Ensure properties maintained through Decompose operation (#794)

* Fix for #786 by setting numProps in GatherFaces method. Added test for Decompose with properties, and ExpectProperties function in test_main in order to check properties in a mesh match those set by SetProperties.

* Correct formatting of ExpectProperties

* Correct more formatting

* Removed ExpectProperties and instead use existing test methods

* Call RelatedGL for each Decomposed Manifold

Author: harmanpa

src/manifold/src/constructors.cpp

@@ -509,7 +509,6 @@ std::vector<Manifold> Manifold::Decompose() const {
 
     impl->GatherFaces(*pImpl_, faceNew2Old);
     impl->ReindexVerts(vertNew2Old, pImpl_->NumVert());
-
     impl->Finish();
 
     meshes.push_back(Manifold(impl));

src/manifold/src/sort.cpp

@@ -464,6 +464,7 @@ void Manifold::Impl::GatherFaces(const Impl& old, const Vec<int>& faceNew2Old) {
     gather(policy, faceNew2Old.begin(), faceNew2Old.end(),
            old.meshRelation_.triProperties.begin(),
            meshRelation_.triProperties.begin());
+    meshRelation_.numProp = old.meshRelation_.numProp;
     meshRelation_.properties = old.meshRelation_.properties;
   }
 

test/manifold_test.cpp

@@ -160,6 +160,29 @@ TEST(Manifold, Decompose) {
   RelatedGL(manifolds, input);
 }
 
+TEST(Manifold, DecomposeProps) {
+  std::vector<MeshGL> input;
+  std::vector<Manifold> manifoldList;
+  auto tet = WithPositionColors(Manifold::Tetrahedron());
+  manifoldList.emplace_back(tet);
+  input.emplace_back(tet);
+  auto cube = WithPositionColors(Manifold::Cube());
+  manifoldList.emplace_back(cube);
+  input.emplace_back(cube);
+  auto sphere = WithPositionColors(Manifold::Sphere(1, 4));
+  manifoldList.emplace_back(sphere);
+  input.emplace_back(sphere);
+  Manifold manifolds = Manifold::Compose(manifoldList);
+
+  ExpectMeshes(manifolds, {{8, 12, 3}, {6, 8, 3}, {4, 4, 3}});
+
+  RelatedGL(manifolds, input);
+
+  for (const Manifold& manifold : manifolds.Decompose()) {
+    RelatedGL(manifold, input);
+  }
+}
+
 /**
  * These tests check the various manifold constructors.
  */