SPE-2709: Fix precision of calculation line cross for thick field creation purpose:

Fixes freezing of SLA support point generator in certain cases

src/libslic3r/SLA/SupportIslands/UniformSupportIsland.cpp

@@ -1164,10 +1164,9 @@ Field create_thick_field(const ThickPart& part, const Lines &lines, const Sample
             assert(!changes.empty());
             size_t change_index = 0;
             if (!points.empty()) { // Not first point, could lead to termination
-                const Point &last_point = points.back();
                 LineUtils::SortFromAToB pred(lines[index]);
                 bool no_change = false;
-                while (pred.compare(changes[change_index].new_b, last_point)) {
+                while (pred.compare(changes[change_index].new_b, points.back())) {
                     ++change_index;
                     if (change_index >= changes.size()) {
                         no_change = true;
@@ -1244,13 +1243,39 @@ Field create_thick_field(const ThickPart& part, const Lines &lines, const Sample
     size_t outline_index = input_index;
     // Done indexes is used to detect holes in field
     std::set<size_t> done_indices; // IMPROVE: use vector(size of lines count) with bools
+#ifdef SLA_SAMPLE_ISLAND_UTILS_STORE_FIELD_TO_SVG_PATH    
+    static int counter = 0;
+    std::string field_to_svg_path = replace_first(
+        SLA_SAMPLE_ISLAND_UTILS_STORE_FIELD_TO_SVG_PATH, "<<COUNTER>>", std::to_string(counter++));
+    {
+        SVG svg(field_to_svg_path.c_str(), LineUtils::create_bounding_box(lines));
+        LineUtils::draw(svg, lines, "black", 0., /*indices*/ true);
+        for(const auto& change_it: wide_tiny_changes)
+            for (const auto& change: change_it.second){
+                Line bisector(change.new_b, change.next_new_a);
+                LineUtils::draw(svg, bisector, "red");
+                std::string text = "from " + std::to_string(change_it.first) 
+                    + " to " + std::to_string(change.next_line_index);
+                svg.draw_text(bisector.a/2 + bisector.b/2, text.c_str(), "orange");
+            }
+    } // flush svg file
+#endif // SLA_SAMPLE_ISLAND_UTILS_STORE_FIELD_TO_SVG_PATH
     do {
         if (!insert_changes(outline_index, points, done_indices, input_index))
             break;        
         inser_point_b(outline_index, points, done_indices);
+
+        if (points.size() > (lines.size() + 2*part.ends.size())){
+            // protection against endless loop
+            assert(false);
+            return {};
+        }
     } while (outline_index != input_index);
 
     assert(points.size() >= 3);
+    if (points.size() < 3)
+        return {}; // invalid field
+
     ExPolygon border{Polygon{points}};
     // finding holes(another closed polygon)
     if (done_indices.size() < field_line_indices.size()) {
@@ -1279,9 +1304,7 @@ Field create_thick_field(const ThickPart& part, const Lines &lines, const Sample
         bool draw_source_line_indexes = true;
         bool draw_border_line_indexes = false;
         bool draw_field_source_indexes = true;
-        static int  counter   = 0;
-        SVG svg(replace_first(SLA_SAMPLE_ISLAND_UTILS_STORE_FIELD_TO_SVG_PATH, 
-        "<<COUNTER>>", std::to_string(counter++)).c_str(),LineUtils::create_bounding_box(lines));
+        SVG svg(field_to_svg_path.c_str(),LineUtils::create_bounding_box(lines));
         LineUtils::draw(svg, lines, source_line_color, 0., draw_source_line_indexes);
         draw(svg, field, border, draw_border_line_indexes, draw_field_source_indexes);
     }

src/libslic3r/SLA/SupportIslands/VoronoiGraphUtils.cpp

@@ -1000,7 +1000,16 @@ std::pair<Slic3r::Point, Slic3r::Point> VoronoiGraphUtils::point_on_lines(
         Line intersecting_line(edge_point, edge_point + PointUtils::perp(dir));
         std::optional<Vec2d> intersection = LineUtils::intersection(line, intersecting_line);
         assert(intersection.has_value());
-        return intersection->cast<coord_t>();
+        Point result = intersection->cast<coord_t>();
+        // result MUST lay on the line, accuracy of float intersection could move point out of line
+        coord_t tolerance = 5; // for sure it is 5 but found case which need tolerance(1) - SPE-2709
+        if (abs(result.x() - line.a.x()) < tolerance && 
+            abs(result.y() - line.a.y()) < tolerance)
+            return line.a; // almost point a
+        if (abs(result.x() - line.b.x()) < tolerance && 
+            abs(result.y() - line.b.y()) < tolerance)
+            return line.b; // almost point b
+        return result;
     };
     
     return {point_on_line(edge), point_on_line(edge->twin())};

tests/sla_print/sla_supptgen_tests.cpp

@@ -587,6 +587,34 @@ TEST_CASE("Uniform sample test islands", "[SupGen], [VoronoiSkeleton]")
     }
 }
 
+TEST_CASE("Sample island with config", "[SupportIsland]") {
+    // set_logging_level(5);
+    SampleConfig cfg{
+        /*thin_max_distance*/ 5832568,
+        /*thick_inner_max_distance*/ 7290710,
+        /*thick_outline_max_distance*/ 5468032,
+        /*head_radius*/ 250000,
+        /*minimal_distance_from_outline*/ 250000,
+        /*maximal_distance_from_outline*/ 1944189,
+        /*max_length_for_one_support_point*/ 1869413,
+        /*max_length_for_two_support_points*/ 7290710,
+        /*max_length_ratio_for_two_support_points*/ 0.250000000f,
+        /*thin_max_width*/ 4673532,
+        /*thick_min_width*/ 4019237,
+        /*min_part_length*/ 5832568,
+        /*minimal_move*/ 100000,
+        /*count_iteration*/ 30,
+        /*max_align_distance*/ 3645355,
+        /*simplification_tolerance*/ 50000.000000000007
+        //*path*/, "C:/data/temp/islands/<<order>>.svg" // need define OPTION_TO_STORE_ISLAND in SampleConfig.hpp
+    };
+    std::string dir = std::string(TEST_DATA_DIR PATH_SEPARATOR) + "sla_islands/";
+    ExPolygon island = load_svg(dir + "SPE-2709.svg"); // Bad field creation
+    SupportIslandPoints points = test_island_sampling(island, cfg);
+    // in time of write poins.size() == 39
+    CHECK(points.size() > 22); // not only thin parts
+}
+
 TEST_CASE("Disable visualization", "[hide]") 
 {
     CHECK(true);