cad-editor/docs/tessellation.md

Entity tessellation paths

How each acadrust EntityType becomes drawable geometry in OpenCADStudio, and whether it passes through the truck modeling kernel.

There are two distinct meanings of "through truck" in the codebase, and they must not be conflated:

1. truck B-rep meshing — the entity is rebuilt as a truck Shell/Solid and triangulated by MeshableShape::triangulation(). This is the only path that produces filled mesh triangles from a kernel.
2. truck curve topology — the entity is expressed as a truck Edge/Wire/ Vertex and sampled into line segments by truck's ParameterDivision1D. It touches truck topology but never triangulates a surface; the output is a polyline.

Everything else is direct: points/segments/triangles are emitted straight from the entity's own data without any truck object.

Entry points: src/scene/convert/tess.rs (tessellate_entity, the per-entity dispatcher) → src/scene/convert/tessellate.rs (tessellate, the TruckObject router) and src/scene/convert/truck_tess.rs (curve sampling).

Summary

Path	Entities
truck B-rep mesh (Shell → triangulation, with a direct per-surface fallback)	Solid3D, Region, Body, Surface
truck curve topology (Edge/Wire → ParameterDivision1D sampling)	Line, Arc, Circle, Ellipse, Spline, LwPolyline, Polyline, Polyline2D, Polyline3D, Point — non-thick variants only
direct (no truck — segments/triangles emitted directly)	everything else

The Model/Design-tab primitives (BOX, SPHERE, CYLINDER, CONE, WEDGE, TORUS, PYRAMID, extrude/revolve/loft/sweep) are also true truck B-rep meshes (truck_tess::tessellate_solid), but they are created as Solid3D placeholders carrying a truck B-rep, not as their own EntityType.

The only call site of shell.triangulation() for an imported entity is acis_to_truck::tessellate_sat_truck. The four ACIS solids dispatch through solid3d_tess::tessellate_acis, which tries that truck kernel first and falls back to a bespoke per-surface LOD sampler (tessellate_sat_lods) when truck cannot rebuild a face — so their path is truck-with-direct-fallback.

Curved-surface tessellation density is radius-relative (CURVE_REL_TOL, a fraction of the surface radius) so a cylinder's facet count matches the circle/arc wire tessellation instead of exploding on large radii.

Full table

Entity	Output	Path	Notes
Arc	wire	direct (truck curve topology)	non-thick: truck Edge (builder::circle_arc) sampled via ParameterDivision1D; thick: direct Lines
AttributeDefinition	wire	direct	routes through the Text/MText LFF glyph stroke pipeline
AttributeEntity	wire	direct	same as AttributeDefinition; values supplied per Insert
Block	none	n/a	block-definition sentinel; not tessellated (referenced via Insert)
BlockEnd	none	n/a	block-definition end marker; no output
Body	mesh	truck-with-direct-fallback	3D ACIS body → truck Shell → triangulation; fallback per-surface sampler
Circle	wire	direct (truck curve topology)	non-thick: truck Wire of two half-circle edges; thick: direct Lines
Dimension	wire	direct	baked-block path recurses on D### sub-entities; synthesis path emits lines/arrows/LFF text
Ellipse	wire	direct (truck curve topology)	truck Wire/Edge over a BSplineCurve, ParameterDivision1D sampled
Face3D	both	direct	edge Lines + direct fan-triangulated fill_tris; no truck B-rep
Hatch	both	direct	boundary outline not emitted to the wire set (#131 OOM); fill rasterized on GPU
Insert	wire	direct	expands block children and tessellates each via its own path; XCLIP filter applied
Leader	wire	direct	leader path + arrowhead + landing, direct Lines
Line	wire	direct (truck curve topology)	non-thick: truck Edge (builder::line); thick: direct Lines
LwPolyline	wire	direct (truck curve topology)	plinegen true: truck Contour with bulge arcs; else direct SegmentedLines
Mesh	both	direct	SubD mesh: edge Lines + direct fan-triangulated fill_tris
MLine	wire	direct	spine + offset lines + caps, direct Lines
MText	wire	direct	wrap-aware multi-line LFF glyph layout; inline formatting codes
MultiLeader	wire	direct	leader + landing + LFF text + frame + fill
Ole2Frame	wire	direct	bounding rectangle + diagonal cross
Point	wire	direct (truck topology)	truck Vertex → cross marker sized by PDSIZE
PolyfaceMesh	both	direct	face list: closed-polyline edges + direct fan-triangulated fill_tris
PolygonMesh	both	direct	M×N grid wireframe + direct fan-triangulated fill_tris
Polyline	wire	direct (truck curve topology)	heavy 3D polyline; truck Contour + bulge arcs
Polyline2D	wire	direct (truck curve topology)	2D polyline with bulge; truck Contour
Polyline3D	wire	direct (truck curve topology)	linear-edge truck Contour, no bulge/thickness
RasterImage	wire	direct	boundary rectangle / clipping polygon
Ray	wire	direct	two-point [base, base + dir×1e6], no sampling
Region	mesh	truck-with-direct-fallback	2D planar ACIS body; same truck path as Solid3D
Seqend	none	n/a	vertex-sequence terminator sentinel; no output
Shape	wire	direct	small diamond marker at the insertion point
Solid	wire	direct	2D SOLID: four quad edges as direct Lines
Solid3D	mesh	truck-with-direct-fallback	3DSOLID: parse SAT/SAB → truck Shell → triangulation; fallback sampler
Spline	wire	direct (truck curve topology)	NURBS/BSpline truck Edge, ParameterDivision1D sampled
Table	both	direct	cell fills (fill_tris) + LFF cell text + grid lines
Text	wire	direct	LFF-font stroked glyph polylines
Tolerance	wire	direct	feature-control frame grid + per-cell LFF symbols
Underlay	wire	direct	boundary rectangle of the PDF/DWF reference
Unknown	none	n/a	unrecognized-entity sentinel; no output
Viewport	wire	direct	content-viewport frame rectangle (sheet viewport skipped)
Wipeout	wire	direct	boundary rectangle / clipping polygon
XLine	wire	direct	three-point infinite line, no sampling