Fix Mesh Boolean negative (cut) text object (#16275)

* Fix Mesh Boolean negative (cut) text object

* Update test_meshboolean.cpp

* missing headers

* Update test_meshboolean.cpp

* Refactor mcut difference test for source splits

* Apply suggestion from @raistlin7447

src/libslic3r/MeshBoolean.cpp

@@ -1045,7 +1045,28 @@ void do_boolean(McutMesh& srcMesh, const McutMesh& cutMesh, const std::string& b
     // But we can force it to work by spliting the src mesh into disconnected components,
     // and do booleans seperately, then merge all the results.
     indexed_triangle_set all_its;
-    if (boolean_opts == "UNION" || boolean_opts == "A_NOT_B") {
+    if (boolean_opts == "A_NOT_B") {
+        // Each cut can leave the source with several disconnected components, which mcut rejects
+        // in the next dispatch, so re-split after every cut part (e.g. each letter of a text).
+        std::vector<indexed_triangle_set> parts = std::move(src_parts);
+        for (size_t j = 0; j < cut_parts.size(); j++) {
+            auto cut_part = triangle_mesh_to_mcut(cut_parts[j]);
+            std::vector<indexed_triangle_set> next_parts;
+            for (indexed_triangle_set &part : parts) {
+                auto src_part = triangle_mesh_to_mcut(part);
+                if (do_boolean_single(*src_part, *cut_part, boolean_opts)) {
+                    TriangleMesh tri_part = mcut_to_triangle_mesh(*src_part);
+                    std::vector<indexed_triangle_set> pieces = its_split(tri_part.its);
+                    std::move(pieces.begin(), pieces.end(), std::back_inserter(next_parts));
+                } else
+                    next_parts.emplace_back(std::move(part));
+            }
+            parts = std::move(next_parts);
+        }
+        for (const indexed_triangle_set &part : parts)
+            its_merge(all_its, part);
+    }
+    else if (boolean_opts == "UNION") {
         for (size_t i = 0; i < src_parts.size(); i++) {
             auto src_part = triangle_mesh_to_mcut(src_parts[i]);
             for (size_t j = 0; j < cut_parts.size(); j++) {

tests/libslic3r/test_meshboolean.cpp

@@ -1,9 +1,11 @@
 #include <catch2/catch_all.hpp>
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/catch_approx.hpp>
-
+#include <catch2/matchers/catch_matchers.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
 #include <libslic3r/TriangleMesh.hpp>
 #include <libslic3r/MeshBoolean.hpp>
+#include <vector>
 
 using namespace Slic3r;
 
@@ -24,3 +26,39 @@ TEST_CASE("CGAL and TriangleMesh conversions", "[MeshBoolean]") {
     
     REQUIRE(! MeshBoolean::cgal::does_self_intersect(M));
 }
+
+TEST_CASE("mcut difference handles source splits between cuts", "[MeshBoolean]") {
+    TriangleMesh body = make_cube(30., 10., 10.);
+
+    TriangleMesh tool;
+
+    // First cut splits the source into two disconnected components.
+    TriangleMesh slab = make_cube(2., 12., 20.);
+    slab.translate(Vec3f(14.f, -1.f, -5.f));
+    its_merge(tool.its, slab.its);
+
+    // These cuts must still be applied after the source has been split.
+    TriangleMesh left_hole = make_cube(4., 4., 20.);
+    left_hole.translate(Vec3f(3.f, 3.f, -5.f));
+    its_merge(tool.its, left_hole.its);
+
+    TriangleMesh right_hole = make_cube(4., 4., 20.);
+    right_hole.translate(Vec3f(21.f, 3.f, -5.f));
+    its_merge(tool.its, right_hole.its);
+
+    std::vector<TriangleMesh> result;
+    MeshBoolean::mcut::make_boolean(body, tool, result, "A_NOT_B");
+
+    REQUIRE(result.size() == 1);
+
+    const std::vector<indexed_triangle_set> components =
+        its_split(result.front().its);
+
+    REQUIRE(components.size() == 2);
+
+    // 3000 - 200 - 160 - 160 = 2480.
+    REQUIRE_THAT(
+        result.front().volume(),
+        Catch::Matchers::WithinRel(2480., 1e-3)
+    );
+}