diff --git a/src/libslic3r/CMakeLists.txt b/src/libslic3r/CMakeLists.txt
index 812d28e088..f7b4de6e25 100644
--- a/src/libslic3r/CMakeLists.txt
+++ b/src/libslic3r/CMakeLists.txt
@@ -149,6 +149,8 @@ set(lisbslic3r_sources
     Fill/FillConcentric.hpp
     Fill/FillConcentricInternal.cpp
     Fill/FillConcentricInternal.hpp
+    Fill/FillCornerSmoothing.cpp
+    Fill/FillCornerSmoothing.hpp
     Fill/Fill.cpp
     Fill/FillCrossHatch.cpp
     Fill/FillCrossHatch.hpp
diff --git a/src/libslic3r/Fill/Fill.cpp b/src/libslic3r/Fill/Fill.cpp
index 88d87ddb26..0888e1bb55 100644
--- a/src/libslic3r/Fill/Fill.cpp
+++ b/src/libslic3r/Fill/Fill.cpp
@@ -970,9 +970,9 @@ std::vector<SurfaceFill> group_fills(const Layer &layer, LockRegionParam &lock_p
                                                                    region_config.sparse_infill_rotate_template.value);
                     params.fixed_angle = !region_config.sparse_infill_rotate_template.value.empty();
 
-                    // Orca: special case; apply smoothing factor only for Hilbert Curve sparse infill.
-                    // FillHilbertCurve::generate clamps and validates the value itself.
-                    if (params.pattern == ipHilbertCurve)
+                    // Orca: the smoothing factor only applies to the sparse infill patterns that
+                    // implement it. The fills clamp and validate the value themselves.
+                    if (is_smoothable_infill_pattern(params.pattern, params.multiline))
                         params.smooth_factor = 0.01 * region_config.sparse_infill_smooth_factor.value;
                 } else {
                     const bool top_layer_direction_set    = surface.is_top() && region_config.top_layer_direction.value >= 0.;
diff --git a/src/libslic3r/Fill/Fill3DHoneycomb.cpp b/src/libslic3r/Fill/Fill3DHoneycomb.cpp
index 5908f854de..ad5f8918fd 100644
--- a/src/libslic3r/Fill/Fill3DHoneycomb.cpp
+++ b/src/libslic3r/Fill/Fill3DHoneycomb.cpp
@@ -2,6 +2,7 @@
 #include "../ShortestPath.hpp"
 #include "../Surface.hpp"
 #include "FillBase.hpp"
+#include "FillCornerSmoothing.hpp"
 #include "Fill3DHoneycomb.hpp"
 
 namespace Slic3r {
@@ -271,6 +272,9 @@ void Fill3DHoneycomb::_fill_surface_single(
     for (Polyline &pl : polylines){
       pl.translate(bb.min);
       pl.simplify(5 * spacing); // simplify to 5x line width
+      // Orca: round the corners of the octahedral wave. The layers where the wave degenerates to a
+      // straight line have no corner to round.
+      smooth_polyline_corners(pl, params.smooth_factor, scaled<double>(params.resolution));
     }
 
     // Apply multiline offset if needed
diff --git a/src/libslic3r/Fill/FillConcentric.cpp b/src/libslic3r/Fill/FillConcentric.cpp
index a75d2ed7d3..1882f7a656 100644
--- a/src/libslic3r/Fill/FillConcentric.cpp
+++ b/src/libslic3r/Fill/FillConcentric.cpp
@@ -5,6 +5,7 @@
 #include "Arachne/WallToolPaths.hpp"
 
 #include "FillConcentric.hpp"
+#include "FillCornerSmoothing.hpp"
 #include <libslic3r/ShortestPath.hpp>
 
 namespace Slic3r {
@@ -32,12 +33,32 @@ void FillConcentric::_fill_surface_single(
 
     Polygons loops = to_polygons(contracted);
 
-    ExPolygons last { std::move(contracted) };
+    ExPolygons last { contracted };
     while (! last.empty()) {
         last = offset2_ex(last, -(distance + min_spacing/2), +min_spacing/2);
         append(loops, to_polygons(last));
     }
 
