OrcaSlicer/docs/HLSD/design-tab.md

Design tab — High Level Design

Purpose and scope

The Design tab is a parametric CAD environment inside the slicer: sketch, constrain, build solid features, commit the result to the plate. It exists because the alternative is a round trip through an external CAD application, and that round trip discards design intent at both ends — a part edited after slicing comes back as a mesh rather than as the feature history that produced it. Keeping the model in the project means a dimension can be changed after the part has been sliced, with the nozzle diameter, the build volume and the material already known.

Its coupling to the rest of the application is deliberately narrow. It adds no stage to the slicing pipeline and touches neither the preset system nor Tab. It reaches the rest of Orca in two places: Commit to Plate, which hands finished solids to Prepare as ordinary model objects, and one optional 3MF archive entry that carries the recipe. Everything else is contained in src/libslic3r/CAD/ and src/slic3r/GUI/CAD/.

The user-facing manual lives in the wiki (Design Tab), not here. This document covers the parts of the design that the code does not make evident.

The model is a recipe

CadDocument holds an ordered list of CadFeature and nothing else that matters. Bodies, meshes and display geometry are derived: recompute() replays the feature list from the start and rebuilds them. Editing a dimension set twenty features ago is therefore an ordinary edit — everything downstream is rebuilt by the same replay that built it the first time.

Two consequences follow from deriving rather than storing:

• Undo is a snapshot of features alone. The caller calls checkpoint() before the mutations that make up one user action; undo restores that snapshot and recomputes. Because everything else is derived, one checkpoint is exactly one Ctrl+Z step and the restored state is exact rather than approximately reconstructed. The tab keeps this stack itself; it is not Orca's project snapshot system, which operates on Model objects the Design tab does not own until Commit.
• Face and edge ids are session-scoped. They are indices into TopExp::MapShapes, so a rebuild invalidates every one of them. CadDocument::topo_generation is bumped on every rebuild so a holder of an id can discover that it is stale instead of silently addressing a different edge. The counter is deliberately not serialized: an id means nothing outside the run that produced it.

Geometry kernel and dependency surface

The kernel is OCCT, which OrcaSlicer already links — Format/STEP.cpp, Format/svg.cpp and Shape/TextShape.cpp use it upstream. The Design tab adds no third-party dependency; it widens the existing OCCT build by one module flag in deps/OCCT/OCCT.cmake:

-DBUILD_MODULE_ModelingAlgorithms=${SLIC3R_CAD}

Most of that module's twelve toolkits were already being built, because DataExchange — the STEP path upstream ships — depends on them. The delta is TKFillet (used through BRepFilletAPI), TKOffset (BRepOffsetAPI) and TKFeat, which nothing here references but which the module flag builds anyway, because OCCT's module flags are all-or-nothing. On macOS and Linux OCCT links statically, so an unreferenced toolkit costs build time and no shipped bytes; on Windows OCCT builds shared, so the cost there is real DLL bytes. That Windows figure has not been measured, and OCCT.cmake says so rather than carrying a number that was derived from an incomplete toolkit list.

On Windows the packaging step asserts that every linked OCCT toolkit has a shipped DLL and fails the configure with the name of any that is missing, because the alternative failure — a deps prefix built with a different SLIC3R_CAD setting than the app — otherwise surfaces as a missing DLL at first launch.

The sketch constraint solver

src/libslic3r/slvs/ is a vendored subset of SolveSpace's libslvs: self-contained, no external dependencies, GPL-3.0, with its LICENSE preserved verbatim in the directory. SketchSolver.cpp is its only consumer and drives every sketch constraint in the tab.

OrcaSlicer is AGPL-3.0. GPLv3 §13 permits combining a GPLv3 work with an AGPLv3 work and AGPLv3 §13 grants the converse, so the combined work is distributable under AGPL-3.0 with the solver's GPLv3 terms preserved. The solver is vendored rather than fetched as a dependency because it is a pinned subset with no build system of its own; the cost of that choice is upstream-sync burden, paid deliberately to keep deps/ unchanged.

The SLIC3R_CAD gate

SLIC3R_CAD (default ON) compiles the tab and selects the OCCT module flag above. With it OFF the tab is not built and the deps prefix matches upstream exactly. The gate is cheap because the hooks the Design tab adds to shared GUI code — chiefly the m_design_sketch_tool member and the render, mouse and key hooks in GLCanvas3D — are null-guarded on the path they extend, so removing the tab removes behaviour rather than requiring the host code to be rewritten.

The flag has to agree between the dependencies and the application; that is what the DLL assertion above is checking.

