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Drawing: validate section geometry contracts (#2053) (#2085)

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# Drawing foundations: first geometry slice
This is the W00 bootstrap ledger for the [published Drawing plan][plan] in
[#860][rfc]. The first implementation is W01a, [#2053][w01-issue] /
[PR #2085][w01-pr], owned by `0xlillith`: neutral section-geometry records,
structural validation and conversion to existing planar types. Its concrete API
and limitations are in [the geometry contract](drawing-geometry.md).
W01a is part of W01, not completion of W00, the full W01 contract, Stage A, or the
Drawing workspace. It does not extract a body section, calculate visibility,
register `inspect.section_curves` or `drawing.view`, export a drawing, or save a
Drawing document. The independent formatter slice W09a is tracked in
[#2054][w09-issue] / [PR #2086][w09-pr]; it is not a module supplied by W01a.
## Scope, sources and status
The initial scope was checked against upstream `f7343e9161bc6243cdb076d3b6c3594db97279ff`.
Recheck the implementation revision, current claims, [AGENTS](../AGENTS.md) and
[CONTRIBUTING](../CONTRIBUTING.md) before taking the next slice. Local `plan/`
documents were unavailable when this work began; this file does not reconstruct
them or replace project instructions.
The published plan reconciles the [original RFC][rfc],
[SychPL's investigation][visibility], [NOWN-DEV's two-stage proposal][fabrication]
and CamWight's [study and questionnaire][study-comment]. The [attachment][study]
retains the detailed requirements, alternatives and sources. Product research is
requirements evidence; its descriptions and proposed standards rules are not
implementation code, independently verified conformance or permission to copy
protected content.
Here, **W01a** means code in this slice; **future** means a requirement still to
implement or decide. A merged representation or validator does not establish a
geometry producer's capability. Drawing milestones in the RFC are distinct from
the repository roadmap's M0-M14; the Drawing workspace remains roadmap M11.
## First-section requirements and fixtures
Fixture IDs below identify checks, not saved designs or registered commands.
The current fixture details and future producer gates are in
[the geometry contract](drawing-geometry.md#fixtures-and-handoff).
| ID | Requirement and source | Package | Fixture / status |
| --- | --- | --- | --- |
| S01 | Model mm/radians, explicit plane axes and correct XZ sign; plan §4.1/W01 | W01a | F01: plane validation and coordinates; W01a |
| S02 | Reuse supported analytic/conic/Bézier representations without silently fitting them; plan §4.1/W01 | W01a, then W02-W03 | F02: representation/serde checks; W01a. Producing exact sections is future |
| S03 | Keep accuracy categories explicit; plan §4.1/W01 | W01a, then W02-W03 | F02/F04: analytic/bounded/sampled metadata and finite values; W01a. Evidence for producer bounds is future |
| S04 | Indexed curves, oriented wire uses and region references; plan W01 and study §§10-12 | W01a | F03: manually constructed rectangle with a hole, orientation-preserving conversion; W01a |
| S05 | Certify closed section boundaries, winding, nesting, material regions and areas; plan W03 | W03 | F05/F06: body-derived hole/oblique-cylinder sections; future. PR #2068 proposes producer tests for review. Structural validation alone does not meet this requirement |
| S06 | Empty, tangent, open, coincident, degenerate and unsupported cuts are distinguishable; plan W03 | W01 follow-up, W02-W03 | F07: producer outcomes; future. Current empty vectors/contact points are only representable data |
| S07 | Complete source/body/occurrence identity, provenance and freshness; plan §4.1/W01, NOWN-DEV Stage A | W01 follow-up, W02-W04 | F08: repeated placements and failure in one selected body; future. Current indices are result-local |
| S08 | Bound input/work and fail without mutation or partial success; project rules, plan W01-W04 | W01a, then W02-W04 | F04: structural bounds; W01a. Byte/allocation limits, producer work budgets and cancellation are future caller responsibilities |
| S09 | Standalone read-only `inspect.section_curves`, existing plane forms, explicit selection and no sketch/viewport prerequisite; NOWN-DEV Stage A | W04 after W03 | F09: command/CLI/MCP parity, unchanged document/undo and error cases; future |
| S10 | SVG/DXF with units, source groups/layers, declared format accuracy and atomic optional file output; NOWN-DEV Stage A | W04, W10 geometry subset | F09: physical coordinates and complete output; future |
| S11 | One section provider shared with the visibility investigation; SychPL/NOWN-DEV and plan W03/W05 | W02-W06 | F05-F08 and a reviewed capability matrix; future |
| S12 | Transient dimensioned snapshots remain distinct from associative drawings; NOWN-DEV Stage B | W07-W08 | Future snapshot/measurement/output fixtures after their contracts settle |
The remaining industrial requirements are retained in the published plan's
coverage ledger and W09-W47: formatting, persistent sheets/references, dimensions,
BOM/balloons, standards, derived views, production output and specialist domains.