+    // Orca: round the corners of the loops. Unlike the other patterns these are never clipped to the
+    // fill region - they are its offsets - so a corner may only be rounded where the curve replacing it
+    // stays inside. Rounding cuts toward the inside of the turn, which around a hole, at a concave
+    // feature or across a thin region is outside the fill and would put the extrusion over a wall.
+    // The reach is capped at half the distance between two loops as well: a loop is as long as the
+    // object, and a corner cut by half of its side would swallow the neighbouring loops.
+    auto corner_stays_inside = [&contracted](const Vec2d &from, const Vec2d &to) {
+        // The straight chord between the ends of the curve is the deepest the curve can cut.
+        for (const double t : { 0.25, 0.5, 0.75 }) {
+            const Vec2d  sample = from + t * (to - from);
+            const Point  point(coord_t(sample.x()), coord_t(sample.y()));
+            if (std::none_of(contracted.begin(), contracted.end(),
+                             [&point](const ExPolygon &region) { return region.contains(point); }))
+                return false;
+        }
+        return true;
+    };
+    smooth_polygons_corners(loops, params.smooth_factor, scaled<double>(params.resolution), 0.5 * distance,
+                            corner_stays_inside);
+
     // generate paths from the outermost to the innermost, to avoid
     // adhesion problems of the first central tiny loops
     loops = union_pt_chained_outside_in(loops);
diff --git a/src/libslic3r/Fill/FillCornerSmoothing.cpp b/src/libslic3r/Fill/FillCornerSmoothing.cpp
new file mode 100644
index 0000000000..2af9f6bb9c
--- /dev/null
+++ b/src/libslic3r/Fill/FillCornerSmoothing.cpp
@@ -0,0 +1,226 @@
+#include <array>
+
+#include "FillCornerSmoothing.hpp"
+
+namespace Slic3r {
+
+// Turns sharper than this are left untouched: both ends of the curve replacing such a corner nearly
+// coincide, so the corner would be rounded into a degenerate loop instead of a hairpin.
+static constexpr const double min_smoothed_turn_cosine = -0.9;
+
+// The control points are expressed in the (incoming, outgoing) basis of the corner, which is not
+// orthonormal for turns other than a right angle.
+using QuinticBezier = std::array<Vec2d, 6>;
+
+static bool is_bezier_flat(const QuinticBezier &curve, const Vec2d &incoming, const Vec2d &outgoing, const double deviation)
+{
+    // A Bezier curve stays inside the convex hull of its control points. Therefore, keeping every
+    // control point within a deviation-wide strip around the endpoint chord conservatively bounds the
+    // flattening error. The cross product is the perpendicular distance scaled by the chord length;
+    // comparing squared values avoids a square root.
+    auto         in_plane        = [&incoming, &outgoing](const Vec2d &c) { return c.x() * incoming + c.y() * outgoing; };
+    const Vec2d  chord           = in_plane(curve.back() - curve.front());
+    const double chord_length_sq = chord.squaredNorm();
+    const double max_cross_sq    = deviation * deviation * chord_length_sq;
+
+    for (size_t i = 1; i + 1 < curve.size(); ++i) {
+        const Vec2d  offset = in_plane(curve[i] - curve.front());
+        const double cross  = chord.x() * offset.y() - chord.y() * offset.x();
+        if (cross * cross > max_cross_sq)
+            return false;
+    }
+    return true;
+}
+
+static void subdivide_bezier(const QuinticBezier &curve, QuinticBezier &left, QuinticBezier &right)
+{
+    // Split the curve at t = 0.5 using de Casteljau's algorithm. Each averaging level contributes one
+    // control point to the left half and one to the right half; the latter is filled backwards to keep
+    // both resulting control polygons in their original parameter direction.
+    QuinticBezier subdivision = curve;
+    left.front() = subdivision.front();
+    right.back() = subdivision.back();
+    for (size_t level = 1; level < curve.size(); ++level) {
+        for (size_t i = 0; i + level < curve.size(); ++i)
+            subdivision[i] = 0.5 * (subdivision[i] + subdivision[i + 1]);
+        left[level] = subdivision.front();
+        right[curve.size() - level - 1] = subdivision[curve.size() - level - 1];
+    }
+}
+
+static void flatten_bezier(
+    const QuinticBezier &curve, const Vec2d &incoming, const Vec2d &outgoing, const double deviation, std::vector<Vec2d> &output)
+{
+    // Subdivide to at least depth 1 so a rounded corner cannot collapse to a single diagonal chord.
+    // A uniform subdivision depth keeps samples at equal parameter intervals t = k / 2^depth,
+    // avoiding abrupt segment-length jumps at adaptive-depth boundaries.
+    static constexpr size_t max_depth = 16;
+
+    std::vector<QuinticBezier> subcurves(2);
+    subdivide_bezier(curve, subcurves[0], subcurves[1]);
+
+    for (size_t depth = 1; depth < max_depth; ++depth) {
+        bool all_flat = true;
+        for (const QuinticBezier &c : subcurves)
+            if (!is_bezier_flat(c, incoming, outgoing, deviation)) {
+                all_flat = false;
+                break;
+            }
+        if (all_flat)
+            break;
+        std::vector<QuinticBezier> finer(subcurves.size() * 2);
+        for (size_t i = 0; i < subcurves.size(); ++i)
+            subdivide_bezier(subcurves[i], finer[i * 2], finer[i * 2 + 1]);
+        subcurves = std::move(finer);
+    }
+
+    // The curve start is deliberately omitted so it can be shared with the straight leg feeding into it.
+    output.clear();
+    output.reserve(subcurves.size());
+    for (const QuinticBezier &c : subcurves)
+        output.emplace_back(c.back());
+}
+
+const std::vector<Vec2d>& CornerSmoother::curve_coefficients(
+    const double corner_distance, const Vec2d &incoming, const Vec2d &outgoing)
+{
+    const double cosine = incoming.dot(outgoing);
+    // Corners of the same size and turn angle are congruent, so they flatten identically. An infill
+    // path walks over the very same corner over and over again, the Hilbert curve over a single one.
+    if (m_has_cached_coefficients && corner_distance == m_cached_distance && cosine == m_cached_cosine)
+        return m_cached_coefficients;
+
+    // One canonical corner running from -corner_distance along the incoming leg to corner_distance
+    // along the outgoing one. At each end, the first three control points are collinear and equally
+    // spaced: the tangent follows the adjoining straight leg and the second derivative is zero. The
+    // endpoint curvature is therefore zero, giving G2 joins to both legs.
+    const double        d = corner_distance;
+    const QuinticBezier corner_curve {{
+        {-d, 0.}, {-0.7 * d, 0.}, {-0.4 * d, 0.}, {0., 0.4 * d}, {0., 0.7 * d}, {0., d}
+    }};
+    // Retain a finite positive tolerance if the smoother was set up with an invalid one.
+    const double deviation = m_tolerance > 0. && std::isfinite(m_tolerance) ? m_tolerance : EPSILON;
+    flatten_bezier(corner_curve, incoming, outgoing, deviation, m_cached_coefficients);
+
+    m_cached_distance         = corner_distance;
+    m_cached_cosine           = cosine;
+    m_has_cached_coefficients = true;
+    return m_cached_coefficients;
+}
+
+void CornerSmoother::round_corner(const Vec2d &previous, const Vec2d &corner, const Vec2d &next)
+{
+    m_corner_points.clear();
+
+    const Vec2d  incoming_leg = corner - previous;
+    const Vec2d  outgoing_leg = next - corner;
+    const double incoming_length = incoming_leg.norm();
+    const double outgoing_length = outgoing_leg.norm();
+    if (incoming_length < EPSILON || outgoing_length < EPSILON) {
+        m_corner_points.emplace_back(corner);
+        return;
+    }
+
+    const Vec2d  incoming = incoming_leg / incoming_length;
+    const Vec2d  outgoing = outgoing_leg / outgoing_length;
+    const double cross    = incoming.x() * outgoing.y() - incoming.y() * outgoing.x();
+    // A collinear vertex is no corner at all, and a hairpin cannot be rounded, see above.
+    if (std::abs(cross) < EPSILON || incoming.dot(outgoing) < min_smoothed_turn_cosine) {
+        m_corner_points.emplace_back(corner);
+        return;
+    }
+
+    // Consuming at most half of the shorter leg keeps the curves of two adjacent corners apart.
+    double corner_distance = m_corner_distance_ratio * std::min(incoming_length, outgoing_length);
+    if (m_max_corner_distance > 0.)
+        corner_distance = std::min(corner_distance, m_max_corner_distance);
+
+    const Vec2d curve_start = corner - corner_distance * incoming;
+    const Vec2d curve_end   = corner + corner_distance * outgoing;
+    if (m_corner_filter && !m_corner_filter(curve_start, curve_end)) {
+        m_corner_points.emplace_back(corner);
+        return;
+    }
+
+    const std::vector<Vec2d> &coefficients = curve_coefficients(corner_distance, incoming, outgoing);
+    m_corner_points.reserve(coefficients.size() + 1);
+    m_corner_points.emplace_back(curve_start);
+    for (const Vec2d &coefficient : coefficients)
+        m_corner_points.emplace_back(corner + coefficient.x() * incoming + coefficient.y() * outgoing);
+}
+
+// Rounds the corners of a scaled point sequence. A polygon closes implicitly, so all of its vertices
+// are corners; a polyline is an open path that keeps both of its ends, even where they coincide - a
+// path returning to where it started retraces its way back and is not a loop.
+static Points smooth_corners(const Points &points, const bool polygon, CornerSmoother &smoother)
+{
+    // A polygon has no free ends, so its first vertex is a corner like any other. Rounding it takes
+    // feeding the smoother the last vertex first, whose own output point is then dropped again.
+    size_t skip = polygon ? 1 : 0;
+
+    Points smoothed;
+    smoothed.reserve(2 * points.size());
+    auto emit = [&smoothed, &skip](const Vec2d &point) {
+        if (skip > 0) {
+            --skip;
+            return;
+        }
+        smoothed.emplace_back(coord_t(std::floor(point.x() + 0.5)), coord_t(std::floor(point.y() + 0.5)));
+    };
+
+    if (polygon)
+        smoother.push(points.back().cast<double>(), emit);
+    for (const Point &point : points)
+        smoother.push(point.cast<double>(), emit);
+    if (polygon)
+        // Wrap the first vertex around, so that the last one is a corner as well.
+        smoother.push(points.front().cast<double>(), emit);
+    smoother.flush(emit);
+
+    if (polygon)
+        // The flushed point is the wrapped first vertex, which a polygon does not store.
+        smoothed.pop_back();
+    return smoothed;
+}
+
+void smooth_polyline_corners(Polyline &polyline, const double smooth_factor, const double tolerance,
+                             const double max_corner_distance, const CornerFilter &corner_filter)
+{
+    CornerSmoother smoother(smooth_factor, tolerance, max_corner_distance, corner_filter);
+    if (!smoother.enabled() || polyline.size() < 3)
+        return;
+
+    polyline.points = smooth_corners(polyline.points, false, smoother);
+    // Rounding back to the integer grid may collapse neighbouring samples of a curve.
+    polyline.remove_duplicate_points();
+}
+
+void smooth_polylines_corners(Polylines &polylines, const double smooth_factor, const double tolerance,
+                              const double max_corner_distance, const CornerFilter &corner_filter)
+{
+    if (sanitize_smooth_factor(smooth_factor) == 0.)
+        return;
+    for (Polyline &polyline : polylines)
+        smooth_polyline_corners(polyline, smooth_factor, tolerance, max_corner_distance, corner_filter);
+}
+
+void smooth_polygons_corners(Polygons &polygons, const double smooth_factor, const double tolerance,
+                             const double max_corner_distance, const CornerFilter &corner_filter)
+{
+    CornerSmoother smoother(smooth_factor, tolerance, max_corner_distance, corner_filter);
+    if (!smoother.enabled())
+        return;
+
+    for (Polygon &polygon : polygons) {
+        if (polygon.size() < 3)
+            continue;
+        polygon.points = smooth_corners(polygon.points, true, smoother);
+        polygon.remove_duplicate_points();
+        // The curves of the first and of the last corner may have met on the segment they share. A
+        // polygon closes implicitly, so it must not repeat its first vertex at the end.
+        if (polygon.points.size() > 1 && polygon.points.front() == polygon.points.back())
+            polygon.points.pop_back();
+    }
+}
+
+} // namespace Slic3r
diff --git a/src/libslic3r/Fill/FillCornerSmoothing.hpp b/src/libslic3r/Fill/FillCornerSmoothing.hpp
new file mode 100644
index 0000000000..1852fc4c67
--- /dev/null
+++ b/src/libslic3r/Fill/FillCornerSmoothing.hpp
@@ -0,0 +1,108 @@
+#pragma once
+
+#include <algorithm>
+#include <cmath>
+#include <functional>
+#include <vector>
+
+#include "../libslic3r.h"
+#include "../Point.hpp"
+#include "../Polygon.hpp"
+#include "../Polyline.hpp"
+
+namespace Slic3r {
+
+// Orca: NaN or infinite factors disable the smoothing, everything else is clamped to <0, 1>.
+inline double sanitize_smooth_factor(double smooth_factor)
+{
+    return std::isfinite(smooth_factor) ? std::clamp(smooth_factor, 0., 1.) : 0.;
+}
+
+// Decides whether a corner may be replaced by the curve that leaves the path at `from` and rejoins it
+// at `to`, both in the coordinate system of the pushed points. Rounding cuts toward the inside of the
+// turn, so a path that is not clipped to the fill region afterwards needs this to stay inside it.
+using CornerFilter = std::function<bool(const Vec2d &from, const Vec2d &to)>;
+
+// Orca: Replaces the sharp vertices of an infill path with curves that join the adjoining straight
+// legs with a continuous curvature, so the toolhead does not have to stop in every corner.
+// Points are pushed one by one, because the plane path fills produce their path on the fly, and
+// every point of the smoothed path is handed over to the caller supplied emit callback.
+// Fully smoothed adjacent corners meet at the midpoint of the segment they share, so the emitted
+// points may collapse onto each other once rounded to the integer grid of the caller. Dropping such
+// duplicates is left to the caller, which is the only one knowing that grid.
+class CornerSmoother
+{
+public:
+    // tolerance is the maximum chordal deviation of the flattened curves, in the units of the pushed
+    // points. max_corner_distance caps how far a curve may reach along a leg, in the same units; it
+    // bounds how far a rounded corner moves away from the original path, which matters where the legs
+    // are much longer than the spacing of the pattern. Zero leaves the reach uncapped.
+    CornerSmoother(double smooth_factor, double tolerance, double max_corner_distance = 0.,
+                   CornerFilter corner_filter = {})
+        : m_corner_distance_ratio(0.5 * sanitize_smooth_factor(smooth_factor)), m_tolerance(tolerance),
+          m_max_corner_distance(max_corner_distance), m_corner_filter(std::move(corner_filter))
+    {}
+
+    bool enabled() const { return m_corner_distance_ratio > 0.; }
+
+    template<typename Emit> void push(const Vec2d &point, Emit &emit)
+    {
+        if (m_pending == 0) {
+            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_corner  = point;
+        m_pending = std::min(m_pending + 1, 2);
+    }
+
+    // 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;
+    }
+
+private:
+    // 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.
+    const double       m_corner_distance_ratio;
+    const double       m_tolerance;
+    const double       m_max_corner_distance;
+    const CornerFilter m_corner_filter;
+    std::vector<Vec2d> m_corner_points;
+    // Cached flattening of the last corner, valid for corners of the same size and turn angle.
+    std::vector<Vec2d> m_cached_coefficients;
+    double             m_cached_distance { 0. };
+    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 };
+};
+
+// Rounds the corners of already scaled paths in place. Paths of less than three points are left alone.
+// Both ends of a polyline are kept where they are, even when they coincide: such a path retraces its
+// way back and joining its ends would turn it into a loop. See CornerSmoother for max_corner_distance.
+void smooth_polyline_corners(Polyline &polyline, double smooth_factor, double tolerance,
+                             double max_corner_distance = 0., const CornerFilter &corner_filter = {});
+void smooth_polylines_corners(Polylines &polylines, double smooth_factor, double tolerance,
+                              double max_corner_distance = 0., const CornerFilter &corner_filter = {});
+// Polygons close implicitly, so every one of their vertices is a corner.
+void smooth_polygons_corners(Polygons &polygons, double smooth_factor, double tolerance,
+                             double max_corner_distance = 0., const CornerFilter &corner_filter = {});
+
+} // namespace Slic3r
diff --git a/src/libslic3r/Fill/FillCrossHatch.cpp b/src/libslic3r/Fill/FillCrossHatch.cpp
index 571095eca4..98be5ef46b 100644
--- a/src/libslic3r/Fill/FillCrossHatch.cpp
+++ b/src/libslic3r/Fill/FillCrossHatch.cpp
@@ -3,6 +3,7 @@
 #include "../Surface.hpp"
 #include <cmath>
 #include "FillBase.hpp"
+#include "FillCornerSmoothing.hpp"
 #include "FillCrossHatch.hpp"
 
