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

package mesh

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
)

// ReadOBJ parses a Wavefront OBJ file. Vertices (v), objects/groups
// (o/g) and faces (f) are honored; polygons are fan-triangulated;
// normals, texture coords and materials are ignored (they are
// recomputed or irrelevant for geometry editing).
func ReadOBJ(r io.Reader) (*Scene, error) {
	var verts []Vec3
	type objFaces struct {
		name string
		tris []Triangle // indices into the global vert list
	}
	objs := []*objFaces{}
	current := func() *objFaces {
		if len(objs) == 0 {
			objs = append(objs, &objFaces{name: "default"})
		}
		return objs[len(objs)-1]
	}

	sc := bufio.NewScanner(r)
	sc.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
	lineNo := 0
	for sc.Scan() {
		lineNo++
		line := strings.TrimSpace(sc.Text())
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		fields := strings.Fields(line)
		switch fields[0] {
		case "v":
			if len(fields) < 4 {
				return nil, fmt.Errorf("obj line %d: vertex needs x y z", lineNo)
			}
			var v Vec3
			var err error
			if v.X, err = strconv.ParseFloat(fields[1], 64); err == nil {
				if v.Y, err = strconv.ParseFloat(fields[2], 64); err == nil {
					v.Z, err = strconv.ParseFloat(fields[3], 64)
				}
			}
			if err != nil {
				return nil, fmt.Errorf("obj line %d: bad vertex: %v", lineNo, err)
			}
			verts = append(verts, v)
		case "o", "g":
			name := "unnamed"
			if len(fields) > 1 {
				name = strings.Join(fields[1:], " ")
			}
			// only open a new object if the current one has faces
			if len(objs) > 0 && len(objs[len(objs)-1].tris) == 0 {
				objs[len(objs)-1].name = name
			} else {
				objs = append(objs, &objFaces{name: name})
			}
		case "f":
			if len(fields) < 4 {
				return nil, fmt.Errorf("obj line %d: face needs at least 3 vertices", lineNo)
			}
			idx := make([]int, 0, len(fields)-1)
			for _, f := range fields[1:] {
				// "v", "v/vt", "v//vn", "v/vt/vn" — we only need v
				vs := strings.SplitN(f, "/", 2)[0]
				i, err := strconv.Atoi(vs)
				if err != nil {
					return nil, fmt.Errorf("obj line %d: bad face index %q", lineNo, f)
				}
				if i < 0 {
					i = len(verts) + i // negative = relative to current count
				} else {
					i-- // obj is 1-based
				}
				if i < 0 || i >= len(verts) {
					return nil, fmt.Errorf("obj line %d: face index %q out of range (have %d vertices)", lineNo, f, len(verts))
				}
				idx = append(idx, i)
			}
			o := current()
			for k := 1; k+1 < len(idx); k++ { // fan triangulation
				o.tris = append(o.tris, Triangle{idx[0], idx[k], idx[k+1]})
			}
		}
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}

	// Compact the global vertex list into per-mesh local lists.
	scene := &Scene{}
	for _, o := range objs {
		if len(o.tris) == 0 {
			continue
		}
		m := &Mesh{Name: o.name}
		remap := map[int]int{}
		for _, t := range o.tris {
			var lt Triangle
			for k, gi := range t {
				li, ok := remap[gi]
				if !ok {
					li = len(m.Verts)
					m.Verts = append(m.Verts, verts[gi])
					remap[gi] = li
				}
				lt[k] = li
			}
			m.Tris = append(m.Tris, lt)
		}
		scene.Meshes = append(scene.Meshes, m)
	}
	if len(scene.Meshes) == 0 {
		return nil, fmt.Errorf("obj contains no faces")
	}
	return scene, nil
}

// WriteOBJ writes the scene as OBJ, one "o" object per mesh.
func WriteOBJ(w io.Writer, s *Scene) error {
	bw := bufio.NewWriter(w)
	fmt.Fprintln(bw, "# exported by mesht (agent-tools)")
	offset := 1 // obj indices are global and 1-based
	for _, m := range s.Meshes {
		fmt.Fprintf(bw, "o %s\n", m.Name)
		for _, v := range m.Verts {
			fmt.Fprintf(bw, "v %g %g %g\n", v.X, v.Y, v.Z)
		}
		for _, t := range m.Tris {
			fmt.Fprintf(bw, "f %d %d %d\n", t[0]+offset, t[1]+offset, t[2]+offset)
		}
		offset += len(m.Verts)
	}
	return bw.Flush()
}

// ReadFile loads a scene, picking the format from the file extension
// (.obj, .stl).
func ReadFile(path string) (*Scene, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	switch ext(path) {
	case "obj":
		s, err := ReadOBJ(f)
		if err != nil {
			return nil, fmt.Errorf("%s: %w", path, err)
		}
		return s, nil
	case "stl":
		s, err := ReadSTL(f)
		if err != nil {
			return nil, fmt.Errorf("%s: %w", path, err)
		}
		return s, nil
	}
	return nil, fmt.Errorf("%s: unsupported format (use .obj or .stl)", path)
}

// WriteFile saves a scene, picking the format from the file extension.
// asciiSTL selects text STL instead of the default binary.
func WriteFile(path string, s *Scene, asciiSTL bool) error {
	f, err := os.Create(path)
	if err != nil {
		return err
	}
	defer f.Close()
	switch ext(path) {
	case "obj":
		err = WriteOBJ(f, s)
	case "stl":
		if asciiSTL {
			err = WriteSTLAscii(f, s)
		} else {
			err = WriteSTLBinary(f, s)
		}
	default:
		err = fmt.Errorf("unsupported output format (use .obj or .stl)")
	}
	if err != nil {
		return fmt.Errorf("%s: %w", path, err)
	}
	return f.Close()
}

func ext(path string) string {
	i := strings.LastIndex(path, ".")
	if i < 0 {
		return ""
	}
	return strings.ToLower(path[i+1:])
}