Derive per-path line height and extrusion width from the moves

[sophiedeziel]

Summary

A file that announces no ;WIDTH: / ;HEIGHT: comments gets one global width and height for the whole print, however much its extrusions actually vary. Everything needed to reconstruct them is already in the moves, so this derives them:

• Height is the Z step between consecutive extrusion moves, which makes z-hop travels invisible to the measurement.
• Width comes from conservation of volume — ΔE × filament cross-section / (length × height) — using a rectangular deposit, matching the box profile ExtrusionGeometry actually extrudes.

Deriving is the default for every dialect, not a Cura feature: Cura is the motivating case, but Simplify3D, Slic3r without verbose comments and hand-written gcode all lack dimension comments too. A dialect only opts out when the arithmetic itself does not hold (Cura's UltiGCode flavor, whose E is cubic millimetres of material rather than millimetres of filament).

Precedence per dimension is announced > supplied > derived > built-in default. The slicer stating what it asked the printer for outranks everything; a value you pass to the constructor is a decision and outranks the inference, so the derivation simply does not run for it; deriving fills what is left. That keeps extrusionWidth and lineHeight meaningful options rather than ones the derivation quietly takes over.

That precedence is also what keeps streaming honest. Gating on "this file announced no dimensions" would decide mid-stream: a PrusaSlicer file whose only fingerprint is ;LAYER_CHANGE is identified after its start gcode has extruded a prime line, so a streamed parse would derive for that path while a one-shot parse would not. Nothing flips here, so streamed and one-shot agree by construction.

Stacked on #461, which supplies the true extruded lengths this needs.

Behavior changes

• Files without dimension comments now render with per-path widths and heights instead of the global fallback.
• Files that announce only some dimensions get the rest derived — a PrusaSlicer print's skirt and prime line, emitted before the first ;WIDTH:, now carry a derived width, unless the caller supplied one.
• Header metadata — the filament diameter and the opt-out — is read from the file's first 200 comments rather than from whichever chunk identified the slicer. Cura writes ;FLAVOR: before the line that identifies it, so under streaming those could land in different chunks: Griffin silently lost its 2.85 mm diameter, scaling every derived width by (2.85/1.75)² = 2.65×, and UltiGCode wrongly enabled derivation.

Implausible results are discarded rather than rendered: heights outside 0.01–1.0 mm, widths outside 0.1–2.0 mm, and segments under 0.05 mm where E quantization dominates. Derived widths are quantized to 0.01 mm with a 2% tolerance so rounding noise cannot shatter the model into micro-paths.

Deriving costs nothing on files that announce their widths — the volume arithmetic is skipped while an announced width is in force, which is worth about a third of the interpret time on 3DBenchy.

Public API changes

Two new overridable hooks on SlicerMetadataParser: derivesExtrusionDimensions (defaults to true; a dialect returns false to opt out) and parseFilamentDiameter. Metadata gains optional deriveExtrusionDimensions and filamentDiameter. State is unchanged: the derived dimensions live inside Job, since nothing outside it reads them. Its surface is now pinned in the public API test all the same, being reachable as job.state. Job accepts extrusionWidth / lineHeight, the dimensions the caller supplied, and Job.deriveMoveDimensions is new. No renames or removals.

Screenshots

Five first-layer lines printed at 1.2 / 0.9 / 0.6 / 0.4 / 0.25 mm, same camera:

Before	After

Heights come out of the Z steps, so the tower's three tiers stack visibly differently — 0.3 mm at the bottom, 0.2 mm in the middle, 0.1 mm on top:

Checks

• npm run check passes (test + typeCheck + lint)
• npm run test:coverage — src/ still at 100%
• Labelled appropriately (enhancement)

Assisted by Claude Code - Fable 5

[github-actions]

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