Expanding this bootstrap into atomic traceability for all 48 packages is remaining
W00 work; absence from this first-section table does not remove a requirement.
## Dependencies, ownership and review
Claim links below identify the bounded slices already tracked here. Other rows do
not reserve work or assign the contributors who proposed it. Check live issues and
draft PRs before claiming. Review columns name the expertise needed, not reviewers
who have already agreed to an assignment.
| Package | Depends on | Ownership / scope here | Review and next handoff |
| --- | --- | --- | --- |
| W00 | RFC and project rules | Bootstrap in W01a; full ledger/decision ownership remains open | Integration review: preserve sources, unresolved decisions and child-issue boundaries |
| W01 | W00 immediate scope | `0xlillith`, [#2053][w01-issue]/[#2085][w01-pr], W01a only | Geometry/kernel and engine review: validate representation limits; finish source/provenance/outcome contracts before producer handoff |
| W02 | Reviewed W01 producer contract and backend placement | Review overlap with NOWN-DEV's [#2068][section-pr] before another claim | Kernel/solidcore review: guarded live-body adapter, supported surfaces, complete source identity and no cache-byte authority |
| W03 | W02 | Review/reuse producer proposal [#2068][section-pr]; exact supported subset remains to be agreed | Geometry/document review: analytic cuts, classified outcomes, winding/nesting/material checks and error evidence |
| W04 | W03 and W10 geometry-only subset | NOWN-DEV's [#2068][section-pr] proposes Stage A command/output; not merged at the recorded review | Engine/IO review: explicit selection, request limits, no mutation, physical SVG/DXF, DTO adaptation and complete-or-error output |
| W05 | Reviewed W01; W02 where extraction is needed | Coordinate with [SychPL's offer][visibility]; this is not an assignment | Geometry/kernel review: bounded HLR investigation and findings in the RFC before a substantial backend proposal |
| W06 | Reviewed W05 findings and W02 | Future projection/visibility slice | Geometry/document review: supported scene-wide visibility, split intervals, provenance and cache keys |
| W07 | W01, W09; W03 for cuts; an agreed preview adapter or W06 | Future transient `drawing.view` | Engine/presentation review: meaningful measurements, snapshot status and explicit approximation |
| W08 | W07 and W10 text/dimension subset; W04 adapters as reused | Future preview and SVG/DXF/PNG slice | UI/IO review: shared navigation/layout, output scale and UI/CLI/MCP parity |
Stage A does not depend on persistent Drawing storage, PDF, fonts, a drafting
profile or the W09 formatter. W10's geometry-only primitives are a separate
increment from its text/dimension layout. W09a's [independent PR][w09-pr] can advance
in parallel without introducing a dependency into W01a.
NOWN-DEV's [PR #2068][section-pr] is an overlapping Stage A producer/export
proposal to review before further W02-W04 claims. It was open and ready, not
merged, when inspected at head `4e945d8af594b8cc252cc4e5ffede3684b7bf3bd` on
2026-10-17. It proposes `doc/project_section.rs` and the
`inspect.section_curves` engine command with mesh/fitted loops and SVG/DXF.
Its body-name/loop JSON and `sweep_deg` angles do not imply compatibility with
W01a's radians, accuracy records and indexed regions. Agree an explicit adapter,
accuracy and source/provenance contract before integration; preserve that
contributor's claim and branch. Its tilted-cylinder test is proposed producer
evidence to review, not an implemented W01a or current-main capability.
