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Make layerOn() preserve orientation #96

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Make layerOn() preserve orientation #96

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8770741
Make layerOn() respect oriented triangulations.
AlexHe98 Oct 30, 2025
c7a82f5
Clarify documentation of layerOn().
AlexHe98 Oct 30, 2025
2045d00
Merge branch 'master' into orient-layer
AlexHe98 Nov 11, 2025
b8e9ef0
Merge branch 'main' into orient-layer
AlexHe98 Mar 8, 2026
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33 changes: 29 additions & 4 deletions engine/triangulation/dim3/insertlayered.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -54,9 +54,34 @@ Tetrahedron<3>* Triangulation<3>::layerOn(Edge<3>* edge) {
// tetrahedron vertices for the second boundary triangle. In each case,
// (0,1) maps to the endpoints of the given edge.
//
// The simplest thing to do is let (0,1,2,3) in the preimages for
// roles1 and roles2 match up with vertices (0,1,2,3) of the new
// In the diagram below, the first triangle is shown on the left, and the
// second triangle is shown on the right.
//
// roles1[0]• •roles2[0]
// /| |\
// / | | \
// roles1[2]• | | •roles2[3]
// \ | | /
// \| |/
// roles1[1]• •roles2[1]
//
// If the triangulation is oriented, then the permutations roles1 and
// roles2 will both have sign +1, so we can ensure that the triangulation
// remains oriented after layering if we attach the new tetrahedron as
// shown below. In terms of the gluing permutations, this amounts to
// letting (0,1,2,3) in the preimages for roles1 and roles2 match up with
// vertices (0,1,3,2) -- note that 2 and 3 are swapped -- of the new
// tetrahedron.
// 0
// •
// /|\
// / | \
// 3•-----•2
// \ | /
// \|/
// •
// 1
//

if (tet1->triangle(roles1[3]) == tet2->triangle(roles2[2]))
throw InvalidArgument("layerOn() requires an edge between two "
Expand All @@ -71,8 +96,8 @@ Tetrahedron<3>* Triangulation<3>::layerOn(Edge<3>* edge) {

Tetrahedron<3>* newTet = newSimplexRaw();

newTet->joinRaw(3, tet1, roles1);
newTet->joinRaw(2, tet2, roles2);
newTet->joinRaw(2, tet1, roles1*Perm<4>(2,3));
newTet->joinRaw(3, tet2, roles2*Perm<4>(2,3));

return newTet;
}
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4 changes: 4 additions & 0 deletions engine/triangulation/dim3/triangulation3.h
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Expand Up @@ -4141,6 +4141,10 @@ class Triangulation<3> : public detail::TriangulationBase<3> {
* edge that it creates will be edge 5 (i.e., the edge joining
* vertices 2 and 3) of this tetrahedron.
*
* If the triangulation (or more generally, the component of the
* triangulation that contains the given edge) is currently oriented,
* then the layering will preserve this orientation.
*
* \pre The given edge is a boundary edge of this triangulation,
* and the two boundary triangles on either side of it are distinct.
*
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