Design: STEP export of the CAD model

Add an "Export STEP…" button to the Design panel that writes every body to a
.step file as native B-rep (not mesh).

- CadDocument::export_step: compound all bodies (applying their per-body Move
  display transform so the STEP matches what Commit ships) and write via OCCT
  STEPControl_Writer (AsIs). Full error handling incl. OCCT Standard_Failure.
- DesignPanel::on_export_step: bake any open preview, wxFileDialog save, export
  at the displayed body positions, status feedback.

Kernel write path verified with a standalone OCCT box->STEP->readback check
(1 solid, non-null) against the same OCCT build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/libslic3r/CadDocument.cpp

@@ -39,6 +39,9 @@
 #include <gp_Pnt2d.hxx>
 #include <gp_Vec.hxx>
 #include <gp_Trsf.hxx>             // pattern: rigid copy transforms
+#include <STEPControl_Writer.hxx>  // STEP export (native B-rep)
+#include <STEPControl_StepModelType.hxx>
+#include <IFSelect_ReturnStatus.hxx>
 #include <gp_Ax1.hxx>             // pattern: rotation axis (circular)
 #include <BRepBuilderAPI_Transform.hxx>
 #include <cmath>
@@ -1780,4 +1783,48 @@ bool CadDocument::deserialize_recipe(const std::string& blob)
     }
 }
 
+bool CadDocument::export_step(const std::string& path,
+                             const std::vector<Transform3d>& body_xforms,
+                             std::string& err) const
+{
+    err.clear();
+    if (bodies.empty()) { err = "nothing to export"; return false; }
+    try {
+        // Compound every body at its displayed (Move-gizmo) position so the STEP matches
+        // what Commit ships. Move transforms are rigid, so gp_Trsf::SetValues is valid.
+        BRep_Builder bld;
+        TopoDS_Compound comp;
+        bld.MakeCompound(comp);
+        for (size_t i = 0; i < bodies.size(); ++i) {
+            if (bodies[i].shape.IsNull()) continue;
+            TopoDS_Shape s = bodies[i].shape;
+            if (i < body_xforms.size() && !body_xforms[i].isApprox(Transform3d::Identity())) {
+                const Transform3d& m = body_xforms[i];
+                gp_Trsf t;
+                t.SetValues(m(0,0), m(0,1), m(0,2), m(0,3),
+                            m(1,0), m(1,1), m(1,2), m(1,3),
+                            m(2,0), m(2,1), m(2,2), m(2,3));
+                s = BRepBuilderAPI_Transform(s, t, true).Shape();
+            }
+            bld.Add(comp, s);
+        }
+        STEPControl_Writer writer;
+        if (writer.Transfer(comp, STEPControl_AsIs) != IFSelect_RetDone) {
+            err = "STEP transfer failed";
+            return false;
+        }
+        if (writer.Write(path.c_str()) != IFSelect_RetDone) {
+            err = "cannot write STEP file";
+            return false;
+        }
+    } catch (const Standard_Failure& e) {
+        err = e.GetMessageString() ? e.GetMessageString() : "OCCT failed to write STEP";
+        return false;
+    } catch (const std::exception& e) {
+        err = e.what();
+        return false;
+    }
+    return true;
+}
+
 } // namespace Slic3r

src/libslic3r/CadDocument.hpp

@@ -360,6 +360,13 @@ public:
     std::string serialize_recipe() const;
     bool deserialize_recipe(const std::string& blob);
 
+    // Export every body to a STEP file as native B-rep (not mesh). body_xforms is the
+    // per-body display transform (Move gizmo); when supplied the bodies are written at
+    // those positions so the STEP matches what Commit ships. false + err on failure.
+    bool export_step(const std::string& path,
+                     const std::vector<Transform3d>& body_xforms,
+                     std::string& err) const;
+
     // Undo/redo of the feature recipe (Onshape-style Ctrl+Z). The caller marks a
     // user-action boundary by calling checkpoint() BEFORE the mutation(s) for that
     // action (add/delete/move/replace, or a direct features edit). undo()/redo() then

src/slic3r/GUI/DesignPanel.cpp

@@ -1514,7 +1514,11 @@ DesignPanel::DesignPanel(wxWindow* parent)
 
     auto* commit = new wxButton(m_form, wxID_ANY, _L("Commit to Plate"));
     commit->Bind(wxEVT_BUTTON, [this](wxCommandEvent&) { on_commit(); });
-    root->Add(commit, 0, wxALL, 12);
+    root->Add(commit, 0, wxLEFT | wxRIGHT | wxTOP, 12);
+
+    auto* export_step = new wxButton(m_form, wxID_ANY, _L("Export STEP…"));
+    export_step->Bind(wxEVT_BUTTON, [this](wxCommandEvent&) { on_export_step(); });
+    root->Add(export_step, 0, wxALL, 12);
 
     m_shape->Bind(wxEVT_CHOICE, [this](wxCommandEvent& e) { on_shape_changed(); e.Skip(); });
     on_shape_changed();
@@ -4766,6 +4770,31 @@ void DesignPanel::on_edit_feature()
     }
 }
 
+void DesignPanel::on_export_step()
+{
+    // Bake any open feature preview first, so the STEP matches what is shown (mirrors on_commit).
+    if (m_active != Tool::None)
+        confirm_tool();
+    if (m_doc.bodies.empty()) {
+        m_status->SetForegroundColour(wxNullColour);
+        m_status->SetLabel(_L("Nothing to export — add a feature first"));
+        m_status->Refresh();
+        return;
+    }
+    wxFileDialog dlg(this, _L("Export STEP"), wxEmptyString, "model.step",
+                     "STEP files (*.step;*.stp)|*.step;*.stp",
+                     wxFD_SAVE | wxFD_OVERWRITE_PROMPT);
+    if (dlg.ShowModal() != wxID_OK)
+        return;
+    sync_body_xform();   // export bodies at their displayed Move-gizmo positions
+    std::string err;
+    const bool ok = m_doc.export_step(dlg.GetPath().ToUTF8().data(), m_body_xform, err);
+    m_status->SetForegroundColour(ok ? wxColour(120, 210, 120) : wxColour(235, 110, 110));
+    m_status->SetLabel(ok ? _L("Exported STEP")
+                          : _L("STEP export failed: ") + wxString::FromUTF8(err));
+    m_status->Refresh();
+}
+
 void DesignPanel::on_commit()
 {
     // A feature tool open with a live preview ghost (e.g. a fillet being previewed) is

src/slic3r/GUI/DesignPanel.hpp

@@ -97,6 +97,7 @@ private:
     // a modal dialog editing the feature's placement transform in place.
     void on_transform_imported(int feat_idx);
     void on_commit();
+    void on_export_step();   // write all bodies to a .step file (native B-rep)
     // Rehydrate the parametric model from a project's saved recipe (3MF
     // Metadata/SnapOrca_cad.bin): deserialize -> recompute -> refresh viewport + tree.
     void load_recipe(const std::string& blob);