## Decision ledger
An **implemented slice choice** below describes W01a, not approval of every
proposal in the RFC. Record later decisions with their evidence, reviewer, date,
affected contract revision and dependent child issues.
| Decision | Current disposition | Next owner/review and gate |
| --- | --- | --- |
| D01: representation placement | W01a uses existing `geom::Plane`, `Seg2`, `Loop2` and `Region2`; neutral section records live at L0 | W01a geometry review. This does not decide the section/HLR algorithm's kernel placement |
| D02: identity/status scope | W01a has only indices within one value and per-curve accuracy metadata; no persistent source names, provenance or freshness record | W01 follow-up with kernel/document/engine reviewers before W02-W04 depend on source contracts |
| D03: validity claim | W01a validates structure and preserves supplied orientation. It does not certify winding, nesting, self-intersections or a material section | W03 owns producer/geometric validity evidence; callers must not promote structure checks into manufacturing certification |
| D04: shared producer/backend | One reusable section provider is the plan; guarded extraction and substantial HLR placement must be coordinated with [solidcore][solidcore] | Kernel/solidcore and integration review before W02/W05 expand into a substantial backend |
| D05: supported geometry/accuracy | Representation support is documented; producer surface/curve coverage and defensible approximation bounds remain future | W02-W03 publish per-case capabilities and F05-F08 results; W05 does the same for visibility |
| D06: public commands | [#2068][section-pr] proposes `inspect.section_curves`; transient `drawing.view` remains future. W01a registers neither | Review #2068's distinct output schema before W04 integration; live registry/claim check in W04/W07; preserve existing viewport commands |
| D07: later Drawing choices | Storage/undo, PDF/fonts, standards/profile semantics and release rules remain in the RFC decision register | Their relevant later packages decide them; none blocks this representation slice or standalone section work |
## Completion and handoff
Each subsequent child issue should state supported cases, owned paths, contract
revision, exact dependencies, analytic expectations, error behavior and required
cross-lane review. Follow current draft-claim, isolation and quality-gate rules;
use `Refs #860` and close the bounded child issue rather than the umbrella RFC.
Report what was actually tested at the proposed merge revision. A structure-only
API has no new CAD result to photograph; later producer, command and UI changes
need real app evidence when their effect can be shown. Keep generated reference
documentation, assets and native-format changes outside this slice. Root project
instructions govern any later change to those areas.
[rfc]: https://github.com/tangency-labs/solidcraft/issues/860
[plan]: https://github.com/tangency-labs/solidcraft/issues/860#issuecomment-6234779815
[visibility]: https://github.com/tangency-labs/solidcraft/issues/860#issuecomment-6167030912
[fabrication]: https://github.com/tangency-labs/solidcraft/issues/860#issuecomment-6231770565
[study-comment]: https://github.com/tangency-labs/solidcraft/issues/860#issuecomment-6216908058
[study]: https://github.com/user-attachments/files/33548772/inventor-drawings-research.md
[w01-issue]: https://github.com/tangency-labs/solidcraft/issues/2053
[w01-pr]: https://github.com/tangency-labs/solidcraft/pull/2085
[w09-issue]: https://github.com/tangency-labs/solidcraft/issues/2054
[w09-pr]: https://github.com/tangency-labs/solidcraft/pull/2086
[section-pr]: https://github.com/tangency-labs/solidcraft/pull/2068
[solidcore]: https://github.com/tangency-labs/solidcraft/issues/1597
docs/drawing-geometry.md
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# Neutral section geometry (W01a)
W01a, [#2053][issue] / [PR #2085][pr], adds the L0
[`solidcraft_geom::section`][section-source] representation and structural validation. See the
[foundation ledger](drawing-foundations.md) for dependencies and the remaining
W01 work. This contract is deliberately smaller than the eventual section and
projection result described in the [Drawing plan][plan].
The module can hold geometry supplied by a caller, validate its structure and
convert selected closed wires/regions to existing `Loop2`/`Region2` values. It does
not intersect model bodies, determine visibility, certify material regions,
attach persistent references or register an engine command. No native design
format, exporter, formatter or new runtime dependency is introduced here.
## Coordinates and frames
Distances and coordinates are millimetres; angular values in `Seg2` are radians.