 namespace Slic3r {
@@ -205,6 +206,9 @@ void FillCrossHatch ::_fill_surface_single(
     // shift the pattern to the actual space
     for (Polyline &pl : polylines) { pl.translate(bb.min); }
 
+    // Orca: round the corners of the transition layers. The repeat layers are straight lines and stay as they are.
+    smooth_polylines_corners(polylines, params.smooth_factor, scaled<double>(params.resolution));
+
     // Apply multiline offset if needed
     multiline_fill(polylines, params, spacing);
 
diff --git a/src/libslic3r/Fill/FillHoneycomb.cpp b/src/libslic3r/Fill/FillHoneycomb.cpp
index a595cdb664..82679541da 100644
--- a/src/libslic3r/Fill/FillHoneycomb.cpp
+++ b/src/libslic3r/Fill/FillHoneycomb.cpp
@@ -2,6 +2,7 @@
 #include "../ShortestPath.hpp"
 #include "../Surface.hpp"
 
+#include "FillCornerSmoothing.hpp"
 #include "FillHoneycomb.hpp"
 
 namespace Slic3r {
@@ -70,6 +71,9 @@ void FillHoneycomb::_fill_surface_single(
             }
             p.rotate(-direction.first, m.hex_center);
             p.simplify(5 * spacing); // simplify to 5x line width
+            // Orca: round the corners of the honeycomb cells. Done before the clipping, so that the
+            // curves are cut by the region boundary just like the sharp path would be.
+            smooth_polyline_corners(p, params.smooth_factor, scaled<double>(params.resolution));
             all_polylines.push_back(p);
         }
     }
diff --git a/src/libslic3r/Fill/FillLightning.cpp b/src/libslic3r/Fill/FillLightning.cpp
index 7937b9d129..77031b42e0 100644
--- a/src/libslic3r/Fill/FillLightning.cpp
+++ b/src/libslic3r/Fill/FillLightning.cpp
@@ -2,6 +2,7 @@
 #include "../Print.hpp"
 #include "../ShortestPath.hpp"
 #include "FillBase.hpp"
+#include "FillCornerSmoothing.hpp"
 #include "FillLightning.hpp"
 #include "Lightning/Generator.hpp"
 
@@ -17,6 +18,19 @@ void Filler::_fill_surface_single(
     const Layer &layer      = generator->getTreesForLayer(this->layer_id);
     Polylines    fill_lines = layer.convertToLines(to_polygons(expolygon), scaled<coord_t>(0.5 * this->spacing - this->overlap));
 
+    // Orca: round the turns of the branches. Hairpins are left sharp, as they cannot be rounded, and
+    // the reach is capped: cutting a corner moves the branch, and a branch is as long as the object
+    // rather than as long as one cell of a pattern, so half of a leg would merge it with its neighbour
+    // instead of rounding the turn between them. Half the distance between two branches keeps them
+    // apart. With more than one line per infill wall the branches are printed as outlines drawn around
+    // them, and the outlines of branches that run into each other merge into a single one; moving a
+    // branch by more than a fraction of its printed width breaks such an outline up into separate
+    // loops, so that width bounds the reach as well.
+    const double branch_width   = scaled<double>(this->spacing) * params.multiline;
+    const double branch_spacing = branch_width / std::max(double(params.density), EPSILON);
+    const double max_reach      = 0.5 * (params.multiline > 1 ? branch_width : branch_spacing);
+    smooth_polylines_corners(fill_lines, params.smooth_factor, scaled<double>(params.resolution), max_reach);
+
     // Apply multiline offset if needed
     multiline_fill(fill_lines, params, spacing);
 
diff --git a/src/libslic3r/Fill/FillPlanePath.cpp b/src/libslic3r/Fill/FillPlanePath.cpp
index 7c4f285ac6..577aef0600 100644
--- a/src/libslic3r/Fill/FillPlanePath.cpp
+++ b/src/libslic3r/Fill/FillPlanePath.cpp
@@ -2,6 +2,7 @@
 #include "../ShortestPath.hpp"
 #include "../Surface.hpp"
 
