mesh-tool: mesht - create/inspect/edit OBJ+STL with ASCII multi-view rendering

- OBJ read/write (multi-object, ngon fan-triangulation, negative indices),
  STL binary+ascii with auto-detect and vertex welding
- info: bbox/size/area/volume + watertightness via edge manifold stats
- view: z-buffered orthographic ASCII renders (front/side/top/iso/back)
- transform: center/mirror/scale/rotate/translate/fit, per-object filter,
  winding auto-flip on negative determinant
- create: box/sphere/cylinder/cone/plane/torus; merge; convert
- go tests: primitive volumes vs analytic values, roundtrips, topology

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmdG9GqfSWCzts7AkDwRDh

mesh-tool/.gitignore

@@ -0,0 +1,2 @@
+build/
+examples/downloads/

mesh-tool/README.md

@@ -0,0 +1,101 @@
+# mesh-tool (`mesht`)
+
+Create, inspect and edit **3D models** from the command line — built so an
+agent can *see* a model (ASCII multi-view rendering + measurements),
+reason about it, and edit it step by step. Written in Go, zero
+dependencies, single static binary.
+
+## Formats
+
+| Format | Read | Write | Notes |
+|--------|------|-------|-------|
+| OBJ    | ✔    | ✔     | multiple named objects, quads/ngons triangulated |
+| STL    | ✔    | ✔     | binary + ASCII, auto-detected; vertices re-welded on load |
+
+OBJ and STL cover the vast majority of simple editing/printing/game
+pipelines and are plain enough to survive round-trips. glTF/GLB is out of
+scope for now (a full JSON+buffer+material model; use Blender for that).
+
+Normals, UVs and materials are ignored on read — this tool edits
+*geometry*. Viewers recompute normals; STL export writes correct face
+normals from the winding.
+
+## Build
+
+```bash
+# Arch/Garuda: sudo pacman -S go
+cd mesh-tool
+go build -o build/mesht .        # or the VS Code task "build mesh-tool"
+go test ./...
+```
+
+## Commands
+
+```bash
+mesht info model.obj           # verts/tris, bbox, size, area, volume, watertight?
+mesht view model.obj           # ASCII render: front, side, top (default)
+mesht view model.obj --views iso --width 100 --object wheel
+
+# creation — box|sphere|cylinder|cone|plane|torus
+mesht create box --size 2,1,1 --name crate -o crate.obj
+mesht create sphere --radius 1 --segments 32 --rings 16 -o ball.stl
+
+# editing (applied in the listed order)
+mesht transform m.obj --center                    # bbox center -> origin
+mesht transform m.obj --mirror x                  # winding auto-corrected
+mesht transform m.obj --scale 2                   # or --scale 1,2,1
+mesht transform m.obj --rotate 0,45,0             # degrees, X then Y then Z
+mesht transform m.obj --translate 0,2,0
+mesht transform m.obj --fit 10                    # largest dimension -> 10 units
+mesht transform m.obj --object wheel --scale 1.2 -o out.obj   # edit one part
+
+mesht merge body.obj wheels.obj -o car.obj        # keeps objects, renames dups
+mesht merge a.stl b.stl --flatten -o one.obj      # single combined object
+mesht convert model.obj -o model.stl              # --ascii for text STL
+```
+
+`-o` picks the output format from the extension. `transform` overwrites
+the input when `-o` is omitted.
+
+## Conventions
+
+- **Axes:** right-handed, **+Y up**, +X right, +Z toward the "front" viewer.
+- Primitives are centered on the origin.
+- Mirroring / negative scaling automatically flips triangle winding so
+  surfaces keep facing outward.
+- `info` reports **volume only for watertight meshes** — otherwise it
+  tells you how many boundary/non-manifold edges the mesh has.
+
+## How an agent should edit a model
+
+1. `mesht info m.obj` — learn size, orientation and object names.
+2. `mesht view m.obj` — see the shape (front/side/top; add `iso` for depth).
+3. Apply **one small transform**, write to a new file.
+4. `mesht view` again and compare — verify before continuing.
+
+The ASCII views are z-buffered orthographic renders; brightness = how
+much the surface faces the light over the viewer's shoulder, so shape
+and curvature read directly from the text. Each view header repeats the
+axis legend and model dimensions.
+
+Example (a 32-segment sphere, front view, width 48):
+
+```
+front view — X→right Y↑up (seen from +Z)
+model 2 x 2 x 2 (XYZ)
+           ==++**####%%%%%%%%%%%%###*
+        -==++**######%%%%%%%%%%%%%%####*
+      :-=+++**#######%%%%%%%%%%%%%%%%%%#**
+     -==++***#######%%%%%%%%%%%%%%%%%%%%##*
+  .:--+++***########%%%%%%%%%%%%%%%%%%%%%%##*+
+```
+
+## Viewing the results as a human
+
+Any of these (Arch/Garuda):
+
+```bash
+sudo pacman -S f3d        # fast minimal viewer:  f3d model.obj
+sudo pacman -S blender    # full editor
+flatpak install org.prusa3d.PrusaSlicer   # if you also want to print
+```

mesh-tool/go.mod

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+module gitea.brasse-pc.eu/brasse/agent-tools/mesh-tool
+
+go 1.24