The model is Z-up. Each result contains the existing [`Plane`][plane-source]:
```text
world(u, v) = origin + u * x + v * y
normal = x cross y
```
`validate_plane(&Plane)` checks a supplied frame without normalizing it. Origin
and both axes must be finite, with every component's absolute value at most
`1e12`. Each axis must be unit length within `1e-8`, and their absolute dot product
must be at most `1e-8`. The normal is derived from the axes; no separate normal or
handedness flag is serialized.
The existing named frames are:
| Plane | Local x | Local y | Derived normal |
| --- | --- | --- | --- |
| XY | +X | +Y | +Z |
| XZ | +X | -Z | +Y |
| YZ | +Y | +Z | +X |
In particular, XZ's local positive y points toward **negative world Z**. Replacing
it with +Z would reverse the normal. `Plane::flipped` reverses an in-plane axis
and therefore the derived normal; validation does not choose an orientation.
The future Stage A request adapter must preserve the existing
[`inspect.section`][section-command] conventions: `offset` follows the named
plane's normal; `at` is the world Z/Y/X coordinate for XY/XZ/YZ respectively;
`offset` and `at` are mutually exclusive; `flip` changes the cut direction without
moving the requested plane. These request fields are not part of W01a's DTO.
Mapping a point already on the plane to local coordinates and back is meaningful.
Mapping an arbitrary 3D point to two coordinates loses its normal component; it
does not have a unique 3D inverse.
## Data and serialization
| Type | Fields and meaning |
| --- | --- |
| `SectionGeometry` | `plane: Plane`, `curves: Vec<SectionCurve>`, `wires: Vec<SectionWire>`, `regions: Vec<SectionRegion>`, `contacts: Vec<Vec2>` |
| `SectionCurve` | `geometry: Seg2`, `accuracy: CurveAccuracy` |
| `SectionWire` | Ordered `curves: Vec<CurveUse>` and explicit `closed: bool` |
| `CurveUse` | `curve: u32` indexes `SectionGeometry.curves`; `forward: bool` selects its direction of use |
| `SectionRegion` | `outer: u32` indexes a wire; `holes: Vec<u32>` indexes its supplied hole wires |
All indices are **local to this one value**. They do not identify a model body,
face, occurrence, revision or an entity across recomputation. W01a has no source
fingerprint, provenance, freshness, persistent-ID or public schema-version field.
Those require a reviewed producer/integration contract in a later W01 slice.
The new section DTOs reject unknown fields through serde. Existing `Plane`,
`Vec2`, `Vec3` and [`Seg2`][segment-source] retain their existing serialization:
vectors are numeric arrays; plane axes are `x_dir` and `y_dir`; segment variants
use a `type` tag with snake-case names. Do not apply a blanket strict-schema claim
to the reused nested types.
`Seg2` represents lines, circular arcs, cubic Béziers, rational quadratic conics
and supported higher-degree polynomial Bézier pieces. This is not every curve in
the neutral B-rep schema, nor evidence of an exact producer for each kind. An
adapter must preserve supported geometry or report its actual conversion and
accuracy; it must not silently coerce unsupported B-rep curves into this shape.
`CurveAccuracy` uses the internally tagged serde field `kind`:
| JSON form | Meaning supplied by the producer |
| --- | --- |
| `{"kind":"analytic"}` | The producer declares an analytic representation without fitting |
| `{"kind":"bounded","max_deviation_mm":0.001}` | The producer claims a deviation bound in model millimetres |
| `{"kind":"sampled","max_sample_deviation_mm":0.001}` | The recorded deviation applies to evaluated samples; it is not a bound between samples |
Validation checks the metadata's numeric domain. It does not independently prove
an analytic classification or a claimed bound against a source surface. Accuracy
is per curve, not a global `exact: true` flag. There is no implicit default tag.
In particular, a requested fitting tolerance is not an established bound, and a
zero sampled deviation remains sampled.
This complete value shows the actual serde spelling for one open line. It is
data, not a request to an implemented Drawing command:
```json
{
"plane": {
"origin": [0, 0, 0],
"x_dir": [1, 0, 0],
"y_dir": [0, 1, 0]
},
"curves": [
{
"geometry": {"type": "line", "a": [0, 0], "b": [10, 0]},
"accuracy": {"kind": "analytic"}
}
],
"wires": [
{"curves": [{"curve": 0, "forward": true}], "closed": false}
],
"regions": [],
"contacts": []
}
```
The value can pass structural validation. Converting wire 0 to a `Loop2` must
fail because it is explicitly open. Merely meeting endpoints does not change a
wire's `closed` flag. Contact points are representable, but W01a does not classify
them as tangent intersections or certify their source.
