cadcraft/docs/geometry-parity.md

Geometry parity

Last reviewed: 2026-10-10 · Last updated: 2026-10-11 · Change: minor (constraint solver: DOF report, G2 Smooth, ellipse/spline/block geometry, CONSTRAINTINFER, #546) · Target: Autodesk AutoCAD 2027

CAD users trust numbers: a trimmed endpoint, an offset, an intersection or an area is either right to the last displayed digit or the tool is unusable for production drawings. This document audits CADCraft's geometry kernel (crates/geom, the curve code in crates/engine/src/cmd, crates/constraints) curve by curve against what AutoCAD users rely on. Snaps and tracking are in ui-parity.md; dimension and hatch behaviour in gaps.md.

Headline (estimated): geometry ≈ 40% (weighted below: 41%), of which the constraint solver ≈ 50%. Everything built from lines, arcs and circles is exact. Ellipses and splines are handled by tessellating them, so intersections, offsets, trims and snaps on them are approximations; polyline offsets don't recompute arc bulges or clean up loops; tolerances are absolute, so very large or very small drawings behave differently from mid-sized ones. 3D geometry doesn't exist. Remaining: ≈ 90–140 Opus 5.5 hours for 2D precision and the solver, plus the 3D kernel (M11, see target-app-parity.md).

How this was measured

Read from the source on 2026-10-10 (main at bd2412f), with file:line evidence in the tables. No numeric comparison against AutoCAD has been run yet: the plan is a set of synthetic drawings whose exact answers are known analytically, measured in both apps black-box (LIST, ID, AREA results) and in CADCraft headless. Until then the percentages are judgements.

Weights (how much AutoCAD users lean on each area in 2D drafting): numeric robustness 15%, curve representation 10%, intersections 15%, offsets 15%, trim/extend/fillet/chamfer/break/join 15%, measurement (closest point, length, area, MASSPROP) 10%, hatch boundaries 10%, constraints 10%.

Numbers and tolerances (≈ 30%)

Item	AutoCAD	CADCraft	Evidence
Representation	IEEE double, 3D throughout	f64, 2D kernel (Vec2); Z kept only on entity points and 3D polylines	geom/src/vec.rs
Tolerance model	Relative to geometry (the ACIS/AcGe tolerance context), stable at survey coordinates	one absolute EPS = 1e-9, plus ad-hoc absolute 1e-12/1e-14 in intersections and 1e-3/1e-4 world units for tessellating curves; a few relative tolerances (line–line denominator, hatch, solver)	geom/src/lib.rs:26, intersect.rs:8,19,34,50, cmd/modify.rs:469-474, snap.rs:98-110
Robust predicates	Exact orientation decisions where it matters	none	—
Large coordinates (UTM, 1e6–1e7)	Works	untested; absolute tolerances will misjudge tangency and coincidence, and absolute tessellation steps create huge sample counts that hit caps	—
Point merging (JOIN, PEDIT Join fuzz, region)	Fuzz distance per command	JOIN 1e-9 absolute; PEDIT has a fuzz; region grid-key rounding at tol	modify.rs:419, region.rs:42
Display precision (LUPREC, AUPREC, units)	Decimal, Engineering, Architectural, Fractional, Scientific; 5 angle formats	units formatting and parsing of all of them	engine/src/units.rs

Curves (≈ 60%)

Curve	AutoCAD	CADCraft
Line, ray, xline	yes	yes, exact
Circle, arc, bulge polyline	yes	yes, exact
Ellipse, elliptical arc	yes	yes (center, major axis, ratio, parameters)
Spline: CV form, rational, any degree	yes	yes: rational B-spline, basis-function evaluation
Spline: fit points	degree 3 with end tangents, knot parameterisation (chord, square root, uniform), fit tolerance	cubic, chord-length, averaged knots, dense solve; no end tangents, no knot parameterisation choice, no fit tolerance (geom/src/spline.rs:31-88)
Spline: closed/periodic	periodic	"closed" appends the first point and refits; not periodic (modify2.rs:647)
Spline editing: knot insertion, split, refine, rebuild, convert to polyline	SPLINEDIT, grips	knot insertion and splitting (curves.rs:539-622); refine/rebuild partial
Helix	yes	no
3D polyline	yes	stored and drawn; no 3D operations
Regions, 3D solids, surfaces, meshes (ACIS/B-rep, NURBS surfaces, subdivision)	yes	no (M11)

Intersections (≈ 40%)