+#include "FillCornerSmoothing.hpp"
 #include "FillPlanePath.hpp"
 
 namespace Slic3r {
@@ -288,145 +289,60 @@ static void generate_hilbert_curve(coord_t min_x, coord_t min_y, coord_t max_x,
     }
 }
 
-using QuinticBezier = std::array<Vec2d, 6>;
-
-static bool is_bezier_flat(const QuinticBezier &curve, const double deviation)
-{
-    // A Bezier curve stays inside the convex hull of its control points. Therefore, keeping every
-    // control point within a deviation-wide strip around the endpoint chord conservatively bounds the
-    // flattening error. The cross product is the perpendicular distance scaled by the chord length;
-    // comparing squared values avoids a square root.
-    const Vec2d  chord           = curve.back() - curve.front();
-    const double chord_length_sq = chord.squaredNorm();
-    const double max_cross_sq    = deviation * deviation * chord_length_sq;
-
-    for (size_t i = 1; i + 1 < curve.size(); ++i) {
-        const Vec2d  offset = curve[i] - curve.front();
-        const double cross  = chord.x() * offset.y() - chord.y() * offset.x();
-        if (cross * cross > max_cross_sq)
-            return false;
-    }
-    return true;
-}
-
-static void subdivide_bezier(const QuinticBezier &curve, QuinticBezier &left, QuinticBezier &right)
-{
-    // Split the curve at t = 0.5 using de Casteljau's algorithm. Each averaging level contributes one
-    // control point to the left half and one to the right half; the latter is filled backwards to keep
-    // both resulting control polygons in their original parameter direction.
-    QuinticBezier subdivision = curve;
-    left.front() = subdivision.front();
-    right.back() = subdivision.back();
-    for (size_t level = 1; level < curve.size(); ++level) {
-        for (size_t i = 0; i + level < curve.size(); ++i)
-            subdivision[i] = 0.5 * (subdivision[i] + subdivision[i + 1]);
-        left[level] = subdivision.front();
-        right[curve.size() - level - 1] = subdivision[curve.size() - level - 1];
-    }
-}
-
-static void flatten_bezier(const QuinticBezier &curve, const double deviation, std::vector<Vec2d> &output)
-{
-    // Subdivide to at least depth 1 so a rounded corner cannot collapse to a single diagonal chord.
-    // A uniform subdivision depth keeps samples at equal parameter intervals t = k / 2^depth,
-    // avoiding abrupt segment-length jumps at adaptive-depth boundaries.
-    static constexpr size_t max_depth = 16;
-
-    std::vector<QuinticBezier> subcurves(2);
-    subdivide_bezier(curve, subcurves[0], subcurves[1]);
-
-    for (size_t depth = 1; depth < max_depth; ++depth) {
-        bool all_flat = true;
-        for (const QuinticBezier &c : subcurves)
-            if (!is_bezier_flat(c, deviation)) {
-                all_flat = false;
-                break;
-            }
-        if (all_flat)
-            break;
-        std::vector<QuinticBezier> finer(subcurves.size() * 2);
-        for (size_t i = 0; i < subcurves.size(); ++i)
-            subdivide_bezier(subcurves[i], finer[i * 2], finer[i * 2 + 1]);
-        subcurves = std::move(finer);
-    }
-
-    // The curve start is deliberately omitted so consecutive curve pieces can share it without duplication.
-    output.reserve(output.size() + subcurves.size());
-    for (const QuinticBezier &c : subcurves)
-        output.emplace_back(c.back());
-}
-
+// Rounds the corners of the generated path on its way to the infill output.
 template<typename Output>
-static void generate_smooth_hilbert_curve(
-    coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double resolution,
-    const double corner_distance, Output &output)
+class SmoothingPolylineOutput
 {
-    // A Hilbert curve is defined on a square grid whose side is a power of two. As in the unsmoothed
-    // generator, expand the larger requested dimension to the next valid Hilbert grid size. The output
-    // clipper or the later region intersection removes the padded part of the traversal.
-    size_t sz = 2;
-    const size_t sz0 = std::max(max_x + 1 - min_x, max_y + 1 - min_y);
-    while (sz < sz0)
-        sz <<= 1;
+public:
+    SmoothingPolylineOutput(Output &output, const double smooth_factor, const double tolerance)
+        : m_output(output), m_smoother(smooth_factor, tolerance) {}
 
-    const size_t point_count = sz * sz;
-    output.reserve(point_count);
+    void reserve(size_t n) { m_output.reserve(n); }
+    void add_point(const Vec2d &pt) { auto emit = emitter(); m_smoother.push(pt, emit); }
+    // The smoother holds back the last point of the path until it knows there is no corner left to round.
+    void finish() { auto emit = emitter(); m_smoother.flush(emit); }
 
-    // The caller normalizes resolution to the unit Hilbert grid; retain a finite positive tolerance
-    // if this helper is invoked with an invalid resolution.
-    const double deviation = resolution > 0. && std::isfinite(resolution) ? resolution : EPSILON;
-    // Construct one canonical 90-degree corner from (-corner_distance, 0) to (0, corner_distance).
-    // At each end, the first three control points are collinear and equally spaced: the tangent follows
-    // the adjoining straight leg and the second derivative is zero. The endpoint curvature is therefore
-    // zero, giving G2 joins to both legs. Every Hilbert turn is an oriented copy of this curve, so flatten
-    // it only once to the requested chordal-deviation tolerance.
-    const QuinticBezier corner_curve {{
-        {-corner_distance, 0.}, {-0.7 * corner_distance, 0.}, {-0.4 * corner_distance, 0.},
-        {0., 0.4 * corner_distance}, {0., 0.7 * corner_distance}, {0., corner_distance}
-    }};
-    std::vector<Vec2d> curve_coefficients;
-    flatten_bezier(corner_curve, deviation, curve_coefficients);
-
-    auto translated_point = [min_x, min_y](size_t idx) {
-        Point p = hilbert_n_to_xy(idx);
-        return Point(p.x() + min_x, p.y() + min_y);
-    };
-    auto to_vec2d = [](const Point &p) { return Vec2d(double(p.x()), double(p.y())); };
-    bool has_last_output = false;
-    Vec2d last_output;
-    // Fully smoothed adjacent corners may meet at the same segment midpoint. Suppress such duplicates
-    // to avoid emitting zero-length extrusion segments.
-    auto add_point = [&output, &has_last_output, &last_output](const Vec2d &point) {
-        if (!has_last_output || point.x() != last_output.x() || point.y() != last_output.y()) {
-            output.add_point(point);
-            last_output     = point;
-            has_last_output = true;
-        }
-    };
-
-    Vec2d previous = to_vec2d(translated_point(0));
-    Vec2d corner   = to_vec2d(translated_point(1));
-    add_point(previous);
-    // Replace each non-collinear Hilbert vertex by the canonical curve expressed in the local basis of
-    // its incoming and outgoing unit vectors. Collinear vertices remain part of the straight polyline.
-    for (size_t i = 1; i + 1 < point_count; ++i) {
-        const Vec2d next     = to_vec2d(translated_point(i + 1));
-        const Vec2d incoming = (corner - previous).normalized();
-        const Vec2d outgoing = (next - corner).normalized();
-        const double cross   = incoming.x() * outgoing.y() - incoming.y() * outgoing.x();
-
-        if (std::abs(cross) < EPSILON) {
-            add_point(corner);
-        } else {
-            add_point(corner - corner_distance * incoming);
-            for (const Vec2d &coefficient : curve_coefficients)
-                add_point(corner + coefficient.x() * incoming + coefficient.y() * outgoing);
-        }
-
-        previous = corner;
-        corner   = next;
+private:
+    // The curves of two adjacent corners meet at the midpoint of the segment they share, where they
+    // may round to the very same output point. Drop those, they would be zero length extrusions.
+    auto emitter()
+    {
+        return [this](const Vec2d &pt) {
+            const Point snapped = m_output.scaled(pt);
+            if (m_has_last_snapped && snapped == m_last_snapped)
+                return;
+            m_last_snapped     = snapped;
+            m_has_last_snapped = true;
+            m_output.add_point(pt);
+        };
     }
-    add_point(corner);
+
+    Output        &m_output;
+    CornerSmoother m_smoother;
+    Point          m_last_snapped { Point::Zero() };
+    bool           m_has_last_snapped { false };
+};
+
+// Runs the path generator against the concrete output type, optionally through the corner smoother.
+// The outputs do not share a virtual add_point(), so the type has to be resolved here.
+template<typename GenerateFn>
+static void generate_path(InfillPolylineOutput &output, const FillParams &params, const double resolution, GenerateFn generate)
+{
+    const double smooth_factor = sanitize_smooth_factor(params.smooth_factor);
+    auto run = [smooth_factor, resolution, &generate](auto &out) {
+        if (smooth_factor == 0.) {
+            generate(out);
+        } else {
+            SmoothingPolylineOutput<std::remove_reference_t<decltype(out)>> smoothing(out, smooth_factor, resolution);
+            generate(smoothing);
+            smoothing.finish();
+        }
+    };
+
+    if (output.clips())
+        run(static_cast<InfillPolylineClipper&>(output));
+    else
+        run(output);
 }
 
