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Fix cgal kernel under-tessellating large-radius arcs (#8051)

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The CGAL kernels (cgal and cgal-simple) allocate arc segments as a
fraction of the full circle via CircleSegments, ignoring the radius.
A large-radius arc that spans a small angle therefore collapsed to a
single chord, turning curved curtain-wall mullions straight while the
OpenCascade kernel (which meshes by deflection) kept them curved.

evaluate_conic now also enforces a deflection-based floor on the number
of segments, keeping the chord deviation within mesher-linear-deflection,
matching OpenCascade. Small circles are unchanged (CircleSegments floor
still dominates); only large-radius curves get denser.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Petru Conduraru
2026-07-07 17:54:21 +03:00
committed by Thomas Krijnen
parent eb7324e7fc
commit dd9fa65629
1 changed files with 27 additions and 1 deletions
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src/ifcgeom/kernels/cgal/CgalKernel.cpp
+27 -1
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@@ -391,6 +391,11 @@ namespace {
}
};
// Representative radius used to size the polygonal approximation of a conic.
// For an ellipse the larger semi-axis is the conservative choice.
inline double conic_radius(const taxonomy::circle::ptr& c) { return c->radius; }
inline double conic_radius(const taxonomy::ellipse::ptr& e) { return e->radius > e->radius2 ? e->radius : e->radius2; }
struct cgal_curve_creation_visitor {
Settings& settings_;
parameter_range param;
@@ -425,7 +430,28 @@ namespace {
if (b <= a) {
b += 2 * M_PI;
}
int num_segments = (int)std::ceil(std::fabs(a - b) / (2 * M_PI) * settings_.get<settings::CircleSegments>().get());
const double span = std::fabs(a - b);
int num_segments = (int)std::ceil(span / (2 * M_PI) * settings_.get<settings::CircleSegments>().get());
// CircleSegments allocates segments as a fraction of the *full* circle and is
// radius-agnostic. A large-radius arc spanning a small angle therefore collapses
// to a single chord (issue #8051: curved curtain-wall mullions became straight in
// the CGAL kernels while OpenCascade, which meshes by deflection, kept them curved).
// Enforce a deflection-based floor so the chord deviation stays within the mesher's
// linear deflection, matching OpenCascade behaviour.
const double radius = conic_radius(t);
const double deflection = settings_.get<settings::MesherLinearDeflection>().get();
if (deflection > 0. && radius > deflection) {
const double max_segment_angle = 2.0 * std::acos(1.0 - deflection / radius);
if (max_segment_angle > 0.) {
const int num_segments_deflection = (int)std::ceil(span / max_segment_angle);
if (num_segments_deflection > num_segments) {
num_segments = num_segments_deflection;
}
}
}
if (num_segments < 1) {
num_segments = 1;
}
double du = (b - a) / num_segments;
taxonomy::point3 P;
// @nb for loop is not inclusive of the both end points