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Add new function mesh/sharp_halfedges.m #175

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dbf5c62
feat: add new function mesh/sharp_halfedges.m
yDF8EPJN8 Jul 4, 2025
8497f57
feat(sharp_edges): improve example
yDF8EPJN8 Jul 4, 2025
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62 changes: 62 additions & 0 deletions mesh/sharp_halfedges.m
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function SHE = sharp_halfedges(V, F, angle, varargin)
% SHARP_HALFEDGES Compute sharp halfedges for a mesh based on dihedral angles.
%
% SHE = sharp_halfedges(V, F, angle)
% SHE = sharp_halfedges(V, F, angle, 'CornerIndexing')
% SHE = sharp_halfedges(V, F, angle, 'CyclicIndexing')
%
% This function marks "sharp" edges in a triangular mesh based on a given dihedral
% angle threshold. An edge is considered sharp if its dihedral angle exceeds the
% specified angle (in radians).
%
% Input:
% V #V by 3 matrix of vertex positions.
% F #F by 3 matrix of triangle vertex indices.
% angle Scalar threshold angle (in radians). An edge with a larger dihedral
% angle is marked as sharp.
%
% Optional argument (mode):
% 'CornerIndexing' (default) - Uses corner-based indexing convention: edge j
% is defined as the edge opposite vertex j in F(i,:).
% 'CyclicIndexing' - Uses edge-based cyclic indexing convention: local edge
% indices follow the order of edges (1→2, 2→3, 3→1) in each triangle.
%
% Output:
% SHE #F by 3 matrix, where SHE(i,j) = 1 if the j-th edge of face i is sharp
% (dihedral angle > angle), and 0 otherwise. Indexing depends on the mode.
%
% Example:
% SHE = sharp_halfedges(V, F, pi/2-0.0001);
% [EF, EI, uE, EMAP] = edge_flaps(F);
% % isSharpEdge - mask of undirected sharp edges in uE.
% isSharpEdge = zeros(size(uE,1),1);
% isSharpEdge(EMAP(SHE==1)) = 1;

% Default mode
mode = 'CornerIndexing';

% Parse optional argument
if ~isempty(varargin)
if ischar(varargin{1}) || isstring(varargin{1})
mode = varargin{1};
else
error('Optional argument must be a string: ''CornerIndexing'' or ''CyclicIndexing''.');
end
end

[A,C] = adjacency_dihedral_angle_matrix(V,F);
[AI,AJ,AV] = find(A);
keep = abs(AV-pi)>(angle) & ~isnan(AV);
A = sparse(AI(keep),AJ(keep),1,size(A,1),size(A,2));
[CI,~,CV] = find(C.*A);
idx = sub2ind(size(C), CI, CV);
SHE = zeros(size(F));
SHE(idx)=1;

% Apply cyclic reordering if requested
if strcmpi(mode, 'CyclicIndexing')
SHE = [SHE(:,3), SHE(:,1), SHE(:,2)];
elseif ~strcmpi(mode, 'CornerIndexing')
error('Unknown indexing mode. Use ''CornerIndexing'' or ''CyclicIndexing''.');
end
end