agent-tools/mesh-tool/mesh/mesh.go

package mesh

import "fmt"

// Triangle indexes three vertices, counter-clockwise seen from outside.
type Triangle [3]int

// Mesh is one named object: a triangle soup over a shared vertex list.
type Mesh struct {
	Name  string
	Verts []Vec3
	Tris  []Triangle
}

// Scene is an ordered list of meshes, matching OBJ objects. STL files
// load as a single-mesh scene.
type Scene struct {
	Meshes []*Mesh
}

func (s *Scene) TotalVerts() int {
	n := 0
	for _, m := range s.Meshes {
		n += len(m.Verts)
	}
	return n
}

func (s *Scene) TotalTris() int {
	n := 0
	for _, m := range s.Meshes {
		n += len(m.Tris)
	}
	return n
}

// Mesh returns the named mesh, or nil.
func (s *Scene) Mesh(name string) *Mesh {
	for _, m := range s.Meshes {
		if m.Name == name {
			return m
		}
	}
	return nil
}

// Merged flattens the scene into a single mesh (copies data).
func (s *Scene) Merged() *Mesh {
	out := &Mesh{Name: "merged"}
	for _, m := range s.Meshes {
		off := len(out.Verts)
		out.Verts = append(out.Verts, m.Verts...)
		for _, t := range m.Tris {
			out.Tris = append(out.Tris, Triangle{t[0] + off, t[1] + off, t[2] + off})
		}
	}
	if len(s.Meshes) == 1 {
		out.Name = s.Meshes[0].Name
	}
	return out
}

// Append adds meshes from another scene, de-duplicating names by
// appending _2, _3, ...
func (s *Scene) Append(other *Scene) {
	taken := map[string]bool{}
	for _, m := range s.Meshes {
		taken[m.Name] = true
	}
	for _, m := range other.Meshes {
		name := m.Name
		for i := 2; taken[name]; i++ {
			name = fmt.Sprintf("%s_%d", m.Name, i)
		}
		m.Name = name
		taken[name] = true
		s.Meshes = append(s.Meshes, m)
	}
}

// FaceNormal returns the (unit) normal of triangle i.
func (m *Mesh) FaceNormal(i int) Vec3 {
	t := m.Tris[i]
	a, b, c := m.Verts[t[0]], m.Verts[t[1]], m.Verts[t[2]]
	return b.Sub(a).Cross(c.Sub(a)).Norm()
}

// FlipWinding reverses the orientation of every triangle.
func (m *Mesh) FlipWinding() {
	for i, t := range m.Tris {
		m.Tris[i] = Triangle{t[0], t[2], t[1]}
	}
}

type edge struct{ a, b int }

func normEdge(a, b int) edge {
	if a > b {
		a, b = b, a
	}
	return edge{a, b}
}

// EdgeStats classifies the mesh topology: boundary edges belong to one
// triangle, manifold edges to two, anything more is non-manifold. A
// closed (watertight) mesh has zero boundary and zero non-manifold edges.
func (m *Mesh) EdgeStats() (boundary, nonManifold int) {
	count := map[edge]int{}
	for _, t := range m.Tris {
		count[normEdge(t[0], t[1])]++
		count[normEdge(t[1], t[2])]++
		count[normEdge(t[2], t[0])]++
	}
	for _, n := range count {
		switch {
		case n == 1:
			boundary++
		case n > 2:
			nonManifold++
		}
	}
	return
}

// Closed reports whether the mesh is watertight.
func (m *Mesh) Closed() bool {
	b, nm := m.EdgeStats()
	return b == 0 && nm == 0
}