 void FillHilbertCurve::generate(coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double /* resolution */, InfillPolylineOutput &output)
@@ -440,19 +356,8 @@ void FillHilbertCurve::generate(coord_t min_x, coord_t min_y, coord_t max_x, coo
 void FillHilbertCurve::generate(coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double resolution,
     const FillParams &params, InfillPolylineOutput &output)
 {
-    const double smooth_factor = std::isfinite(params.smooth_factor) ?
-        std::clamp(params.smooth_factor, 0., 1.) : 0.;
-    if (smooth_factor == 0.) {
-        this->generate(min_x, min_y, max_x, max_y, resolution, output);
-        return;
-    }
-
-    const double corner_distance = 0.5 * smooth_factor;
-    if (output.clips())
-        generate_smooth_hilbert_curve(
-            min_x, min_y, max_x, max_y, resolution, corner_distance, static_cast<InfillPolylineClipper&>(output));
-    else
-        generate_smooth_hilbert_curve(min_x, min_y, max_x, max_y, resolution, corner_distance, output);
+    generate_path(output, params, resolution,
+        [min_x, min_y, max_x, max_y](auto &out) { generate_hilbert_curve(min_x, min_y, max_x, max_y, out); });
 }
 
 template<typename Output>
@@ -495,4 +400,11 @@ void FillOctagramSpiral::generate(coord_t min_x, coord_t min_y, coord_t max_x, c
         generate_octagram_spiral(min_x, min_y, max_x, max_y, output);
 }
 
+void FillOctagramSpiral::generate(coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double resolution,
+    const FillParams &params, InfillPolylineOutput &output)
+{
+    generate_path(output, params, resolution,
+        [min_x, min_y, max_x, max_y](auto &out) { generate_octagram_spiral(min_x, min_y, max_x, max_y, out); });
+}
+
 } // namespace Slic3r
diff --git a/src/libslic3r/Fill/FillPlanePath.hpp b/src/libslic3r/Fill/FillPlanePath.hpp
index b4b25b73ae..a1e9068ca9 100644
--- a/src/libslic3r/Fill/FillPlanePath.hpp
+++ b/src/libslic3r/Fill/FillPlanePath.hpp
@@ -21,10 +21,10 @@ public:
     void            add_point(const Vec2d& pt) { m_out.emplace_back(this->scaled(pt)); }
     Points&& result() { return std::move(m_out); }
     virtual bool    clips() const { return false; }
-
-protected:
+    // The output grid the generated points are snapped to.
     const Point     scaled(const Vec2d& fpt) const { return { coord_t(floor(fpt.x() * m_scale_out + 0.5)), coord_t(floor(fpt.y() * m_scale_out + 0.5)) }; }
 
+protected:
     // Output polyline.
     Points          m_out;
 
@@ -93,6 +93,8 @@ public:
 protected:
     bool centered() const override { return true; }
     void generate(coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double resolution, InfillPolylineOutput &output) override;
+    void generate(coord_t min_x, coord_t min_y, coord_t max_x, coord_t max_y, const double resolution,
+                              const FillParams &params, InfillPolylineOutput &output) override;
 };
 
 } // namespace Slic3r
diff --git a/src/libslic3r/Fill/FillRectilinear.cpp b/src/libslic3r/Fill/FillRectilinear.cpp
index 8b40b8753c..0c82354b38 100644
--- a/src/libslic3r/Fill/FillRectilinear.cpp
+++ b/src/libslic3r/Fill/FillRectilinear.cpp
@@ -18,6 +18,7 @@
 #include "../ShortestPath.hpp"
 #include "../VariableWidth.hpp"
 
+#include "FillCornerSmoothing.hpp"
 #include "FillRectilinear.hpp"
 
 // #define SLIC3R_DEBUG
@@ -3364,6 +3365,10 @@ bool FillRectilinear::fill_surface_trapezoidal(
         for (Polyline &pl : polylines)
             pl.translate(rotate_vector.second);
 
+    // Orca: round the corners of the trapezoids. The straight base lines of the triangular family
+    // have no corner to round.
+    smooth_polylines_corners(polylines, params.smooth_factor, scaled<double>(params.resolution));
+
     // Apply multiline fill
     multiline_fill(polylines, params, spacing);
 
diff --git a/src/libslic3r/PrintConfig.cpp b/src/libslic3r/PrintConfig.cpp
index f9d895332a..8083da954e 100644
--- a/src/libslic3r/PrintConfig.cpp
+++ b/src/libslic3r/PrintConfig.cpp
@@ -3469,9 +3469,8 @@ void PrintConfigDef::init_fff_params()
     def = this->add("sparse_infill_smooth_factor", coPercent);
     def->label = L("Sparse infill smooth factor");
     def->category = L("Strength");
-    def->tooltip = L("Controls how strongly sparse infill corners are rounded. 0% keeps the original right-angle path, "
-                     "while 100% produces the largest possible curves between adjacent infill lines. "
-                     "Currently applies only to the Hilbert Curve.");
+    def->tooltip = L("Controls how strongly sparse infill corners are rounded. 0% keeps the original sharp path, "
+                     "while 100% produces the largest possible curves between adjacent infill lines.");
     def->sidetext = "%";
     def->min = 0;
     def->max = 100;
diff --git a/src/libslic3r/PrintConfig.hpp b/src/libslic3r/PrintConfig.hpp
index f51c1c6411..26a708b78d 100644
--- a/src/libslic3r/PrintConfig.hpp
+++ b/src/libslic3r/PrintConfig.hpp
@@ -146,6 +146,29 @@ inline bool is_separable_infill_pattern(InfillPattern pattern)
     }
 }
 
