Csg2g

[erikg]

Summary

Bidirectional OpenSCAD converters that preserve CSG structure:

• csg-g — imports OpenSCAD's evaluated .csg format into BRL-CAD .g files
• g-scad — exports BRL-CAD .g objects to .scad files

Both converters map primitives and boolean operations directly to their equivalents rather than tessellating to mesh, producing clean analytic geometry wherever possible.

csg-g (OpenSCAD → BRL-CAD)

.csg input	BRL-CAD output
cube()	RPP
sphere()	SPH
cylinder()	TGC
polyhedron()	ARB4/5/6/7/8 if convex match, else BOT
rotate_extrude(circle())	TOR
linear_extrude(square())	RPP
linear_extrude(circle())	TGC
linear_extrude(polygon())	sketch + extrusion
import("file.stl")	BOT (STL inlined during conversion)
difference/union/intersection/group	boolean combinations
multmatrix()	transformation matrix
color()	color/alpha attributes
render()	pass-through

Polyhedron import detects all ARB types (4–8) with a convexity check — non-convex polyhedra fall back to BOT. Tetrahedra skip the convexity check since they're always convex.

g-scad (BRL-CAD → OpenSCAD)

BRL-CAD input	.scad output
SPH/ELL	sphere() + multmatrix()
TGC/REC	cylinder() + multmatrix()
ARB4–8	polyhedron() (degenerate faces stripped)
BOT	polyhedron()
TOR	rotate_extrude() circle()
PARTICLE	hull() { sphere(); sphere(); }
PIPE	union() of cylinders + joint spheres
REVOLVE	rotate_extrude(angle) polygon()
EXTRUDE	linear_extrude() polygon()
HALF	multmatrix() cube(2e10)
combinations	difference/union/intersection
combination colors	color()
everything else	tessellated via ft_tessellate → polyhedron()

Tessellation uses a fresh rt_db_get_internal copy to avoid heap corruption from ft_tessellate modifying the caller's internal.

Usage

# OpenSCAD to BRL-CAD (two-step via .csg)
openscad -o model.csg model.scad
csg-g model.csg model.g

# BRL-CAD to OpenSCAD
g-scad -o model.scad model.g object_name

Testing

Round-trip tested across 59 BRL-CAD sample models (.g → .scad → .csg → .g):

• 55 of 59 models completed the full circuit successfully
• 43 models had zero warnings in both directions — lossless round-trip
• 4 timeouts on large BREP/DSP tessellation (bishop, terra, 2 NIST models)
• All round-tripped .scad files verified to render in OpenSCAD
• regress-repository passes (API wrapper compliance)
• Full ninja test suite passes (1093/1095, 2 pre-existing non-issues)
• CI passes on Ubuntu GCC, MSVC Ninja, MSVC Standard Tools, macOS Clang

Solid count mismatches in round-trip are structural, not data loss:

• PIPE primitives expand to multiple cylinders+spheres in OpenSCAD (e.g., toyjeep 321→475)
• One hull() node (from PARTICLE) is not parsed back by csg-g

Known Limitations

• hull() and minkowski() nodes in .csg are skipped (no CSG equivalent without shelling out to OpenSCAD)
• linear_extrude with twist parameter has no BRL-CAD equivalent
• Binary STL in import() not yet handled (ASCII only)
• Named hierarchy and region IDs are lost through the .csg format (OpenSCAD doesn't preserve them)
• Transforms get baked into primitives by OpenSCAD's .csg export — geometry is equivalent but representation differs

[starseeker]

@erikg Is this ready to merge yet, or are you still working on it?

[erikg]

@erikg Is this ready to merge yet, or are you still working on it?

ready, just wanted review/confirmation since this is a new capability

[starseeker]

@erikg we're about to release, so we'll review it and plan to merge next week

[brlcad]

Pretty sweet. Just fyi arbs can be planar concave now (it turned them into bots). We're also moving them all into gcv but that can happen later. Would be good to get the ones that failed attached as an issue with detail on what it needs like Minkowski or whatever.