Project persistence

A project stores the recipe as one optional archive entry, Metadata/orca_cad.bin, backed by a single std::string cad_recipe on Model. The entry is written only when the string is non-empty, and readers that do not know it ignore it, so projects that contain no CAD model are byte-identical to what upstream would have written and older readers are unaffected.

The blob is a cereal binary archive whose layout is the field order of CadFeature's save/load. That makes the format the one irreversible decision in the subsystem, and the rules that keep it survivable are:

• Append only, never reorder. Enums serialize positionally as their underlying integer, so inserting a value in the middle of SketchConstraintType or CadFeatureType reinterprets every constraint in every saved project. New fields go at the end.
• Features are length-framed. Since v5 each feature is a length-prefixed, self-contained cereal stream, so a reader can skip a feature written by a newer build and stop cleanly on an older one. This is what makes appending a field a non-breaking change from here on. v4 and earlier still open through the pre-framing flat path; v1 is deliberately not loadable and has no migration path.
• A newer stamp is refused, not guessed at. deserialize_recipe rejects a blob whose version exceeds ORCA_CAD_RECIPE_VERSION with a message naming both versions.
• The rules are held by fixtures, not by discipline. tests/data/cad_recipe_v{3,4,5}.bin are checked-in blobs from the builds that wrote them, and the tests that load them fail if a field is reordered — which the in-memory round-trip test cannot detect. A regeneration test ([.regen], not run by default) produces a fresh fixture when a new version is stamped.

Import features embed the imported solid as an OCCT BRep string inside the recipe rather than referencing the source file, so a project opens without the STEP or mesh it was built from. The cost is that saved projects are coupled to an OCCT BRep revision. A Text feature follows the same rule: it stores the outlines it was vectorised into alongside its string, font and height, so the project opens identically on a machine that lacks the font; the three parameters are only what an edit reopens the dialog with.

The interaction contract

Three inputs carry the whole modelling loop — left click, right click and Esc — and the contract between them is stated in code rather than spread across handlers.

DesignInteraction.hpp defines a four-level LIFO stack whose enum value is the depth, so "which level does this press belong to" is a comparison:

Level	Holds	One Esc press
Transient	a value field or a popup menu	closes it; the tool stays armed
Gesture	an uncommitted delta — an entity being drawn, a body being dragged	reverts it; committed work is untouched
Tool	a feature card, an armed sketch tool, a constrain session	exits it; drawn entities survive
Idle	nothing transient	clears the selection; leaves a sketch session only if it is empty

cad_escape_level() is a constexpr free function over a POD of four booleans rather than a method on the panel, so the ordering that is the entire contract is checkable without a window, a GL context or an event loop — five static_asserts in the header do exactly that at compile time.

The strict invariant: no level of Esc deletes a feature, discards a sketch that holds geometry, or rolls history back. Destroying work needs a gesture that says so — Del on an explicit selection, the sketch ribbon's Cancel, which asks first, or Ctrl+Z. A sketch session is deliberately not a Tool level; it is the environment the Idle level lives in, which makes the destructive path unrepresentable rather than merely unlikely.

Right-click is read at button-up against one budget, 3 px of drift, applied to the whole press rather than to its end points: a press that wandered past the budget at any moment is navigation, even if it comes back to where it started, which is what stops a slow, careful orbit from ending in a menu. There is no time budget — a gesture that means something different when it is slow is exactly what the interaction charter rules out. The raycast uses the press position, not the release. An armed sketch tool that already consumed the right button (to terminate a chain, say) declines to also open a menu, through a read-and-clear flag. Past either budget the event is navigation, and navigation does not transition the state machine.

Navigation itself is Prepare's: the camera reads the drag actions set in Preferences > Control for each button. The left button is shared with picking, so a whole body is swept with a rectangle on plain left-drag only while no camera action is assigned to it, and with Shift+left-drag otherwise — Prepare's own rectangle selection.

Entering a sketch changes three things at once so the mode is legible: a banner above the canvas (a sibling of the canvas, not a child over it — on GTK a child window over a wxGLCanvas is a native window and does not reliably stack over GL), the printer bed muted so a plate grid is never read as a sketch grid, and N to look normal to the plane. Code that changes any of the three belongs with a change to this section.

Rendering the bodies

The tab draws its bodies through the same GLCanvas3D object path as Prepare, so how they look is decided in the shared object shader, not in the tab. The slicer's two lights both sit near the camera, which leaves the sides of a part in nearly one tone; the Design canvas asks for a studio model instead — a world-space sky/ground hemisphere, a key and a fill light, a plastic-like highlight and a darker silhouette — through GLCanvas3D::set_studio_lighting() and the phong shader's lighting_model uniform. The program is shared by every canvas, so each use sets the uniform (0 for the slicer's canvases) rather than relying on a default: a canvas that left it alone would inherit whatever the last canvas chose.

