solid-diff

Renders visual diffs between revisions of SolidWorks part files (.SLDPRT).

Given two versions of a part, produce image renderings that highlight what changed — added/removed/modified geometry — suitable for PR-style review of CAD changes out of the PDM vault.

Status

We read modern (2015+) SLDPRT files end to end with no SolidWorks, no Windows and no commercial SDK: chunk container → Parasolid B-rep → tessellation → OBJ/STL/SVG. On the full 1536-part vault export, 98.9% of files parse and mesh; the rest are pre-2015 OLE2 containers.

solid-diff diff compares two revisions and colours what changed: unchanged material muted, moved amber, added green, removed red, both sides rendered to one shared scale. Faces are matched geometrically -- by surface type, that surface's parameters and the extent of the boundary -- because SolidWorks' per-face ids turn out to be a feature-provenance chain that no face survives an edit with.

Give diff a .gif and it renders a turntable instead of a still: a full turn about the vertical axis, then a tumble, both ending where they began so the loop is seamless. since walks a PDM backup at any depth, groups the revisions of each part and compares the newest against the last one from before the cutoff, reading dates out of each file's dcterms:modified so that copying the backup around does not disturb them.

Everything lives in the Rust crate under rust/. Tessellation runs through OpenCASCADE: the Parasolid B-rep is exported to STEP (rust/src/step.rs, exact surfaces + sampled boundary polylines) and meshed by OCCT (rust/src/occt.rs), whose shape healing closes trimmed periodic faces properly.

The tests assert invariants the part files state about themselves rather than any recorded reference output — see rust/tests/invariants.rs. For coverage numbers and every known gap, see docs/STATUS.md for coverage numbers and every known gap.

cd rust && cargo build --release
R=target/release/solid-diff

$R scan    part.SLDPRT --streams        # container streams + Parasolid found
$R extract part.SLDPRT -o out/          # carve embedded .x_b transmits
$R mesh    part.SLDPRT -o part.obj --stl part.stl --stats
$R render  *.SLDPRT -o sheet.svg --size 360     # translucent x-ray SVG
$R iso     *.SLDPRT -o sheet.png --size 420     # matte isometric PNG
$R diff    old.SLDPRT new.SLDPRT -o diff.png    # what changed between revisions
$R diff    old.SLDPRT new.SLDPRT -o diff.gif    # ...as a looping turntable
$R since   /pdm/backup --since 2026-07-01 -o out/   # everything that changed

Two rendering styles, for different jobs:

	render (SVG)	iso (PNG)
look	translucent x-ray, feature edges	matte solid, soft shading
shows	internal structure through the part	outward form and surface finish
method	painter's algorithm over exact BSP order	area-sampled point splats, z-buffered
output	vector, scales losslessly	raster, transparent background
cost	grows with triangle count	grows with pixels, not triangles

iso also builds annotated contact sheets — part name, bounding box in cm, triangle count and an auto-scaled scale bar per tile — matching the visual style of the GLB tooling in tamalpais-configuration.

Docs

• docs/STATUS.md — current coverage, mesh-quality numbers, and the full list of open work.
• docs/FORMAT.md — the format spec: byte-level description of everything reverse-engineered, from the container through the Parasolid XT node stream to the tessellation algorithm.
• docs/PARASOLID.md — how the B-rep is embedded and extracted (container streams → zlib sections → Parasolid transmit).
• docs/BREP2MESH.md — the tessellator: curve/surface evaluation, universal trimming, validation results.
• REFERENCES.md — survey of everything else that can read SLDPRT (open-source, commercial, cloud) and prior art in CAD diffing.
• docs/RENDERING.md — notes on the rendering approach.

Layout

path	what
rust/src/container.rs	2015+ chunk container (ROL names, deflate chunks)
rust/src/sections.rs	zlib carving + Parasolid transmit sniffing
rust/src/xt/	Parasolid XT decoder (schema, reader, node records)
rust/src/graph.rs	node graph: topology, attributes, face colours/ids
rust/src/geom/	curve and surface evaluators
rust/src/tess.rs	tessellation (parity trimming, shared edge sampling)
rust/src/render.rs	painter's-algorithm SVG renderer (BSP ordering)
rust/src/iso.rs	isometric point-splat PNG renderer + contact sheets
rust/src/font.rs	5x7 bitmap font for image annotations
tools/	contact-sheet batching, render gallery server
samples/fetch.sh	fetch the public test corpus (16 parts)

Reference repos (openswx, ps-parser, sldprt-format-research) are cloned into vendor/, which is gitignored.