+// Orca: Infill patterns that round their corners by the "sparse_infill_smooth_factor" option.
+// Grid, Triangles and Tri-hexagon only do so in their trapezoidal form, which is generated with more
+// than one line per infill wall; a single line makes them plain crossing lines with nothing to round.
+inline bool is_smoothable_infill_pattern(InfillPattern pattern, int multiline = 1)
+{
+    switch (pattern) {
+    case ipHilbertCurve:
+    case ipOctagramSpiral:
+    case ipLightning:
+    case ipHoneycomb:
+    case ip3DHoneycomb:
+    case ipConcentric:
+    case ipCrossHatch:
+        return true;
+    case ipGrid:
+    case ipTriangles:
+    case ipStars:
+        return multiline > 1;
+    default:
+        return false;
+    }
+}
+
 enum class IroningType {
     NoIroning,
     TopSurfaces,
diff --git a/src/slic3r/GUI/ConfigManipulation.cpp b/src/slic3r/GUI/ConfigManipulation.cpp
index 3885a391b8..de94bb6b4b 100644
--- a/src/slic3r/GUI/ConfigManipulation.cpp
+++ b/src/slic3r/GUI/ConfigManipulation.cpp
@@ -752,7 +752,7 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig *config, in
     bool has_top_shell    = has_top_shell_layers && config->option<ConfigOptionPercent>("top_surface_density")->value > 0;
     bool has_bottom_shell = config->opt_int("bottom_shell_layers") > 0;
     bool has_solid_infill = has_top_shell_layers || has_bottom_shell;
-    toggle_line("sparse_infill_smooth_factor", pattern == ipHilbertCurve);
+    toggle_line("sparse_infill_smooth_factor", is_smoothable_infill_pattern(pattern, config->opt_int("fill_multiline")));
     toggle_field("top_surface_pattern", has_top_shell);
     toggle_field("bottom_surface_pattern", has_bottom_shell);
     toggle_field("top_surface_density", has_top_shell_layers);
diff --git a/tests/fff_print/test_fill.cpp b/tests/fff_print/test_fill.cpp
index 5fbce5a342..07460d3990 100644
--- a/tests/fff_print/test_fill.cpp
+++ b/tests/fff_print/test_fill.cpp
@@ -698,3 +698,290 @@ TEST_CASE("Solid infill direction offsets every layer when no template is set",
         CHECK(delta == 30);
     }
 }
+
+TEST_CASE("Honeycomb infill rounds its cell corners with the smooth factor", "[Fill]")
+{
+    // A cell whose sides are several times the line width, so that the corners have room to be rounded.
+    const double spacing = 0.45;
+    const double density = 0.1;
+    auto         fill    = [spacing, density](double smooth_factor) {
+        std::unique_ptr<Slic3r::Fill> filler(Slic3r::Fill::new_from_type("honeycomb"));
+        filler->spacing = spacing;
+
+        FillParams params;
+        params.density = float(density);
+        params.dont_adjust = true;
+        // Keep the fragments apart, so that only the turns of the pattern itself are measured.
+        params.anchor_length_max = 0.f;
+        params.smooth_factor     = smooth_factor;
+
+        Slic3r::ExPolygon square{ Slic3r::Points{
+            Point::new_scale(0., 0.), Point::new_scale(50., 0.), Point::new_scale(50., 50.), Point::new_scale(0., 50.) } };
+        Slic3r::Surface surface(stInternal, square);
+        return filler->fill_surface(&surface, params);
+    };
+
+    // Cosine of the sharpest turn of any of the paths, 1 meaning none of them turns at all.
+    auto sharpest_turn_cosine = [](const Slic3r::Polylines &polylines) {
+        double sharpest = 1.;
+        for (const Polyline &polyline : polylines)
+            for (size_t i = 1; i + 1 < polyline.size(); ++i) {
+                const Vec2d incoming = (polyline[i] - polyline[i - 1]).cast<double>().normalized();
+                const Vec2d outgoing = (polyline[i + 1] - polyline[i]).cast<double>().normalized();
+                sharpest = std::min(sharpest, incoming.dot(outgoing));
+            }
+        return sharpest;
+    };
+    auto point_count = [](const Slic3r::Polylines &polylines) {
+        return std::accumulate(polylines.begin(), polylines.end(), size_t(0),
+                               [](size_t count, const Polyline &polyline) { return count + polyline.size(); });
+    };
+
+    const Slic3r::Polylines sharp  = fill(0.);
+    const Slic3r::Polylines smooth = fill(1.);
+
+    REQUIRE(!sharp.empty());
+    REQUIRE(smooth.size() == sharp.size());
+    REQUIRE(point_count(smooth) > point_count(sharp));
+    // The cell corners turn by 60 degrees; smoothing replaces them by gentle curves.
+    REQUIRE(sharpest_turn_cosine(sharp) < 0.6);
+    REQUIRE(sharpest_turn_cosine(smooth) > 0.9);
+}
+
+// Point count, number of turns sharper than 25 degrees and length of the sparse infill of a print.
+// A rounded corner is a run of much gentler turns, so smoothing shows up as fewer sharp ones.
+struct SparseInfillShape {
+    size_t point_count { 0 };
+    size_t sharp_turns { 0 };
+    size_t path_count { 0 };
+    double length { 0. };
+};
+
+static SparseInfillShape sparse_infill_shape(const Print &print)
+{
+    SparseInfillShape shape;
+
+    auto account = [&shape](const ExtrusionPath &path) {
+        if (!sparse_role(path.role()))
+            return;
+        const Points3 &pts = path.polyline.points;
+        ++shape.path_count;
+        shape.point_count += pts.size();
+        for (size_t i = 1; i < pts.size(); ++i)
+            shape.length += (pts[i] - pts[i - 1]).head<2>().cast<double>().norm();
+        for (size_t i = 1; i + 1 < pts.size(); ++i) {
+            const Vec2d incoming = (pts[i] - pts[i - 1]).head<2>().cast<double>();
+            const Vec2d outgoing = (pts[i + 1] - pts[i]).head<2>().cast<double>();
+            if (incoming.squaredNorm() > 0. && outgoing.squaredNorm() > 0. &&
+                incoming.normalized().dot(outgoing.normalized()) < 0.9)
+                ++shape.sharp_turns;
+        }
+    };
+
+    for (const Layer *layer : print.objects().front()->layers())
+        for (const LayerRegion *region : layer->regions())
+            for (const ExtrusionEntity *entity : region->fills.flatten().entities) {
+                if (auto *path = dynamic_cast<const ExtrusionPath *>(entity))
+                    account(*path);
+                else if (auto *multi = dynamic_cast<const ExtrusionMultiPath *>(entity))
+                    for (const ExtrusionPath &p : multi->paths)
+                        account(p);
+                else if (auto *loop = dynamic_cast<const ExtrusionLoop *>(entity))
+                    for (const ExtrusionPath &p : loop->paths)
+                        account(p);
+            }
+    return shape;
+}
+
+TEST_CASE("Lightning infill rounds the turns of its branches with the smooth factor", "[Fill]")
+{
+    auto shape_for = [](const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "lightning"},
+                                             {"sparse_infill_density", "15%"},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for("0%");
+    const SparseInfillShape smooth = shape_for("100%");
+
+    REQUIRE(sharp.point_count > 0);
+    // The branch turns are replaced by curves, which cut the corners off and take more points to
+    // describe. The turns where two branches are joined into one path stay sharp.
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+    REQUIRE(smooth.length < sharp.length);
+}
+
+TEST_CASE("Concentric infill rounds its loops with the smooth factor", "[Fill]")
+{
+    auto shape_for = [](const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "concentric"},
+                                             {"sparse_infill_density", "20%"},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for("0%");
+    const SparseInfillShape smooth = shape_for("100%");
+
+    REQUIRE(sharp.point_count > 0);
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+    REQUIRE(smooth.length < sharp.length);
+}
+
+TEST_CASE("Cross hatch infill rounds its transition layers with the smooth factor", "[Fill]")
+{
+    auto shape_for = [](const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "crosshatch"},
+                                             {"sparse_infill_density", "20%"},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for("0%");
+    const SparseInfillShape smooth = shape_for("100%");
+
+    REQUIRE(sharp.point_count > 0);
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+    REQUIRE(smooth.length < sharp.length);
+}
+
+TEST_CASE("Trapezoidal grid infill rounds its corners only with more than one line", "[Fill]")
+{
+    auto shape_for = [](int multiline, const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "grid"},
+                                             {"sparse_infill_density", "20%"},
+                                             {"fill_multiline", multiline},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for(2, "0%");
+    const SparseInfillShape smooth = shape_for(2, "100%");
+
+    REQUIRE(sharp.point_count > 0);
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+    REQUIRE(smooth.length < sharp.length);
+
+    // A single line per infill wall is the plain crossing line grid, which has no corner of its own.
+    const SparseInfillShape single_sharp  = shape_for(1, "0%");
+    const SparseInfillShape single_smooth = shape_for(1, "100%");
+    REQUIRE(single_sharp.point_count > 0);
+    REQUIRE(single_smooth.point_count == single_sharp.point_count);
+    REQUIRE(single_smooth.length == single_sharp.length);
+}
+
+TEST_CASE("3D honeycomb infill rounds its octahedral waves with the smooth factor", "[Fill]")
+{
+    auto shape_for = [](const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "3dhoneycomb"},
+                                             {"sparse_infill_density", "20%"},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for("0%");
+    const SparseInfillShape smooth = shape_for("100%");
+
+    REQUIRE(sharp.point_count > 0);
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+    REQUIRE(smooth.length < sharp.length);
+}
+
+TEST_CASE("Smoothed concentric infill stays inside the fill region", "[Fill][Regression]")
+{
+    // The concentric loops are offsets of the fill region and are never clipped to it, so a corner
+    // rounded across its boundary ends up in a hole or over a wall. Rounding cuts toward the inside of
+    // the turn, which leaves the region at every corner of a hole, and in a region thinner than the
+    // curve even at a corner turning inwards.
+    const bool  thin_region = GENERATE(false, true);
+    ExPolygon   region;
+    if (thin_region) {
+        // An L of two 1.2mm wide arms: cutting the corner they meet at crosses both of them.
+        region = ExPolygon{ Slic3r::Points{
+            Point::new_scale(0., 0.), Point::new_scale(20., 0.), Point::new_scale(20., 1.2),
+            Point::new_scale(1.2, 1.2), Point::new_scale(1.2, 20.), Point::new_scale(0., 20.) } };
+    } else {
+        region = ExPolygon{ Slic3r::Points{ Point::new_scale(0., 0.), Point::new_scale(50., 0.),
+                                            Point::new_scale(50., 50.), Point::new_scale(0., 50.) },
+                            Slic3r::Points{ Point::new_scale(30., 20.), Point::new_scale(30., 30.),
+                                            Point::new_scale(20., 30.), Point::new_scale(20., 20.) } };
+    }
+    CAPTURE(thin_region);
+
+    auto fill = [&region](double smooth_factor) {
+        std::unique_ptr<Slic3r::Fill> filler(Slic3r::Fill::new_from_type("concentric"));
+        filler->spacing = 0.45;
+
+        FillParams params;
+        params.density       = 0.1f;
+        params.dont_adjust   = true;
+        params.smooth_factor = smooth_factor;
+
+        Slic3r::Surface surface(stInternal, region);
+        return filler->fill_surface(&surface, params);
+    };
+    auto point_count = [](const Slic3r::Polylines &polylines) {
+        return std::accumulate(polylines.begin(), polylines.end(), size_t(0),
+                               [](size_t count, const Polyline &polyline) { return count + polyline.size(); });
+    };
+
+    const Slic3r::Polylines sharp  = fill(0.);
+    const Slic3r::Polylines smooth = fill(1.);
+    REQUIRE(!sharp.empty());
+
+    // Nothing leaves the fill region, which the unrounded loops already touch from the inside.
+    const ExPolygons bounds = offset_ex(region, float(SCALED_EPSILON));
+    REQUIRE(diff_pl(sharp, bounds).empty());
+    REQUIRE(diff_pl(smooth, bounds).empty());
+    // The corners that the region has room for are still rounded.
+    if (!thin_region)
+        REQUIRE(point_count(smooth) > point_count(sharp));
+}
+
+TEST_CASE("Smoothing multiline lightning infill keeps its outlines connected", "[Fill][Regression]")
+{
+    // With more than one line per infill wall, the branches are printed as outlines drawn around them,
+    // and the outlines of branches that run close to each other merge into one. Rounding the branches
+    // before those outlines are built moves them apart, which breaks the merged outlines up into
+    // separate loops - many more of them, each needing its own travel move.
+    auto shape_for = [](const std::string &smooth_factor) {
+        Print print;
+        Slic3r::Test::init_and_process_print({Slic3r::Test::cube(20)}, print,
+                                            {{"sparse_infill_pattern", "lightning"},
+                                             {"sparse_infill_density", "50%"},
+                                             {"fill_multiline", 2},
+                                             {"sparse_infill_smooth_factor", smooth_factor},
+                                             {"layer_height", 0.2}});
+        return sparse_infill_shape(print);
+    };
+
+    const SparseInfillShape sharp  = shape_for("0%");
+    const SparseInfillShape smooth = shape_for("100%");
+
+    REQUIRE(sharp.path_count > 0);
+    REQUIRE(smooth.path_count <= sharp.path_count);
+    // The outlines are still rounded.
+    REQUIRE(smooth.point_count > sharp.point_count);
+    REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
+}
diff --git a/tests/libslic3r/CMakeLists.txt b/tests/libslic3r/CMakeLists.txt
index 1ad299473c..bc10bb4f73 100644
--- a/tests/libslic3r/CMakeLists.txt
+++ b/tests/libslic3r/CMakeLists.txt
@@ -18,6 +18,7 @@ add_executable(${_TEST_NAME}_tests
     test_preset_setting_id.cpp
     test_preset_diff.cpp
     test_elephant_foot_compensation.cpp
+    test_fill_corner_smoothing.cpp
     test_fill_plane_path.cpp
     test_geometry.cpp
     test_multimaterial_segmentation.cpp
diff --git a/tests/libslic3r/test_fill_corner_smoothing.cpp b/tests/libslic3r/test_fill_corner_smoothing.cpp
new file mode 100644
index 0000000000..f2c25e816d
--- /dev/null
+++ b/tests/libslic3r/test_fill_corner_smoothing.cpp
@@ -0,0 +1,173 @@
+#include <catch2/catch_all.hpp>
+
+#include <algorithm>
+#include <cmath>
+#include <limits>
+
+#include "libslic3r/Fill/FillCornerSmoothing.hpp"
+#include "libslic3r/Polyline.hpp"
+#include "libslic3r/libslic3r.h"
+
+using namespace Slic3r;
+
+namespace {
+
+// A right angle turn, with the outgoing leg ten times longer than the incoming one.
+Polyline asymmetric_corner()
+{
+    return Polyline{ Point::new_scale(0., 0.), Point::new_scale(10., 0.), Point::new_scale(10., 100.) };
+}
+
+double max_turn_cosine(const Polyline &polyline)
+{
+    double sharpest = 1.;
+    for (size_t i = 1; i + 1 < polyline.size(); ++i) {
+        const Vec2d incoming = (polyline[i] - polyline[i - 1]).cast<double>().normalized();
+        const Vec2d outgoing = (polyline[i + 1] - polyline[i]).cast<double>().normalized();
+        sharpest = std::min(sharpest, incoming.dot(outgoing));
+    }
+    return sharpest;
+}
+
+bool contains(const Polyline &polyline, const Point &point)
+{
+    return std::find(polyline.points.begin(), polyline.points.end(), point) != polyline.points.end();
+}
+
+const double tolerance = scaled<double>(0.0125);
+
+} // namespace
+
+TEST_CASE("Corner smoothing replaces a sharp vertex by a curve", "[FillCornerSmoothing]")
+{
+    const Polyline sharp  = asymmetric_corner();
+    Polyline       smooth = sharp;
+    smooth_polyline_corners(smooth, 1., tolerance);
+
+    REQUIRE(smooth.size() > sharp.size());
+    REQUIRE(smooth.front() == sharp.front());
+    REQUIRE(smooth.back() == sharp.back());
+    // The right angle is gone, every remaining turn is a gentle one.
+    REQUIRE(max_turn_cosine(sharp) < 0.1);
+    REQUIRE(max_turn_cosine(smooth) > 0.9);
+    REQUIRE(smooth.length() < sharp.length());
+}
+
+TEST_CASE("Corner smoothing keeps the path untouched at a zero factor", "[FillCornerSmoothing]")
+{
+    const Polyline sharp = asymmetric_corner();
+
+    Polyline none = sharp;
+    smooth_polyline_corners(none, 0., tolerance);
+    REQUIRE(none.points == sharp.points);
+
+    Polyline invalid = sharp;
+    smooth_polyline_corners(invalid, std::numeric_limits<double>::quiet_NaN(), tolerance);
+    REQUIRE(invalid.points == sharp.points);
+}
+
+TEST_CASE("Corner smoothing consumes at most half of the shorter leg", "[FillCornerSmoothing]")
+{
+    // The curve must not reach beyond the middle of either adjoining segment, otherwise the curves of
+    // two adjacent corners would overlap. The shorter leg is 10mm long, so the corner at (10, 0) is
+    // left 5mm before it and rejoined 5mm past it, even though the other leg is 100mm long.
+    Polyline smooth = asymmetric_corner();
+    smooth_polyline_corners(smooth, 1., tolerance);
+
+    REQUIRE(contains(smooth, Point::new_scale(5., 0.)));
+    REQUIRE(contains(smooth, Point::new_scale(10., 5.)));
+    // A Bezier curve stays within the convex hull of its control points, so the rounded path stays
+    // inside the box spanned by the two legs.
+    for (const Point &point : smooth.points) {
+        REQUIRE(point.x() >= 0);
+        REQUIRE(point.y() >= 0);
+        REQUIRE(point.x() <= Point::new_scale(10., 0.).x());
+        REQUIRE(point.y() <= Point::new_scale(0., 100.).y());
+    }
+}
+
+TEST_CASE("Corner smoothing scales the curve with the factor", "[FillCornerSmoothing]")
+{
+    Polyline half = asymmetric_corner();
+    smooth_polyline_corners(half, 0.5, tolerance);
+    Polyline full = asymmetric_corner();
+    smooth_polyline_corners(full, 1., tolerance);
+
+    // Half of the factor leaves the 10mm leg half as far from the corner.
+    REQUIRE(contains(half, Point::new_scale(7.5, 0.)));
+    REQUIRE(contains(full, Point::new_scale(5., 0.)));
+    // A larger factor rounds a wider portion of the legs, cutting more of the corner off.
+    REQUIRE(full.length() < half.length());
+}
+
+TEST_CASE("Corner smoothing leaves hairpins sharp", "[FillCornerSmoothing]")
+{
+    // Both ends of a curve replacing a nearly reversing turn coincide, which would round the hairpin
+    // into a degenerate loop instead of a tip.
+    Polyline hairpin{ Point::new_scale(0., 0.), Point::new_scale(10., 0.), Point::new_scale(0., 0.5) };
+    const Polyline sharp = hairpin;
+    smooth_polyline_corners(hairpin, 1., tolerance);
+    REQUIRE(hairpin == sharp);
+}
+
+TEST_CASE("Corner smoothing follows the flattening tolerance", "[FillCornerSmoothing]")
+{
+    Polyline coarse = asymmetric_corner();
+    smooth_polyline_corners(coarse, 1., scaled<double>(0.2));
+    Polyline fine = asymmetric_corner();
+    smooth_polyline_corners(fine, 1., scaled<double>(0.001));
+
+    REQUIRE(fine.size() > coarse.size());
+    REQUIRE(fine.front() == coarse.front());
+    REQUIRE(fine.back() == coarse.back());
+}
+
+TEST_CASE("Corner smoothing emits no zero length segments", "[FillCornerSmoothing]")
+{
+    // Fully smoothed adjacent corners meet at the midpoint of the segment they share.
+    Polyline zigzag;
+    for (int i = 0; i < 8; ++i)
+        zigzag.points.emplace_back(Point::new_scale(i, i % 2 ? 1. : 0.));
+    smooth_polyline_corners(zigzag, 1., tolerance);
+
+    for (size_t i = 1; i < zigzag.size(); ++i)
+        REQUIRE((zigzag[i] - zigzag[i - 1]).cast<double>().squaredNorm() > 0.);
+}
+
+TEST_CASE("Corner smoothing rounds every vertex of a polygon", "[FillCornerSmoothing]")
+{
+    // A polygon closes implicitly, so none of its corners may stay sharp, not even the first one.
+    const Polygon square{ Point::new_scale(0., 0.), Point::new_scale(10., 0.), Point::new_scale(10., 10.),
+                          Point::new_scale(0., 10.) };
+    Polygons smooth{ square };
+    smooth_polygons_corners(smooth, 1., tolerance);
+    const Polyline rounded = smooth.front().split_at_first_point();
+
+    REQUIRE(smooth.front().size() > square.size());
+    REQUIRE(max_turn_cosine(rounded) > 0.9);
+    // The turn from the closing segment back into the first one must be gentle as well.
+    const Vec2d incoming = (rounded[rounded.size() - 1] - rounded[rounded.size() - 2]).cast<double>().normalized();
+    const Vec2d outgoing = (rounded[1] - rounded[0]).cast<double>().normalized();
+    REQUIRE(incoming.dot(outgoing) > 0.9);
+    // None of the corners is cut by more than half of a 10mm side.
+    for (const Point &point : smooth.front().points) {
+        REQUIRE(point.x() >= 0);
+        REQUIRE(point.y() >= 0);
+        REQUIRE(point.x() <= Point::new_scale(10., 0.).x());
+        REQUIRE(point.y() <= Point::new_scale(0., 10.).y());
+    }
+}
+
+TEST_CASE("Corner smoothing keeps the ends of a path that returns to its start", "[FillCornerSmoothing][Regression]")
+{
+    // A branch of a lightning tree walks out and retraces its way back, ending where it started. Its
+    // ends are two free ends that happen to coincide, and joining them would close it into a loop.
+    Polyline retrace{ Point::new_scale(0., 0.), Point::new_scale(10., 0.), Point::new_scale(10., 10.),
+                      Point::new_scale(5., 10.), Point::new_scale(0., 0.) };
+    const Polyline sharp = retrace;
+    smooth_polyline_corners(retrace, 1., tolerance);
+
+    REQUIRE(retrace.size() > sharp.size());
+    REQUIRE(retrace.front() == sharp.front());
+    REQUIRE(retrace.back() == sharp.back());
+}
diff --git a/tests/libslic3r/test_fill_plane_path.cpp b/tests/libslic3r/test_fill_plane_path.cpp
index bbb75dce58..7fc7f4a6b0 100644
--- a/tests/libslic3r/test_fill_plane_path.cpp
+++ b/tests/libslic3r/test_fill_plane_path.cpp
@@ -27,6 +27,31 @@ public:
     }
 };
 
