[CURA-9474] (Some) Seams Not On Vertices (#2087)

include/InsetOrderOptimizer.h

@@ -113,6 +113,15 @@ class InsetOrderOptimizer
     Shape retraction_region_; // After printing an outer wall, move into this region so that retractions do not leave visible blobs. Calculated lazily if needed (see
                               // retraction_region_calculated).
 
+    /*!
+     * Given a closed polygon, insert a seam point at the point where the seam should be placed.
+     * This should result in the seam-finding algorithm finding that exact point, instead of the
+     * 'best' vertex on that polygon. Under certain circumstances, the seam-placing algorithm can
+     * however still deviate from this, for example when the seam-point placed here isn't suppored
+     * by the layer below.
+     */
+    void insertSeamPoint(ExtrusionLine& closed_line);
+
     /*!
      * Determine if the paths should be reversed
      * If there is one extruder used, and we're currently printing the inner walls then Reversing the insets now depends on the inverse of

src/InsetOrderOptimizer.cpp

@@ -110,10 +110,14 @@ bool InsetOrderOptimizer::addToLayer()
         group_outer_walls,
         disallowed_areas_for_seams_);
 
-    for (const auto& line : walls_to_be_added)
+    for (auto& line : walls_to_be_added)
     {
         if (line.is_closed_)
         {
+            if (! settings_.get<bool>("z_seam_on_vertex"))
+            {
+                insertSeamPoint(line);
+            }
             order_optimizer.addPolygon(&line);
         }
         else
@@ -164,6 +168,56 @@ bool InsetOrderOptimizer::addToLayer()
     return added_something;
 }
 
+void InsetOrderOptimizer::insertSeamPoint(ExtrusionLine& closed_line)
+{
+    assert(closed_line.is_closed_);
+    assert(closed_line.size() >= 3);
+
+    Point2LL request_point;
+    switch (z_seam_config_.type_)
+    {
+    case EZSeamType::USER_SPECIFIED:
+        request_point = z_seam_config_.pos_;
+        break;
+    case EZSeamType::SHORTEST:
+        request_point = gcode_layer_.getLastPlannedPositionOrStartingPosition();
+        break;
+    default:
+        return;
+    }
+
+    // NOTE: Maybe rewrite this once we can use C++23 ranges::views::adjacent
+    size_t closest_junction_idx = 0;
+    coord_t closest_distance_sqd = std::numeric_limits<coord_t>::max();
+    for (const auto& [i, junction] : closed_line.junctions_ | ranges::views::enumerate)
+    {
+        const auto& next_junction = closed_line.junctions_[(i + 1) % closed_line.junctions_.size()];
+        const coord_t distance_sqd = LinearAlg2D::getDist2FromLineSegment(junction.p_, request_point, next_junction.p_);
+        if (distance_sqd < closest_distance_sqd)
+        {
+            closest_distance_sqd = distance_sqd;
+            closest_junction_idx = i;
+        }
+    }
+
+    const auto& start_pt = closed_line.junctions_[closest_junction_idx];
+    const auto& end_pt = closed_line.junctions_[(closest_junction_idx + 1) % closed_line.junctions_.size()];
+    const auto closest_point = LinearAlg2D::getClosestOnLineSegment(request_point, start_pt.p_, end_pt.p_);
+    constexpr coord_t smallest_dist_sqd = 25;
+    if (vSize2(closest_point - start_pt.p_) <= smallest_dist_sqd || vSize2(closest_point - end_pt.p_) <= smallest_dist_sqd)
+    {
+        return;
+    }
+
+    // NOTE: This could also be done on a single axis (skipping the implied sqrt), but figuring out which one and then using the right values became a bit messy/verbose.
+    const coord_t total_dist = vSize(end_pt.p_ - start_pt.p_);
+    const coord_t start_dist = vSize(closest_point - start_pt.p_);
+    const coord_t end_dist = vSize(closest_point - end_pt.p_);
+    const coord_t w = ((end_pt.w_ * end_dist) / total_dist) + ((start_pt.w_ * start_dist) / total_dist);
+
+    closed_line.junctions_.insert(closed_line.junctions_.begin() + closest_junction_idx + 1, ExtrusionJunction(closest_point, w, start_pt.perimeter_index_));
+}
+
 InsetOrderOptimizer::value_type InsetOrderOptimizer::getRegionOrder(const std::vector<ExtrusionLine>& extrusion_lines, const bool outer_to_inner)
 {
     if (extrusion_lines.empty())