G-STEP(1)

g-step writes selected BRL-CAD objects to an ISO 10303-21 exchange file. It supports AP203, AP203 edition 2, AP214 edition 3, and AP242 editions 1 through 4 when their schema plugins are installed. AP203 is the default; --schema ap242 selects AP242 edition 4.

Named object arguments limit the export. Without them, every top-level object in input.g is selected. The input may be - for standard input. The required -o option names the output, which is replaced if it already exists.

The default path writes B-reps and mechanical product structure for maximum interoperability. Union-only combinations become assemblies with repeated rigid occurrences preserved. Native CSG export is optional for AP203e2, AP214, and AP242. Unsupported content is reported rather than silently approximated; permissive mode may publish useful partial output with status 1, while --strict makes any omission transactional.

The exporter also re-authors supported product identity, presentation, material, property, unit, occurrence, and configuration data retained by step-g. See STEP(5) for detailed representation and metadata coverage.

The ordinary path uses an existing B-rep or a primitive’s B-rep conversion. It does not run the STEP importer’s topology-repair pipeline on an existing BRL-CAD B-rep. A primitive for which neither the requested native mapping nor a B-rep fallback is available is omitted explicitly; an empty placeholder is never written.

Until evaluated B-rep export is available, a non-union Boolean combination cannot be flattened on the ordinary path. It is reported as partial rather than misrepresented as an assembly union. Use --native-csg when its tree and leaves have exact mappings in the selected schema. Operations which cannot be represented without changing their meaning remain failures rather than being approximated.

Permissive output is useful when supported objects should still be exchanged despite explicit omissions. Use --report to audit coverage, or --strict when the file must represent every selected and reachable object and requested metadata value.

With --native-csg, union, intersection, and difference map directly and XOR is lowered to an equivalent expression. AP203e2 and AP214 share mappings for common STEP primitives; AP242 adds exact mappings such as ellipsoid, rectangular pyramid, tetrahedron, and convex hexahedron. Finite unsupported leaves may use an exact B-rep inside an otherwise preserved tree. BRL-CAD NOT and guard/no-op nodes have no lossless finite STEP encoding. See STEP(5) for the complete mapping policy.

Union-only combinations become nested product assemblies. Repeated children remain repeated occurrences, and rigid member transforms become occurrence placements. Scaling cannot be expressed by this product-structure mapping and is reported as an omission.

When a database came from step-g, retained role and source provenance lets the exporter distinguish a product’s representation items from its assembly occurrences. Secondary placed geometry is restored with a transforming representation relationship instead of manufacturing another product usage. With native CSG, non-region union structure remains an assembly while a region or non-union Boolean shape receives its own CSG representation. Embedded CSG operands are not also emitted as unrelated products.

Occurrence identity is retained on the parent combination with guarded step:occurrence:N:…​ attributes. The exporter applies those values only while the recorded child still matches the current tree member, preventing stale imported metadata from moving to another occurrence after edits.

Product names, identifiers, descriptions, revisions, and definition fields are restored from step: object attributes when present; otherwise the BRL-CAD object name supplies a product name and identifier. Part 21 strings preserve apostrophes and Unicode.

AP203e2, AP214, and AP242 can re-author retained colour, transparency, layers, materials, and typed product or material properties. Native BRL-CAD colour takes precedence over retained STEP colour. Supported numeric properties keep their measure type, dimensional exponents, and structured SI, conversion-based, context-dependent, or derived unit graph. Unsupported or malformed metadata is reported rather than emitted with guessed semantics.

Reference-linked approvals, people, organizations, dates, documents, classifications, categories, effectivities, and related records retained by step-g are validated against the target schema before re-authoring. Every source reference is remapped to a newly emitted entity. AP203’s narrower vocabularies, assignments, and mandatory management closure are enforced; records that cannot be translated legally remain explicit omissions. The export report gives each retained configuration node an emitted, unsupported, or malformed disposition.

The exporter does not treat unrelated opaque STEP::…​ attributes as entities, and source IDs are never reused as output Part 21 instance numbers. AP242 PMI retained by the importer is not yet re-authored.

BRL-CAD geometry is stored internally in millimetres. g-step therefore defaults to millimetres, degrees, and a physical uncertainty of 0.05 mm. Changing --output-units rescales coordinates, B-rep control geometry, analytic dimensions, assembly translations, and the default uncertainty together. Recognized SI lengths use SI_UNIT; other recognized lengths use exact CONVERSION_BASED_UNIT values.

An explicit --uncertainty value is interpreted in the selected output unit. A .g database has no authoritative model-wide construction tolerance, so display units and primitive-local tolerances are not silently promoted to a STEP uncertainty. Imported step:source_context:N:…​ attributes remain provenance rather than automatic export instructions; select the output policy explicitly when source settings should be restored.

The JSON report covers every selected and reachable object as handled, deliberately skipped, unsupported, or failed, with its BRL-CAD name and reason. Separate counters summarize products, occurrences, presentation, materials, properties, and retained configuration records. Records recognized but not supported for authoring are explicit omissions, not silent losses.

Status 0 means complete output. Status 1 means permissive partial output was published. Other nonzero statuses mean no STEP output was published; the JSON report, when requested, distinguishes empty input coverage from conversion, input, and output failures.

step-g(1), STEP(5), brlcad(1), mged(1), g-iges(1)

BRL-CAD Team

This software is Copyright (c) 1989-2026 by the United States Government as represented by U.S. Army Research Laboratory.

Reports of bugs or problems should be submitted via electronic mail to devs@brlcad.org