+class TestableOctagramSpiral : public FillOctagramSpiral
+{
+public:
+    Points generate_points(double resolution, double smooth_factor = 0., coord_t max_coordinate = 7)
+    {
+        InfillPolylineOutput output(output_scale);
+        FillParams params;
+        params.smooth_factor = smooth_factor;
+        FillOctagramSpiral::generate(-max_coordinate, -max_coordinate, max_coordinate, max_coordinate, resolution, params, output);
+        return std::move(output.result());
+    }
+};
+
+// Cosine of the sharpest turn of a path, 1 meaning it has no turn at all.
+double sharpest_turn_cosine(const Points &points)
+{
+    double sharpest = 1.;
+    for (size_t i = 1; i + 1 < points.size(); ++i) {
+        const Vec2d incoming = (points[i] - points[i - 1]).cast<double>().normalized();
+        const Vec2d outgoing = (points[i + 1] - points[i]).cast<double>().normalized();
+        sharpest = std::min(sharpest, incoming.dot(outgoing));
+    }
+    return sharpest;
+}
+
 double path_length(const Points &points)
 {
     double length = 0.;
@@ -146,6 +171,35 @@ TEST_CASE("Hilbert smoothing joins straight segments with continuous curvature",
     REQUIRE(fine_entry_curvature < 0.25 * coarse_entry_curvature);
 }
 
