IfcOpenShell/docs/dev-notes/profile-length-per-instance.md

Profile length per-instance — typed vs per-instance length for profile types

Living dev note for the profile-length-per-instance branch/PR. Read before working on the feature; append decisions and findings as the PR is refined. This is not user documentation — at merge it is removed or its durable parts promoted to code comments. See README.md for the convention.

Tracks feature #8657; depends on bug fixes #8655 (change_data None) and #8656 (mapped-profile assign cascade).

Problem

Revit-exported profile members (e.g. curtain-wall mullions) come in typed-length: the extrusion — including its length — lives on the IfcMemberType as an IfcRepresentationMap, and every occurrence shares it via an IfcMappedItem. Bonsai's profile tools assume each occurrence owns an editable extrusion (IfcMemberStandardCase style), so on these mapped occurrences the length UI never appears, editing/joining crashes, changing the type-material bleeds across all instances, and extend_profile flips the origin.

Goal: make Length a typed-vs-per-instance property of a profile type, toggle-able in both directions, without corrupting the shared geometry.

• Typed length = geometry (incl. length) on the type's RepresentationMap; occurrences IfcMappedItem it. One length for all instances.
• Per-instance length = occurrence has its own SweptSolid body + its own IfcMaterialProfileSetUsage. Each instance edits its own length (IfcMemberStandardCase).

Key facts established

• tool.Model.get_usage_type(el) returns "PROFILE" iff el carries an IfcMaterialProfileSet(Usage) (checked with should_inherit=False). Bonsai gates the per-instance profile UI (Length control) on this. A mapped occurrence that only inherits the type's profile set reports None → no Length UI. This is why un-mapping must also give the occurrence its own IfcMaterialProfileSetUsage.
• The mapped representation is shared. bonsai.core.geometry.remove_representation, when handed a mapped rep, resolves it and loops get_elements_of_type → switch_from_representation, i.e. it cascades to every sibling instance of the type, leaving them empty. Any code that removes an occurrence's mapped body triggers this. This is the single biggest hazard and the root of most crashes we saw.
• Typed length is spec-valid. A plain IfcMember with a mapped body + a profile-set usage is valid IFC (just not the IfcMemberStandardCase subtype). IfcMaterialProfileSet has no length; length is always in the geometry. So offering typed length is a legitimate mode, not a workaround (verified against local IFC4 ADD2 TC1).
• Two origins, on opposite ends. Revit places the object's ObjectPlacement origin at one end of the mullion and the extrusion's own Position origin at the other, with the object's local +Z pointing away from the sweep (local Z runs -depth → 0). get_profile_axis (object bound-box local-Z range) + DumbProfileJoiner.recreate_profile (which plants the new origin at body[0] = min-local-Z) then relocate the origin to the far end on extend/join — the "flip". Natively-authored profiles run local Z 0 → depth, so body[0] is already the origin and nothing moves.
• create_profile double-bodies mapped types. assign_type (default should_map_representations=True) maps the type's shared body onto a new occurrence, then DumbProfileGenerator.create_profile adds a per-instance extrusion on top → two Body reps / typed-by-default.
• The wrapper/core were not the cause of the crashes we chased for a while — a git reset --hard v0.8.0 reproduced clean, our applied changes reproduced the crash. The installed environment is now matched (repo source + release wrapper 3e7b739, via dev_environment.py).

Design

Two explicit operators + a Type-panel toggle, plus defaults/guards so the mapped hazard is never hit implicitly.

