Fix uneven corner rounding in multiline infill (#15352)

* Skip straight-run splits in corner smoothing

Teach `CornerSmoother` to treat vertices that only continue a straight segment as part of the same leg instead of rounding them as corners. The smoother now keeps a three-point window so it can emit a corner only once both adjoining legs are known, which avoids unnecessary corner processing while preserving real turns such as hairpins.

* Add regression test for split-leg smoothing

Adds a FillCornerSmoothing regression test covering polylines with an extra collinear vertex in a straight run. The test ensures corner smoothing treats split and unsplit geometry identically, preventing inconsistent rounding radii in triangular/grid infill paths.

src/libslic3r/Fill/FillCornerSmoothing.cpp

@@ -108,6 +108,22 @@ const std::vector<Vec2d>& CornerSmoother::curve_coefficients(
     return m_cached_coefficients;
 }
 
+bool CornerSmoother::is_on_straight_run(const Vec2d &previous, const Vec2d &vertex, const Vec2d &next)
+{
+    const Vec2d  incoming_leg    = vertex - previous;
+    const Vec2d  outgoing_leg    = next - vertex;
+    const double incoming_length = incoming_leg.norm();
+    const double outgoing_length = outgoing_leg.norm();
+    // A vertex repeating one of its neighbours carries no direction of its own.
+    if (incoming_length < EPSILON || outgoing_length < EPSILON)
+        return true;
+
+    const Vec2d incoming = incoming_leg / incoming_length;
+    const Vec2d outgoing = outgoing_leg / outgoing_length;
+    return incoming.dot(outgoing) > 0. &&
+           std::abs(incoming.x() * outgoing.y() - incoming.y() * outgoing.x()) < EPSILON;
+}
+
 void CornerSmoother::round_corner(const Vec2d &previous, const Vec2d &corner, const Vec2d &next)
 {
     m_corner_points.clear();

src/libslic3r/Fill/FillCornerSmoothing.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <algorithm>
+#include <array>
 #include <cmath>
 #include <functional>
 #include <vector>
@@ -47,36 +48,57 @@ public:
 
     template<typename Emit> void push(const Vec2d &point, Emit &emit)
     {
-        if (m_pending == 0) {
+        if (m_held == 0) {
+            // The first point of a path is an end, not a corner, and stays where it is.
             emit(point);
-            m_previous = point;
-        } else if (m_pending > 1) {
-            round_corner(m_previous, m_corner, point);
-            for (const Vec2d &corner_point : m_corner_points)
-                emit(corner_point);
-            m_previous = m_corner;
+            m_window[m_held++] = point;
+            return;
         }
-        m_corner  = point;
-        m_pending = std::min(m_pending + 1, 2);
+        if (m_held > 1 && is_on_straight_run(m_window[m_held - 2], m_window[m_held - 1], point)) {
+            // The newest vertex only splits a straight leg, so the leg runs on to this point instead.
+            m_window[m_held - 1] = point;
+            return;
+        }
+        if (m_held < 3) {
+            m_window[m_held++] = point;
+            return;
+        }
+        // Both legs of the middle vertex are complete now, so its curve can no longer grow.
+        emit_corner(m_window[0], m_window[1], m_window[2], emit);
+        m_window[0] = m_window[1];
+        m_window[1] = m_window[2];
+        m_window[2] = point;
     }
 
     // Emits the last point of the path and prepares the smoother for a new one.
     template<typename Emit> void flush(Emit &emit)
     {
-        if (m_pending > 1)
-            emit(m_corner);
-        m_pending = 0;
+        if (m_held > 2)
+            emit_corner(m_window[0], m_window[1], m_window[2], emit);
+        if (m_held > 1)
+            emit(m_window[m_held - 1]);
+        m_held = 0;
     }
 
 private:
+    template<typename Emit> void emit_corner(const Vec2d &previous, const Vec2d &corner, const Vec2d &next, Emit &emit)
+    {
+        round_corner(previous, corner, next);
+        for (const Vec2d &corner_point : m_corner_points)
+            emit(corner_point);
+    }
+
+    // Tells a vertex that only continues a straight leg (or repeats its predecessor) from a corner.
+    // A path doubling back on itself is not one, that vertex is a hairpin and stays where it is.
+    static bool is_on_straight_run(const Vec2d &previous, const Vec2d &vertex, const Vec2d &next);
     // Fills m_corner_points with the points replacing the corner vertex.
     void round_corner(const Vec2d &previous, const Vec2d &corner, const Vec2d &next);
     // Flattens the canonical corner curve of the given size and turn into coordinates of the
     // (incoming, outgoing) basis of the corner. Cached, as an infill path repeats the same corner.
     const std::vector<Vec2d>& curve_coefficients(double corner_distance, const Vec2d &incoming, const Vec2d &outgoing);
 
-    // Fraction of the shorter adjoining segment consumed on each side of a corner. Half of a segment
-    // is the maximum, otherwise the curves of two adjacent corners would overlap.
+    // Fraction of the shorter adjoining leg consumed on each side of a corner. Half of a leg is the
+    // maximum, otherwise the curves of two adjacent corners would overlap.
     const double       m_corner_distance_ratio;
     const double       m_tolerance;
     const double       m_max_corner_distance;
@@ -88,10 +110,11 @@ private:
     double             m_cached_cosine { 0. };
     bool               m_has_cached_coefficients { false };
 
-    Vec2d m_previous { Vec2d::Zero() };
-    Vec2d m_corner { Vec2d::Zero() };
-    // Number of points held back: none, the first point of a path, or a corner candidate.
-    int   m_pending { 0 };
+    // The corners seen last, kept free of vertices that merely split a straight leg. The middle one
+    // is rounded once the third arrives, which is what makes its outgoing leg final.
+    std::array<Vec2d, 3> m_window { Vec2d::Zero(), Vec2d::Zero(), Vec2d::Zero() };
+    // How many of them are filled in.
+    int                  m_held { 0 };
 };
 
 // Rounds the corners of already scaled paths in place. Paths of less than three points are left alone.

tests/libslic3r/test_fill_corner_smoothing.cpp

@@ -171,3 +171,24 @@ TEST_CASE("Corner smoothing keeps the ends of a path that returns to its start",
     REQUIRE(retrace.front() == sharp.front());
     REQUIRE(retrace.back() == sharp.back());
 }
+
+TEST_CASE("Corner smoothing ignores vertices splitting a straight leg", "[FillCornerSmoothing][Regression]")
+{
+    // The triangular and grid infills emit a vertex halfway along the straight run joining two of
+    // their corners. Measuring the legs up to that vertex instead of up to the next corner let the
+    // rounding reach only half as far there as it did into the very same run elsewhere in the
+    // pattern, so geometrically identical corners came out rounded to different radii.
+    const Polyline plain{ Point::new_scale(0., 20.), Point::new_scale(10., 0.),
+                          Point::new_scale(20., 0.), Point::new_scale(30., 20.) };
+    Polyline split = plain;
+    split.points.insert(split.points.begin() + 2, Point::new_scale(15., 0.));
+
+    Polyline smooth_plain = plain;
+    smooth_polyline_corners(smooth_plain, 1., tolerance);
+    Polyline smooth_split = split;
+    smooth_polyline_corners(smooth_split, 1., tolerance);
+
+    REQUIRE(smooth_split.points == smooth_plain.points);
+    // Both corners reach the middle of the 10mm run they share, which the extra vertex sat on.
+    REQUIRE(contains(smooth_plain, Point::new_scale(15., 0.)));
+}