono::steady_clock> m_preview_interaction_until{};
+    // whether the frame that restores the toolpaths once that time is up is still owed
+    bool m_preview_settle_pending{ false };
 
     //BBS:add plate related logic
     mutable std::vector<int> m_hover_volume_idxs;
@@ -1222,6 +1226,10 @@ public:
     void msw_rescale() { m_gcode_viewer.invalidate_legend(); }
 
     void request_extra_frame() { m_extra_frame_requested = true; }
+    // whether the user is holding the camera, the navigator, a gizmo, the rectangle selection or a preview slider
+    bool is_user_interacting() const;
+    // a wheel step is over before the next frame, so it holds the preview's reduced set for a settle time
+    void note_preview_interaction();
 
     void schedule_extra_frame(int milliseconds);
 
@@ -1372,6 +1380,9 @@ private:
     //BBS: GUI refactor: add canvas size as parameters
     void _render_gcode(int canvas_width, int canvas_height);
     void _render_gcode_overlay(int canvas_width, int canvas_height);
+    // decides whether the preview draws its reduced set this frame and returns whether what the scene
+    // pass draws changed; runs before the cached scene is consulted
+    bool _update_preview_interaction();
     //BBS: render a plane for assemble
     void _render_plane() const;
     void _render_selection();

src/slic3r/GUI/IMSlider.cpp

@@ -483,6 +483,11 @@ void IMSlider::draw_background_and_groove(const ImRect& bg_rect, const ImRect& g
     ImGui::RenderFrame(groove.Min, groove.Max, groove_col, false, 0.5 * groove.GetWidth());
 }
 
+bool IMSlider::is_dragging() const
+{
+    return GImGui != nullptr && m_imgui_id != 0 && GImGui->ActiveId == m_imgui_id && GImGui->IO.MouseDown[0];
+}
+
 bool IMSlider::horizontal_slider(const char* str_id, int* value, int v_min, int v_max, const ImVec2& size, float scale)
 {
     ImGuiWindow* window = ImGui::GetCurrentWindow();
@@ -491,6 +496,7 @@ bool IMSlider::horizontal_slider(const char* str_id, int* value, int v_min, int
 
     ImGuiContext& context = *GImGui;
     const ImGuiID id = window->GetID(str_id);
+    m_imgui_id = id;
 
     const ImVec2 pos = window->DC.CursorPos;
     const ImRect draw_region(pos, pos + size);
@@ -883,6 +889,7 @@ bool IMSlider::vertical_slider(const char* str_id, int* higher_value, int* lower
 
     ImGuiContext& context = *GImGui;
     const ImGuiID id = window->GetID(str_id);
+    m_imgui_id = id;
 
     const ImVec2 pos = window->DC.CursorPos;
     const ImRect draw_region(pos, pos + size);

src/slic3r/GUI/IMSlider.hpp

@@ -118,6 +118,9 @@ public:
 
     //BBS update scroll value changed
     bool is_dirty() { return m_dirty; }
+    // whether the mouse is holding this slider's handle, read from ImGui's active id rather than
+    // from the dirty flag, which is raised and consumed inside a single frame
+    bool is_dragging() const;
     void set_as_dirty(bool dirty = true) { m_dirty = dirty; }
     bool is_need_post_tick_event() { return m_is_need_post_tick_changed_event; }
     void reset_post_tick_event(bool val = false) {
@@ -182,6 +185,8 @@ private:
     int  m_higher_value;
     int  m_one_layer_value; // ORCA
     bool m_dirty = false;
+    // the ImGui id of the slider widget, as of its last render
+    unsigned int m_imgui_id = 0;
 
     bool m_render_as_disabled{ false };
 

src/slic3r/GUI/Preferences.cpp

@@ -322,7 +322,7 @@ wxBoxSizer* PreferencesDialog::create_item_combobox(wxString title, wxString too
     return sizer;
 }
 
-wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, const wxString wiki_url)
+wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, std::function<void(std::string)> onchange, const wxString wiki_url)
 {
     assert(vlist.size() == config_name_index.size());
     unsigned int current_index = 0;
@@ -338,8 +338,9 @@ wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString too
     auto [sizer, combobox] = create_item_combobox_base(title, tooltip, param, vlist, current_index);
 
     //// save config
-    combobox->GetDropDown().Bind(wxEVT_COMBOBOX, [this, param, config_name_index](wxCommandEvent& e) {
+    combobox->GetDropDown().Bind(wxEVT_COMBOBOX, [this, param, config_name_index, onchange](wxCommandEvent& e) {
         app_config->set(param, config_name_index[e.GetSelection()]);
+        if (onchange != nullptr) onchange(config_name_index[e.GetSelection()]);
         e.Skip();
     });
 
@@ -2044,6 +2045,32 @@ void PreferencesDialog::create_items()
         "preview_default_view_type", PreviewViewTypeLabels, PreviewViewTypeValues);
     g_sizer->Add(item_preview_view_type);
 
+    auto item_reduced_detail_mode = create_item_combobox(
+        _L("Simplify preview while dragging"),
+        _L("What the sliced preview draws while you drag the camera or a preview slider, or zoom with the mouse wheel, so that large prints stay responsive. "
+           "The full toolpaths are restored as soon as you let go.\n"
+           "Off: the full toolpaths.\n"
+           "Solid model: the sliced objects and the prime tower as solid shapes in their filament colors, cut to the visible layer range, "
+           "with its bottom and top layers drawn as toolpaths. Supports are not shown, and negative volumes are not cut out.\n"
+           "Shell only: every layer without its sparse infill, internal solid infill and gap fill, which lie under the walls and skins. "
+           "Walls, top and bottom surfaces, bridges, supports and the prime tower are drawn whole, so the print looks the same from outside.\n"
+           "The bottom and top of the visible layer range are always drawn whole."),
+        "preview_reduced_detail_mode",
+        {_L("Off"), _L("Solid model"), _L("Shell only")},
+        {"off", "solid", "shell"},
+        // apply the new mode immediately to the currently loaded preview
+        [](std::string value) {
+            if (Plater* plater = wxGetApp().plater()) {
+                if (GLCanvas3D* canvas = plater->get_preview_canvas3D()) {
+                    canvas->get_gcode_viewer().set_reduced_detail_mode(value);
+                    canvas->set_as_dirty();
+                    canvas->request_extra_frame();
+                }
+            }
+        }
+    );
+    g_sizer->Add(item_reduced_detail_mode);
+
     auto item_dim_previous_layers = create_item_checkbox(
         _L("Dim lower layers"),
         _L("When scrubbing the layer slider in the sliced preview, render the layers below the current one darkened so that only the layer being viewed is shown at full brightness."),

src/slic3r/GUI/Preferences.hpp

@@ -93,7 +93,7 @@ public:
     wxBoxSizer *create_item_title(wxString title);
     wxBoxSizer *create_item_label(wxString label, const wxString tooltip = "", const wxString wiki_url = "");
     wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::function<void(wxString)> onchange = {}, const wxString wiki_url = "");
-    wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, const wxString wiki_url = "");
+    wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, std::function<void(std::string)> onchange = {}, const wxString wiki_url = "");
     wxBoxSizer *create_item_region_combobox(wxString title, wxString tooltip);
     wxBoxSizer *create_item_language_combobox(wxString title, wxString tooltip);
     wxBoxSizer *create_item_loglevel_combobox(wxString title, wxString tooltip, std::vector<wxString> vlist);