Pair	AutoCAD	CADCraft
line–line, line–circle/arc, circle–circle	exact	closed form, exact; tangency decided by a fixed `
line/arc–ellipse	exact (quartic)	tessellated: 1e-4 world units in TRIM, 1e-3 in snaps; TRIM refines with 720 samples + 60-step bisection, near-exact against line/arc cutters (modify2.rs:1946-1995)
line/arc–spline	exact to tolerance (subdivision + Newton)	tessellated as above
ellipse–ellipse, ellipse–spline, spline–spline	exact to tolerance	polygon approximations only
Self-intersections (curves, offset loops)	yes	no routine
Apparent intersection (projected)	yes	treated as plain intersection

Offsets (≈ 30%)

Curve	AutoCAD OFFSET	CADCraft (cmd/modify.rs:276-424)
Line, ray, xline	exact	exact
Circle, arc	exact	exact (radius change)
Ellipse	spline approximation within tolerance	a polyline from tessellation at 1e-3 × major axis
Spline	spline approximation within tolerance	tessellate at 1e-3, offset vertices along normals, keep about 40 points, refit: lossy
Polyline with arcs	exact, with self-intersection trimming, OFFSETGAPTYPE (extend, fillet, chamfer)	segments offset and joined at neighbours' intersections; arc bulges not recomputed after endpoints move (comment at L413), no loop cleanup, collapsed segments not removed; OFFSETGAPTYPE only echoed
Options: Through, Erase, Layer, Multiple	yes	see gaps.md

Trim, extend, fillet, chamfer, break, join, lengthen (≈ 70%)

Command	AutoCAD supports	CADCraft supports
TRIM	every curve type, quick and standard modes	line, arc, circle, polyline; ellipse and spline via sampled cutting (modify.rs:485-656, modify2.rs trim_other); a closed spline leaves one open spline
EXTEND	every open curve	line, arc, polyline, elliptical arc, open spline (runs on along its end tangent, modify2/curve_edits.rs)
FILLET	lines, arcs, circles, polylines, ellipses, splines, xlines, rays; Trim/No trim	lines, arcs, circles (TTR circle), polyline mode, Trim/No trim, Multiple; elliptical arcs and splines with any of these (tangent circle from sampled offsets refined by Newton's method, geom/src/fillet.rs; arcs trimmed or extended round the ellipse, splines split at the touch point); two straight segments of one polyline (adjacent, or with one between that the corner replaces), a polyline's straight end segment with a line (the line joins it); not polyline arc segments, xlines or rays
CHAMFER	lines, polylines, xlines, rays	lines, Distance/Angle, whole polylines (Polyline option), polyline segments as for FILLET (modify2/fillet_pline.rs); refuses arcs, ellipses and splines with a message, as AutoCAD; not xlines or rays
BREAK	every curve	line, arc, circle, polyline
JOIN	lines, arcs, polylines, ellipses, splines, helixes	line, arc, polyline into a polyline; elliptical arcs on one ellipse into a larger arc or the full ellipse (gaps close); splines with lines, arcs, polylines and elliptical arcs into one exact NURBS (degree elevation, rational arcs; geom/src/nurbs.rs); no helixes
LENGTHEN	lines, arcs, polylines, elliptical arcs, splines	line, arc, polyline; elliptical arc (arc length along the ellipse); open spline (cut along the curve, or run on along its end tangent)

Where line and arc are supported the results are exact.

Measurement (≈ 40%)

Quantity	AutoCAD	CADCraft
Closest point: line, arc, circle, polyline	exact	exact
Closest point: ellipse, spline	exact to tolerance	sampled (curve.rs:264, curves.rs:637)
Tangent points, common tangents	every curve	circles and arcs only (curve.rs:170-183)
AREA: circle, ellipse	exact	exact area (ellipse perimeter by Ramanujan, not exact); an open elliptical arc has no area, like an arc
AREA: polyline with arcs	exact (bulge segments)	tessellated at 1e-4 (curve.rs:491)
MASSPROP (regions: area, perimeter, centroid, moments)	exact on regions and solids	shoelace over a tessellated circle, polyline or first hatch loop (utility.rs:310-352); no regions
LIST/DIST/ID precision	full double	full double

Hatch boundaries (≈ 45%)

geom/src/region.rs: curves are polygonised, crossings split on a grid, vertices merged at tol, a ray picks the edge and the face is traced by sharpest left turn; islands by connectivity (including islands made of separate lines, #163). Caps: 60,000 segments, 4,096 islands. Missing: gap tolerance (HPGAPTOL), true arcs and curves in boundaries (boundaries come out tessellated, so a hatched circle's boundary is a polygon), island detection modes beyond the default, boundary sets, and retained associative boundaries for every case. Pattern lines are scanline families clipped against the tessellated loops (render/src/hatch.rs).

