Exact planes from snapped vertices, so faces that meet at a corner meet exactly

Aperta
  • Qualcosa non va?
#252 0 commenti 0 reazioni 0 assegnatari Vedi su GitHub

I maintainer di solito rispondono entro 1 giorno

Nessuno ha ancora preso questa issue.

Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
35/100
Tipo di issue
Funzionalità
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
csharp, unity
Ambito
computer-graphics

Direzione di ricerca

Start with ExactBrushes.AddPlanes, ExactPlane.Quantize, and ExactJudge to trace how brush planes are created and validated; the issue also names OutputModelWeld and ChiselGeneratorSizeTests.SpiralStairsFilledDown_HaveWallsDownToTheFloor. The change requires snapping brush vertices, deriving shared face planes consistently, and revisiting integer widths and validation. Done when the spiral stairs tests pass with corners meeting exactly and the input check matches the new plane construction.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Problem

The exact CSG rounds every plane of a brush on its own (ExactBrushes.AddPlanes -> ExactPlane.Quantize: unit normal to 2^-20, offset to 1/16384 m). Planes that meet in one point in the brush's float mesh generally no longer meet in one point after rounding. Where more than three planes meet at a corner, the exact polytope gets small extra edges near that corner.

When two of those planes are nearly parallel, the extra edge is long: about 0.03 mm / sin(angle between them). A spiral stairs rail piece is one convex brush whose twisted top is two triangles. At both ends of the fold between them, four planes meet: the end, the side and both triangles, which are only a degree or two apart. The fold then ends 1.65 mm from the corner instead of at it. That leaves a vertex in the middle of the rail's top edge.

The output weld collapses edges shorter than 1 mm (OutputModelWeld.kCollapseLength) to hide these slivers after the fact. It can't reach this one.

Failing tests that show it: ChiselGeneratorSizeTests.SpiralStairsFilledDown_HaveWallsDownToTheFloor (Up and Down).

What the literature does

Plane-based exact Booleans import a mesh by rounding its vertices to an integer grid, and derive each face's plane exactly from three of them (normal = e12 x e13, d = -v.n). Every face plane then passes exactly through its vertices, so faces meeting at a corner meet there exactly, however many there are.

  • Bernstein & Fussell 2009, Fast, Exact, Linear Booleans
  • Campen & Kobbelt 2010, Exact and Robust (Self-)Intersections for Polygonal Meshes
  • Nehring-Wirxel, Trettner & Kobbelt 2021, Fast Exact Booleans for Iterated CSG using Octree-Embedded BSPs (arXiv 2103.02486). Section 1 describes this failure for planes made separately, and section 3.2 gives the bit bounds (27 bit positions, 55 bit normals, 256 bit arithmetic).
  • Trettner, Nehring-Wirxel & Kobbelt 2022, EMBER

What it would take in Chisel

  • Snap each brush's vertices to a grid in tree space, and make each face's exact plane from three of its snapped vertices. Brushes that share a face must pick the same three, by position, so their planes come out exactly opposite.
  • Bit widths: a cross-product normal needs about twice the bits of the coordinates. ExactPlane allows normals below 2^32 and offsets below 2^56 (Int128/Int192 maths). That leaves about 15-16 bits of coordinates across a brush, about 2.8 m on the current 1/16384 m grid. So the exact maths needs wider integers, or the vertex grid needs to be coarser.
  • Snapping nearly-coplanar faces of different brushes onto one plane would then come from the vertex grid, not from rounding the planes.
  • The ExactJudge input check, which rebuilds the expected planes with Quantize, has to follow.
  • Once corners are exact, OutputModelWeld's short-edge collapse may no longer be needed.
Lingua principale
C#
Stelle
135
Fork
15
Merge medio
33m
PR unite (30g)
122

Preparare l'ambiente

Questo progetto non fornisce container di sviluppo, Dockerfile né guida per i contributori, quindi l'ambiente è a tuo carico: parti dal suo README e consulta la nostra guida al primo contributo per i passaggi generali.

Come iniziare

  1. Leggi tutta la issue e poi la guida ai contributi del progetto.
  2. Commenta sulla issue per dire che te ne occupi tu — evita che due persone facciano lo stesso lavoro.
  3. Fai un fork del repository e lavora su un branch.
  4. Apri una pull request che faccia riferimento al numero della issue.

Altre issue di RadicalCSG/com.chisel

Tutte le issue di RadicalCSG/com.chisel

Issue simili

Altre issue su C#

Ricevi le nuove issue nella tua casella

Un breve riepilogo di issue GitHub adatte ai principianti.