## Validation and conversion boundary
The public operations are `validate_plane(&Plane)` and
`SectionGeometry::validate_structure(tol)`, `to_loop(wire, tol)` and
`to_region(region, tol)`. Errors use the module's `Invalid` error type. There is
no geometric `bounds()` API or batch region-expansion API in this slice.
`tol` must be finite and in `1e-9..=1` mm. It is a structural endpoint-connectivity
tolerance, separate from each curve's accuracy metadata and from future model
intersection tolerances, paper deviation and dimension display precision.
The public `MAX_ITEMS` is `100_000`. Validation counts the aggregate of curves,
wires, regions, contacts, all curve-use references and all hole references before
walking the geometry. Counting references prevents a small curve set from hiding
an unbounded number of uses. This limit is not a maximum JSON byte size.
Structural validation checks the frame, numeric domains, finite curve/contact
data, bounded collection counts and local reference integrity. It checks all
serialized Bézier control slots, including unused tail slots. Every wire must be
nonempty; oriented consecutive endpoints must meet within `tol`. A wire declared
closed must also meet last-to-first. Region boundaries must reference closed
wires; a region cannot list its outer wire as a hole or repeat the same hole.
Different regions may share a wire.
The arithmetic domains are explicit, not a geometric precision certificate:
| Stored value | Accepted domain |
| --- | --- |
| Coordinates and control slots | Finite; absolute component at most `1e12` mm |
| Arc radius | `1e-12..=1e12` mm |
| Arc start | `-1e6..=1e6` radians |
| Absolute arc sweep | `1e-12..=2*pi + 1e-12` radians |
| Conic weight | `1e-12..=1e12`; only positive weights are supported here |
| Higher-degree Bézier `n` | `4..=9`, using the existing ten-slot control array |
| Bounded/sample deviation | Finite, `0..=1e12` mm |
Line and Bézier/conic controls must not all coincide; evaluated segment endpoints
must be finite. These tests do not certify a minimum physical feature size or a
small absolute arithmetic error at extreme magnitudes. In particular, the stored
coordinate limit is not a computed geometric bounding box.
`to_loop` and `to_region` revalidate the entire value, check the selected index and
preflight the selected output's segment expansion before conversion. A
`forward: false` use copies the reversed segment. They preserve the supplied wire
orientation, outer/hole classification and geometry; they do not fit, repair,
close or correct a boundary's winding to make it acceptable. Repeated conversions
repeat validation. Callers planning many conversions must account for that work;
this API does not provide a shared validation certificate or a batch result.
Conversion also checks the actual converted loop's closure with the same `tol`.
Reversing an arc computes a new start angle; at large radii, floating-point
rounding can make its evaluated endpoints disagree even when the original wire
passed validation. The converter then returns
`converted loop is not closed within tolerance`; it does not snap endpoints.
`Loop2`/`Region2` contain geometry only, not `CurveAccuracy` or the result-local
indices. A caller that needs those records must retain the original metadata;
conversion neither carries an accuracy certificate nor promotes accuracy.
**Structural success does not certify winding, nesting, self-intersection,
separation of regions or a physically valid material section.** Converters do not
establish those properties either. The future section producer must supply and
verify outer-CCW/hole-CW boundaries in `(x,y)` coordinates, nesting, signed/net area
semantics and legitimate material regions. A result may be structurally valid
while violating those future geometric requirements.
Existing profile area and containment methods also have their own curve-specific
limitations; converting to `Loop2` or `Region2` does not make conic/freeform area or
containment exact or certified.
Deserialization does not call structural validation automatically. Validate before
use or serialization; the public data may have changed since an earlier check.
Serde can allocate vectors before these validators run. A future file, command
or transport adapter must impose byte, allocation, nesting and request limits
before or during decoding, then call structural validation before downstream
geometry operations. Producer traversal/iteration budgets, cancellation and
complete source selection are also future caller responsibilities. None is
provided by the `MAX_ITEMS` check after deserialization.
## Fixtures and handoff
These labels connect to the [requirement ledger](drawing-foundations.md).
