viewport: wire up the backface-culling setting

The "Backface Culling" checkbox persisted a value and reflected it, but
nothing consumed AppSettings::backfaceCulling — the opaque pipeline
hardcoded cullMode = Back, so toggling had no effect.

Build a second opaque pipeline (cullMode None) alongside the culled one and
pick between them per-frame from a backface_culling_ flag; setBackfaceCulling
flips the flag and requests a redraw (no rebuild). ViewportWindow forwards
it, and MainWindow applies the persisted value at startup and re-applies on
change — same wiring as the nav preset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bonsaiviewer/MainWindow.cpp

@@ -510,6 +510,15 @@ void MainWindow::setupStatus() {
                     vp->applyNavPreset(AppSettings::navPresetName(preset));
             });
 
+    // Backface culling: apply the persisted choice and re-apply live on change.
+    if (auto* vp = viewport_widget_->viewport())
+        vp->setBackfaceCulling(AppSettings::instance().backfaceCulling());
+    connect(&AppSettings::instance(), &AppSettings::backfaceCullingChanged, this,
+            [this](bool enabled) {
+                if (auto* vp = viewport_widget_->viewport())
+                    vp->setBackfaceCulling(enabled);
+            });
+
     connect(session_state_, &bonsaiviewer::SessionState::statusMessageChanged,
             this, [this](const QString& mode, const QString& detail) {
         status_mode_label_->setText(mode);

src/ifcviewer/ViewportCore.cpp

@@ -479,6 +479,12 @@ void ViewportCore::setNavPreset(const char* name) {
         nav_bindings_ = { B::Middle, M::Plain,  B::Middle, M::Shift, B::Left,  M::Plain };
 }
 
+void ViewportCore::setBackfaceCulling(bool enabled) {
+    if (backface_culling_ == enabled) return;
+    backface_culling_ = enabled;
+    host_->requestFrame();
+}
+
 bool ViewportCore::frameSelection() {
     if (selection_.count() == 0) return false;
     float lo[3] = {  std::numeric_limits<float>::infinity(),
@@ -1150,6 +1156,19 @@ bool ViewportCore::buildPipelines() {
         return false;
     }
 
+    // ---- Backface-culling-off variant of the opaque pipeline -----------
+    // The "Backface Culling" setting picks between this and main_pipeline_ at
+    // draw time (opaque pass). Identical but cullMode None, so single-sided
+    // IFC meshes show their back faces.
+    WGPURenderPipelineDescriptor rp_desc_nc = rp_desc;
+    rp_desc_nc.label = svFromCStr("ifcviewer-wgpu.main_pipeline_no_cull");
+    rp_desc_nc.primitive.cullMode = WGPUCullMode_None;
+    main_pipeline_no_cull_ = wgpuDeviceCreateRenderPipeline(device_, &rp_desc_nc);
+    if (!main_pipeline_no_cull_) {
+        Log::warn() << "wgpu main no-cull render pipeline creation failed";
+        return false;
+    }
+
     // ---- Transparent variant of the main pipeline ----------------------
     // Same shader, same layout, same vertex pulling, same depth test —
     // differs only in:
@@ -1796,6 +1815,7 @@ void ViewportCore::shutdown() {
     if (selection_flags_buffer_)  { wgpuBufferRelease(selection_flags_buffer_);       selection_flags_buffer_ = nullptr; }
     selection_flags_capacity_ = 0;
     if (main_pipeline_)              { wgpuRenderPipelineRelease(main_pipeline_);             main_pipeline_ = nullptr; }
+    if (main_pipeline_no_cull_)      { wgpuRenderPipelineRelease(main_pipeline_no_cull_);     main_pipeline_no_cull_ = nullptr; }
     if (main_pipeline_transparent_)  { wgpuRenderPipelineRelease(main_pipeline_transparent_); main_pipeline_transparent_ = nullptr; }
     section_gizmo_.destroy();
     if (main_shader_module_)   { wgpuShaderModuleRelease(main_shader_module_);    main_shader_module_ = nullptr; }
@@ -6400,9 +6420,10 @@ void ViewportCore::render() {
     WGPURenderPassEncoder pass = wgpuCommandEncoderBeginRenderPass(enc, &pass_desc);
 
     // Two-pass main render: opaque first, then transparent.
-    if (main_pipeline_ && main_pipeline_transparent_
+    if (main_pipeline_ && main_pipeline_no_cull_ && main_pipeline_transparent_
         && frame_bind_group_ && !models_gpu_.empty()) {
-        wgpuRenderPassEncoderSetPipeline(pass, main_pipeline_);
+        wgpuRenderPassEncoderSetPipeline(pass,
+            backface_culling_ ? main_pipeline_ : main_pipeline_no_cull_);
         wgpuRenderPassEncoderSetBindGroup(pass, 0, frame_bind_group_, 0, nullptr);
 
         for (const auto& [session_model_id, m] : models_gpu_) {

src/ifcviewer/ViewportCore.h

@@ -223,6 +223,12 @@ public:
     void setNavPreset(const char* name);
     const NavBindings& navBindings() const { return nav_bindings_; }
 
+    // Toggle backface culling of opaque geometry. Off draws back faces too
+    // (useful for single-sided IFC meshes). Switches the opaque pipeline at
+    // draw time — no rebuild.
+    void setBackfaceCulling(bool enabled);
+    bool backfaceCulling() const { return backface_culling_; }
+
     // Frame the current selection: union the selected objects' world AABBs and
     // fit the camera to them (same 1.30 padding as the desktop "F" hotkey).
     // No-op with an empty selection or no resolvable AABBs; returns whether it
@@ -909,7 +915,9 @@ private:
     WGPUBindGroupLayout model_bgl_                = nullptr;  // group 1
     WGPUPipelineLayout  pipeline_layout_          = nullptr;
     WGPURenderPipeline  main_pipeline_            = nullptr;
+    WGPURenderPipeline  main_pipeline_no_cull_    = nullptr;  // backface culling off
     WGPURenderPipeline  main_pipeline_transparent_ = nullptr;
+    bool                backface_culling_         = true;
     // Section-plane gizmo, shared by desktop + web (both render via render()).
     // Lifted out of the Qt-coupled OverlayRenderer so one identical gizmo draws
     // everywhere; the desktop's OverlayRenderer no longer draws it.

src/ifcviewer/ViewportWindow.cpp

@@ -1499,6 +1499,10 @@ void ViewportWindow::applyNavPreset(const char* name) {
     select_button_ = toQtBtn(b.select);  select_mods_ = toQtMod(b.select_mod);
 }
 
+void ViewportWindow::setBackfaceCulling(bool enabled) {
+    core_.setBackfaceCulling(enabled);
+}
+
 // -----------------------------------------------------------------------------
 // One-shot framebuffer capture → PNG
 // -----------------------------------------------------------------------------

src/ifcviewer/ViewportWindow.h

@@ -247,6 +247,7 @@ public:
     // Sources the shared binding table from ViewportCore; called from init
     // (env / persisted setting) and live from the Settings dialog.
     void applyNavPreset(const char* name);
+    void setBackfaceCulling(bool enabled);
 
 
     // Queue a one-shot framebuffer capture: the next rendered frame is