subdivide non-manifold 2d meshes

model2d/mesh_ops.go

@@ -93,6 +93,53 @@ func (m *Mesh) Subdivide(iters int) *Mesh {
 	return current
 }
 
+// SubdividePath is like Subdivide, except that it is meant
+// to be applied to meshes that include unclosed paths
+// rather than closed, manifold meshes.
+//
+// There should be no cycles in the paths in the mesh, i.e.
+// no vertices with more than two attached segments.
+func (m *Mesh) SubdividePath(iters int) *Mesh {
+	if iters < 1 {
+		panic("must subdivide for at least one iteration")
+	}
+	current := m
+	for i := 0; i < iters; i++ {
+		next := NewMesh()
+		current.Iterate(func(s *Segment) {
+			len0 := len(current.Find(s[0]))
+			len1 := len(current.Find(s[1]))
+			if len0 > 2 || len1 > 2 {
+				panic("mesh does not consist of manifold sub-meshes or unclosed loops")
+			}
+			mp1 := s[0].Scale(0.75).Add(s[1].Scale(0.25))
+			mp2 := s[1].Scale(0.75).Add(s[0].Scale(0.25))
+			if len0 == 2 && len1 == 2 {
+				next.Add(&Segment{mp1, mp2})
+			}
+
+			if len1 == 2 {
+				others := current.Find(s[1])
+				s1 := others[0]
+				if s1 == s {
+					s1 = others[1]
+				}
+				mp3 := s1[0].Scale(0.75).Add(s1[1].Scale(0.25))
+				next.Add(&Segment{mp2, mp3})
+				if len0 == 1 {
+					next.Add(&Segment{s[0], mp2})
+				}
+			} else if len0 == 2 {
+				next.Add(&Segment{mp1, s[1]})
+			} else {
+				next.Add(&Segment{s[0], s[1]})
+			}
+		})
+		current = next
+	}
+	return current
+}
+
 // Manifold checks if the mesh is manifold, i.e. if every
 // vertex has two segments.
 func (m *Mesh) Manifold() bool {

model2d/mesh_ops_test.go

@@ -56,6 +56,32 @@ func TestMeshSubdivide(t *testing.T) {
 	}
 }
 
+func TestMeshSubdividePath(t *testing.T) {
+	mesh := MarchingSquaresSearch(&Circle{Radius: 0.9}, 0.01, 8)
+
+	// Remove some segment and subdivide it separately,
+	// which should actually leave it unchanged.
+	seg := mesh.SegmentsSlice()[0]
+	mesh.Remove(seg)
+	mesh1 := mesh.SubdividePath(2)
+	mesh2 := NewMeshSegments([]*Segment{seg}).SubdividePath(2)
+	mesh1.AddMesh(mesh2)
+	mesh.Add(seg)
+
+	MustValidateMesh(t, mesh1, true)
+
+	sdf := MeshToSDF(mesh)
+	sdf1 := MeshToSDF(mesh1)
+	for i := 0; i < 1000; i++ {
+		c := NewCoordRandNorm()
+		s1 := sdf.SDF(c)
+		s2 := sdf1.SDF(c)
+		if math.Abs(s1-s2) > 0.01 {
+			t.Errorf("bad SDF at %v: expected %f but got %f", c, s1, s2)
+		}
+	}
+}
+
 func TestMeshRepair(t *testing.T) {
 	original := NewMesh()
 	for i := 0.0; i < 2*math.Pi; i += 0.1 {