The B-rep edges of every body are drawn over it by the sketch overlay as thin view-facing ribbons, depth tested and pulled a few pixels toward the eye so they win against the faces that meet at them and still hide behind faces in front; lines are not used because they do not rasterise under the software GL context the tab also supports. Seams of closed surfaces and degenerate edges are left out (GeometryEngine::display_edges), and the polylines are sampled once per shape, keyed by its TShape, because a recompute that leaves a body unchanged is the common case.

Following the app

The tab is a page of Orca's main window and answers to the same settings as Prepare.

• Theme. Its chrome is coloured from a table of light/dark token pairs. A theme switch reaches DesignPanel::on_sys_color_changed from MainFrame, which moves every colour that is one theme's token onto the other theme's and then runs the app's own dark pass; the icons are Orca's sidebar grey, which the icon cache maps per theme, so they are re-rasterised rather than re-tinted.
• Scale. Sizes are in DIP, and a DPI change reaches DesignPanel::msw_rescale, which re-rasterises every icon (button faces, flyout rows, card headers, the tree's image list).
• Viewport text. The status line and the active tool's values are drawn by the canvas in its ImGui pass, so they go with the canvas: a top-level window over GL does not follow its frame and was left floating over other applications.
• Undo. The tab keeps its own history (the recipe is not part of Prepare's snapshots), but it has no Undo/Redo of its own: the top bar, Ctrl+Z and Edit drive it while the tab is shown, greyed to what an undo would actually do.

The offer is generated, not hand-written

Right-clicking geometry opens the offer: eight families in a fixed order, each verb at a permanent row index, verbs that do not apply shown disabled in place with their reason rather than removed. The invariant is that a verb's row index is identical in every selection where it appears and that adding a verb never moves an existing one — the hand learns the position, so the menu is never re-sorted, compacted or adaptively ordered.

An invariant across 92 verbs and 20 selection kinds does not survive by review, so the map exists once, as data: scripts/CAD/tool_atlas.json carries every verb with its row, key, icon, accepted selections, preconditions and refusal string, and scripts/CAD/gen_offer_table.py emits src/slic3r/GUI/CAD/DesignOffer.hpp from it. The header is checked in and never hand-edited; scripts/CAD/run-all-checks.sh runs the generator with --check as its first rung, which is what makes "GENERATED — DO NOT EDIT" a fact rather than a request. The generator also refuses an atlas with a duplicate verb id, since mcp_run_verb resolves a verb by id and would make the second one unreachable.

The atlas and its generator sit in scripts/CAD/ rather than in docs/: they are build inputs for a checked-in header, not documentation.

Automation surface

McpControl exposes the document over JSON-RPC when ORCA_CAD_MCP is set in the environment, with tools/orca_cad_mcp_bridge.py as the client side. It describes the scene, queries topology, measures, and runs the same verbs the offer does — it re-implements nothing, so a scripted action and a clicked one cannot diverge. It is off unless the variable is set.

Where the code lives

Path	Role
src/libslic3r/CAD/CadDocument.*	the feature recipe, its replay, undo and serialization
src/libslic3r/CAD/GeometryEngine.*	OCCT wrapper — faces, edges, booleans, healing
src/libslic3r/CAD/SketchEngine.*	profile → wire → solid
src/libslic3r/CAD/SketchSolver.*	constraint solving, over the vendored solver
src/libslic3r/slvs/	vendored 2D constraint solver (GPLv3)
src/slic3r/GUI/CAD/DesignPanel.*	the tab: toolbar, feature cards, tree, key maps
src/slic3r/GUI/CAD/DesignCanvas.*	viewport integration
src/slic3r/GUI/CAD/DesignSketchTool.*	in-canvas sketching
src/slic3r/GUI/CAD/DesignInteraction.hpp	the Esc level contract
src/slic3r/GUI/CAD/DesignOffer.hpp	generated offer table
scripts/CAD/tool_atlas.json	source of truth for the offer

Verification

The kernel is covered by Catch2 suites in tests/libslic3r/ (test_caddocument, test_sketchconstraints, test_sketchedit, test_sketchimport, test_sketchinference, test_sketchprofile, test_slvs_constraints), which need no display; scripts/CAD/run-kernel-tests.sh builds only libslic3r_tests and runs them headless.

The GUI half is not covered by CI, which has no OpenGL canvas or synthetic input: the ladders in scripts/CAD/ drive a running application in a local rig instead, and scripts/CAD/run-all-checks.sh is the gate that runs all of them. A green kernel run says nothing about the viewport, so the two are reported separately rather than as one number.