+TEST_CASE("Octagram spiral smoothing rounds the turns of the spiral", "[FillPlanePath]")
+{
+    const Points sharp  = TestableOctagramSpiral().generate_points(0.005);
+    const Points smooth = TestableOctagramSpiral().generate_points(0.005, 1.);
+
+    REQUIRE(smooth.size() > sharp.size());
+    REQUIRE(smooth.front() == sharp.front());
+    REQUIRE(smooth.back() == sharp.back());
+    // The spiral alternates between 90 and 135 degree turns; both are rounded into gentle ones.
+    REQUIRE(sharpest_turn_cosine(sharp) < -0.7);
+    REQUIRE(sharpest_turn_cosine(smooth) > 0.9);
+
+    for (size_t i = 1; i < smooth.size(); ++i)
+        REQUIRE((smooth[i] - smooth[i - 1]).cast<double>().squaredNorm() > 0.);
+}
+
+TEST_CASE("Octagram spiral smooth factor controls corner curvature", "[FillPlanePath]")
+{
+    const Points sharp          = TestableOctagramSpiral().generate_points(0.005);
+    const Points half_smooth    = TestableOctagramSpiral().generate_points(0.005, 0.5);
+    const Points full_smooth    = TestableOctagramSpiral().generate_points(0.005, 1.);
+    const Points invalid_factor = TestableOctagramSpiral().generate_points(
+        0.005, std::numeric_limits<double>::quiet_NaN());
+
+    REQUIRE(path_length(full_smooth) < path_length(half_smooth));
+    REQUIRE(path_length(half_smooth) < path_length(sharp));
+    REQUIRE(invalid_factor == sharp);
+}
+
 TEST_CASE("Hilbert curve smooth factor controls corner curvature", "[FillPlanePath]")
 {
     const Points sharp       = TestableHilbertCurve().generate_points(0.005);