Constraint solver (≈ 50%)

Item	AutoCAD	CADCraft (crates/constraints)
Geometric constraints (12)	coincident, collinear, concentric, fix, parallel, perpendicular, horizontal, vertical, tangent, smooth (G2), symmetric, equal	all 12; Smooth is G2 (common point, tangent and curvature, curves.rs) between a spline and a line, arc, elliptical arc, polyline segment or spline; between two non-spline curves it stays tangency
Dimensional constraints	linear, horizontal, vertical, aligned, angular, radial, diameter; dynamic and annotational; reference	distance (aligned/horizontal/vertical), angular, radius, diameter; parameters and expressions (expr.rs)
Geometry	lines, arcs, circles, polylines, ellipses, splines, blocks	line, circle, arc, polyline, point; ellipse and elliptical arc (centre, axis ends and arc ends as points; tangent, coincident, concentric, horizontal/vertical on the major axis); spline (fit or control points as points; tangent and smooth at its ends; open clamped or fit splines); block reference (insertion point, and the block's own points riding rigidly with position and rotation) (model.rs)
Algorithm	proprietary	damped Gauss–Newton / Levenberg–Marquardt, numeric Jacobian, per connected component; MAX_ITER 200, MAX_ROWS 800; curve–curve tangency with an ellipse through auxiliary contact parameters
Over-constraint	refused with a message	redundancy by Jacobian rank (Householder QR with column pivoting, rows and columns balanced), including Fix; the message names the constraints the new one duplicates; conflicts by removal probing (up to 48 probes)
Degrees of freedom, fully constrained state	shown (constraint bars, status)	constraints.dof and constraints.inspect: free parameters minus Jacobian rank per component, per-object DOF from the null space, under/fully/over-constrained status (dof.rs); a status strip on each constraint bar and the drawing's state on the canvas
Inference while drawing (CONSTRAINTINFER)	yes	CONSTRAINTINFER (variable and command): after a drawing or editing command, what it placed exactly onto existing geometry (snaps, ortho, polar) gets Coincident (end, mid, centre, on object), Tangent, Perpendicular, Parallel and Horizontal/Vertical, filtered by the AutoConstrain types (auto.rs infer_placed)

Tests

#[test] counts on 2026-10-10: geom 28, constraints 22, render 50, engine 174. No property-based or fuzz tests of geometry, and no known-answer suite at large coordinates. Hostile-input tests exist for parsing, not for numerics.

What to do first (ranked)

1. Exact ellipse and spline intersection and closest point (subdivision + Newton with a relative tolerance); use them in snaps, TRIM, EXTEND, BREAK. 15–25 h.
2. Polyline offset with bulge recomputation, self-intersection cleanup and OFFSETGAPTYPE; spline/ellipse offset as fitted splines within a tolerance. 15–25 h.
3. A tolerance context relative to the drawing's extents, and a known-answer test suite at origin and at 1e6–1e7 coordinates. 8–12 h.
4. Exact areas and MASSPROP for bulge polylines, regions (2D) and hatch loops. 6–10 h.
5. Hatch gap tolerance and curved boundaries. 8–12 h.
6. Solver: DOF reporting, G2 Smooth, ellipse/spline geometry, constraint inference. Done (#546).
7. Spline fit options (end tangents, knot parameterisation, fit tolerance, periodic). 6–10 h.

Total ≈ 90–140 h; items 1–6 parallelise across two or three agents.

Revision history

Date	Change	Summary
2026-10-10	major	First geometry-precision audit: tolerances, curves, intersections, offsets, modify commands, measurement, hatch, solver
2026-10-11	minor	EXTEND, JOIN and LENGTHEN on elliptical arcs and splines; closed splines trim to one piece; trim/extend section 45% → 55% (#466)
2026-10-11	minor	FILLET with elliptical arcs and splines; trim/extend section 55% → 65% (#466)
2026-10-11	minor	FILLET/CHAMFER with polyline segments (same polyline, end segment with a line); section 65% → 70% (#466)
2026-10-11	minor	Constraint solver (#546): DOF report, G2 Smooth, ellipse/spline/block geometry, CONSTRAINTINFER