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Make CGAL circle-segments 0-default deflection-driven (rework #8368)

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Address maintainer request on #8368: instead of a deflection floor on top
of a fixed CircleSegments count, use one mode or the other. When
CircleSegments == 0 (the new default) the CGAL kernel derives the conic
segment count from MesherLinearDeflection, matching the deflection based
meshing OpenCascade already does and fixing #8051. When CircleSegments is
non zero it is used directly as a fixed, radius independent count.

CircleSegments is only read by the CGAL kernel; OpenCascade meshes by
deflection and never reads it, so the new default has no effect there.

Update the setting description and the ifcconvert / geometry-settings docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit 0d70812641)
This commit is contained in:
Petru Conduraru
2026-07-07 22:11:34 +03:00
committed by Dion Moult
parent b774c9ad18
commit c57da082a5
4 changed files with 34 additions and 22 deletions
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src/ifcgeom/ConversionSettings.h
+2 -2
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@@ -373,8 +373,8 @@ namespace ifcopenshell {
struct CircleSegments : public SettingBase<CircleSegments, int> {
static constexpr const char* const name = "circle-segments";
static constexpr const char* const description = "Number of segments to approximate full circles in CGAL kernel.";
static constexpr int defaultvalue = 16;
static constexpr const char* const description = "Number of segments to approximate full circles in the CGAL kernel. When 0 (the default) the segment count is derived from mesher-linear-deflection instead, so curves stay within the deflection tolerance regardless of radius.";
static constexpr int defaultvalue = 0;
};
struct CgalSmoothAngleDegrees : public SettingBase<CgalSmoothAngleDegrees, double> {
src/ifcgeom/kernels/cgal/CgalKernel.cpp
+24 -16
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@@ -436,22 +436,30 @@ namespace {
b += 2 * M_PI;
}
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;
}
// CircleSegments controls how conics (circles, ellipses, arcs) are approximated
// in the CGAL kernel. Two modes, one or the other:
// - CircleSegments == 0 (the default): the segment count is derived from
// MesherLinearDeflection, so the chord deviation stays within the mesher's
// linear deflection regardless of radius. This matches the deflection based
// meshing the OpenCascade kernel already does and fixes issue #8051, where
// large radius arcs (curved curtain wall mullions) collapsed to straight chords
// because a fixed segment count is radius agnostic.
// - CircleSegments > 0: it is used directly as the number of segments for a full
// circle, giving deterministic, radius independent output.
int num_segments;
const int circle_segments = settings_.get<settings::CircleSegments>().get();
if (circle_segments > 0) {
num_segments = (int)std::ceil(span / (2 * M_PI) * circle_segments);
} else {
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);
num_segments = (int)std::ceil(span / max_segment_angle);
} else {
// Radius within the deflection tolerance (or no deflection set): a chord per
// quarter turn already keeps the deviation within tolerance.
num_segments = (int)std::ceil(span / (M_PI / 2.));
}
}
if (num_segments < 1) {
src/ifcopenshell-python/docs/ifcconvert/usage.rst
+6 -2
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@@ -310,8 +310,12 @@ CLI Manual
output.
--force-space-transparency arg Overrides transparency of spaces in
geometry output.
--circle-segments arg (= 16) Number of segments to approximate full
circles in CGAL kernel.
--circle-segments arg (= 0) Number of segments to approximate full
circles in the CGAL kernel. When 0 (the
default) the segment count is derived from
mesher-linear-deflection instead, so curves
stay within the deflection tolerance
regardless of radius.
--cgal-smooth-angle-degrees arg (= -1)
Angle in degrees under which adjacent
facets will have averaged vertex
src/ifcopenshell-python/docs/ifcopenshell/geometry_settings.rst
+2 -2
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@@ -228,10 +228,10 @@ circle-segments
+------+-----------------------+---------+
| Type | IfcConvert Option | Default |
+======+=======================+=========+
| INT | ``--circle-segments`` | 16 |
| INT | ``--circle-segments`` | 0 |
+------+-----------------------+---------+
Number of segments to approximate full circles in CGAL kernel.
Number of segments to approximate full circles in the CGAL kernel. When 0 (the default) the segment count is derived from mesher-linear-deflection instead, so curves stay within the deflection tolerance regardless of radius.
context-identifiers
^^^^^^^^^^^^^^^^^^^