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

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Evaluación

Dificultad
5/5
Tiempo estimado
Más de una semana
Aptitud para principiantes
35/100
Tipo de issue
Nueva funcionalidad
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
csharp, unity
Área
computer-graphics

Línea de trabajo

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.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

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.
Lenguaje dominante
C#
Estrellas
135
Forks
15
Merge medio
33 min
PR fusionados (30 d)
122

Preparar el entorno

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Primeros pasos

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  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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