• bim.make_profile_length_per_instance (un-map; MakeProfileLengthPerInstance):
  1. Copy the mapped extrusion items into a new per-instance SweptSolid body, keeping the shared IfcProfileDef (copy_deep(..., exclude=["IfcProfileDef"])).
  2. Orphan the old mapped body (retarget the product shape, do not delete it) — deleting cascades to siblings, and the raw delete dangles Bonsai's Blender-side links.
  3. Give the occurrence its own IfcMaterialProfileSetUsage (else no Length UI).
  4. Normalize placement: if the object's local +Z points away from the sweep (origin at the max-local-Z end), flip 180° about local X and rebuild the extrusion via add_profile_representation so local Z runs 0 → depth from the (unchanged) origin. This is what stops extend_profile flipping the origin.
  • Idempotent / repair-capable: re-running adds a missing usage to an already-un-mapped occurrence. Only identity mapping transforms are handled (others are skipped).
• bim.make_profile_length_type_driven (re-map; MakeProfileLengthTypeDriven): drop the occurrence's own usage, then ifcopenshell.api.type.map_type_representations to map the type's shared geometry back on. If the type has no RepresentationMap (Bonsai-authored profile type), first promote a copy of the occurrence's body onto the type as a RepresentationMap (so the first toggled occurrence defines the type's length; siblings snap to it).
• UI toggle (type/ui.py draw_product_ui, type/data.py, type/prop.py): a single Per-instance Length checkbox. Backed by a get/set BoolProperty (length_per_instance) — get reads the current mode from cached TypeData flags (is_typed_length_profile / can_make_length_type_driven), set runs the matching operator. No stored state to desync.
• New occurrences default to per-instance (DumbProfileGenerator.create_profile): after assign_type, drop the inherited mapped reps so only the per-instance extrusion remains. No-op for types without a RepresentationMap.
• recreate_profile mapped guard (DumbProfileJoiner.recreate_profile): if the body is mapped, skip the per-instance rebuild (leaving typed geometry alone — a length-driven type should stay typed) but switch_representation to reload so the Blender mesh reflects a just-mapped type's geometry. This kills the "assign a length-driven type ⇒ Failed to set value + sibling turns empty" cascade and keeps the typed display fresh. Fixes issue #8656.
• assign_type preserves per-instance (core/type.py): if the occurrence is already per-instance (own non-mapped body + own profile usage), pass should_map_representations=False so reassigning a type keeps its own geometry/length instead of converting it to typed. (Side effect: it also keeps its own profile/material; fine when all types share one profile — revisit if reassigning across different profiles.)

Supporting bug fixes (separate from the feature)

• tool/geometry.py change_data — guard has_data_users/delete_data against a None old_data (empty→mesh reload path). Real Bonsai bug, exposed by the re-map reload; filed as #8655 — worth its own commit.
• ifcopenshell/util/placement.py get_axis2placement — numpy-2.x x.resize(3) fix for 2D RefDirection. Duplicate of open PRs #8307 / #8586 — kept locally only so profile editing works during testing; do not commit, drop when #8307 merges.

Dead ends (ruled out)

• Auto-un-mapping during type.assign_type/regenerate_profile → sibling cascade. Un-map is an explicit action only.
• Deriving depth from obj.bound_box for un-map → unreliable when several instances share a Blender mesh; use the copied extrusion / native rebuild instead.
• Blaming the compiled wrapper / core (3e7b739) for the profile crashes — it was our code.

Test checklist / what's left before merge

• Strip debug prints: make_length_per_instance / make_length_type_driven, DumbProfileJoiner.recreate_profile + get_profile_axis, and core/type.py assign_type.
• Un-map a mullion → own length, Length UI, geometry unchanged, siblings untouched.
• extend_profile('T') on an un-mapped mullion → origin stays (no flip).
• Toggle checkbox both ways → round-trips; typed snaps to type length.
• New occurrence of a mapped type → defaults to per-instance.
• Assign a length-driven type to a typed occurrence → adopts type length, no cascade.
• Assign another type to a per-instance occurrence → stays per-instance, keeps length.
• Edge cases untested: non-identity mapping transforms; non-centroid cardinal point vs the 180° flip; reassigning across types with different profiles.
• Commit layout: feature on 1c421d0; separate change_data fix; exclude placement.py.