Current checks exercise manually supplied representation values, not extracted
model geometry; see [`section_tests.rs`][tests-source]. Test results belong to the
tested PR revision, not to this list.
| Fixture | Current check or remaining acceptance gate |
| --- | --- |
| F01: frames | W01a: finite/orthonormal plane validation, XZ sign and plane-coordinate mapping. Degenerate, nonfinite and out-of-range frames fail |
| F02: representation/accuracy | W01a: supported segments and actual serde shape; accuracy numeric-domain checks; sampled error remains distinct from a bound |
| F03: rectangle with a hole | W01a: manual `100 x 60 mm` rectangle and radius-5 circular hole at `(50,30)`, in `Plane::XY.offset(10)`. Conversion preserves signed areas `6000` and `-25*pi`, and net area `6000 - 25*pi mm²`. It is not a body-section or nesting certificate |
| F04: malformed structure/conversion | W01a: invalid indices, disconnected/empty wires, closure, invalid region references, excessive aggregate items, nonfinite data, malformed curves and post-conversion endpoint agreement; no mutation |
| F05: plate through a hole | W03: a `100 x 60 x 20 mm` box with a radius-5 through hole parallel to Z, cut by an XY plane strictly between its end faces, has section area `6000 - 25*pi mm²`; producer must identify and certify outer/hole boundaries |
| F06: tilted cylinder | W03: radius 10, axis Z, plane normal 30° from Z, away from caps: ellipse semiaxes `10` and `10/cos(30°)`, area `100*pi/cos(30°)`. Analytic representation or justified conversion evidence is required |
| F07: section outcomes | W03: empty/tangent/coincident/open/degenerate/unsupported cases, with explicit outcome semantics rather than a guessed empty success |
| F08: source completeness | W02-W04: repeated/nested/mirrored occurrences retain source identity; one unsupported selected body cannot silently disappear from a complete result |
| F09: command/output | W04: explicit plane/selection, same CLI/MCP/engine result, no model/undo mutation, complete physical SVG/DXF output and bounded cancellation/error behavior |
F03 is `line_arc_region_conversion_preserves_winding_and_analytic_area`: one
rectangle edge is stored backwards and used in reverse; two semicircular arcs
are used in reverse order/direction to form the hole. It checks orientation and
unchanged input as well as these analytic line/arc areas. In contrast,
`connected_bow_tie_and_wrong_winding_are_not_certified_or_silently_repaired`
deliberately accepts and preserves structurally valid but geometrically unsuitable
boundaries. Neither test extracts a cut from a solid.
`reversed_large_arc_conversion_rechecks_actual_connectivity_without_snapping`
checks the converted result too: a reversed radius-`1e12` arc with start `0.1` and
sweep `1.0` radians fails at `tol=1e-9` mm but is accepted at `0.01` mm. This is an
arithmetic/closure regression, not evidence of that accuracy against a source.
[NOWN-DEV's PR #2068][producer-pr], reviewed as an open proposal rather than
merged code, contains Stage A producer/export work and a tilted-cylinder test.
Review that evidence and its actual mesh/fitting guarantees before reusing it.
Its body-name/loop/`sweep_deg` output is a distinct schema; W01a supplies no adapter
and promises no automatic compatibility. Agree conversion, accuracy and source
identity handling with that contributor before further overlapping claims.
W01a can unblock review of representation and structural invariants. W02-W04 still
need the producer contract for source/provenance, outcomes, completeness and
accuracy, plus an agreed kernel/solidcore boundary. Persistent Drawing files,
fonts, profiles and PDF are not prerequisites for standalone section work.
[issue]: https://github.com/tangency-labs/solidcraft/issues/2053
[pr]: https://github.com/tangency-labs/solidcraft/pull/2085
[plan]: https://github.com/tangency-labs/solidcraft/issues/860#issuecomment-6234779815
[producer-pr]: https://github.com/tangency-labs/solidcraft/pull/2068
[section-source]: ../crates/geom/src/section.rs
[tests-source]: ../crates/geom/src/section_tests.rs
[plane-source]: ../crates/geom/src/plane.rs
[segment-source]: ../crates/geom/src/profile.rs
[section-command]: ../crates/engine/src/cmd/section.rs