mesh-tool/main.go

@@ -0,0 +1,406 @@
+// mesht creates, inspects and edits 3D models (OBJ and STL) from the
+// command line, with ASCII multi-view rendering so agents can "see"
+// a model before and after editing it.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"os"
+	"strconv"
+	"strings"
+
+	"gitea.brasse-pc.eu/brasse/agent-tools/mesh-tool/mesh"
+)
+
+var version = "dev"
+
+const usage = `mesht - 3D model tool for agents (OBJ + STL)
+
+Usage:
+  mesht info <file>                     stats: size, volume, watertightness
+  mesht view <file> [flags]             ASCII render from several angles
+  mesht create <primitive> [flags]      box|sphere|cylinder|cone|plane|torus
+  mesht transform <file> [flags]        scale/rotate/translate/mirror/center/fit
+  mesht merge <a> <b> ... -o <out>      combine several files into one
+  mesht convert <in> -o <out>           obj <-> stl
+  mesht version
+
+View flags:
+  --views front,side,top,iso,back       which views to draw (default front,side,top)
+  --width <n>                           characters per view (default 64)
+  --object <name>                       only draw one object from the file
+
+Create flags (always with -o <out.obj|out.stl>):
+  box:      --size x,y,z          (default 1,1,1)
+  sphere:   --radius r --segments n --rings n     (default 1, 24, 12)
+  cylinder: --radius r --height h --segments n    (default 0.5, 1, 24)
+  cone:     --radius r --height h --segments n    (default 0.5, 1, 24)
+  plane:    --size w,d            (default 1,1)
+  torus:    --radius R --tube r --segments n --rings n (default 1, 0.25, 24, 12)
+  --name <s>                      object name in the output
+
+Transform flags (applied in this order):
+  --center                 move bounding-box center to the origin
+  --mirror x|y|z           mirror across that axis' plane
+  --scale s | x,y,z        uniform or per-axis scale
+  --rotate x,y,z           degrees around X, then Y, then Z
+  --translate x,y,z        move
+  --fit n                  uniformly scale so the largest dimension = n
+  --object <name>          only transform the named object (obj files)
+  -o <out>                 output file (default: overwrite input)
+
+Common:
+  --ascii                  write text STL instead of binary
+  Formats are picked from file extensions (.obj, .stl).
+
+Axes: +Y is up, +X right, +Z toward the front viewer (right-handed).
+`
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Print(usage)
+		os.Exit(2)
+	}
+	switch os.Args[1] {
+	case "info":
+		cmdInfo(os.Args[2:])
+	case "view":
+		cmdView(os.Args[2:])
+	case "create":
+		cmdCreate(os.Args[2:])
+	case "transform":
+		cmdTransform(os.Args[2:])
+	case "merge":
+		cmdMerge(os.Args[2:])
+	case "convert":
+		cmdConvert(os.Args[2:])
+	case "version", "--version", "-v":
+		fmt.Println("mesht", version)
+	case "help", "--help", "-h":
+		fmt.Print(usage)
+	default:
+		die("unknown command %q — run 'mesht help'", os.Args[1])
+	}
+}
+
+// parseInterspersed lets flags appear before or after positional args.
+func parseInterspersed(fs *flag.FlagSet, args []string) {
+	var flags, pos []string
+	for i := 0; i < len(args); i++ {
+		a := args[i]
+		if len(a) > 1 && a[0] == '-' {
+			flags = append(flags, a)
+			name := strings.TrimLeft(a, "-")
+			if !strings.Contains(name, "=") {
+				f := fs.Lookup(name)
+				isBool := false
+				if f != nil {
+					if bv, ok := f.Value.(interface{ IsBoolFlag() bool }); ok && bv.IsBoolFlag() {
+						isBool = true
+					}
+				}
+				if !isBool && i+1 < len(args) {
+					i++
+					flags = append(flags, args[i])
+				}
+			}
+		} else {
+			pos = append(pos, a)
+		}
+	}
+	fs.Parse(append(flags, pos...))
+}
+
+func parseVec(s string, uniformOK bool) (mesh.Vec3, error) {
+	parts := strings.Split(s, ",")
+	switch len(parts) {
+	case 1:
+		if !uniformOK {
+			return mesh.Vec3{}, fmt.Errorf("%q: want x,y,z", s)
+		}
+		v, err := strconv.ParseFloat(strings.TrimSpace(parts[0]), 64)
+		return mesh.Vec3{X: v, Y: v, Z: v}, err
+	case 3:
+		var v [3]float64
+		for i, p := range parts {
+			f, err := strconv.ParseFloat(strings.TrimSpace(p), 64)
+			if err != nil {
+				return mesh.Vec3{}, fmt.Errorf("%q: bad number %q", s, p)
+			}
+			v[i] = f
+		}
+		return mesh.Vec3{X: v[0], Y: v[1], Z: v[2]}, nil
+	}
+	return mesh.Vec3{}, fmt.Errorf("%q: want one value or x,y,z", s)
+}
+
+func cmdInfo(args []string) {
+	if len(args) != 1 {
+		die("info takes exactly one file")
+	}
+	scene, err := mesh.ReadFile(args[0])
+	if err != nil {
+		die("%v", err)
+	}
+	merged := scene.Merged()
+	mn, mx := merged.BBox()
+	size := mx.Sub(mn)
+	center := mn.Add(size.Mul(0.5))
+	fmt.Printf("file:     %s\n", args[0])
+	fmt.Printf("objects:  %d\n", len(scene.Meshes))
+	fmt.Printf("verts:    %d\n", scene.TotalVerts())
+	fmt.Printf("tris:     %d\n", scene.TotalTris())
+	fmt.Printf("bbox min: (%.4g, %.4g, %.4g)\n", mn.X, mn.Y, mn.Z)
+	fmt.Printf("bbox max: (%.4g, %.4g, %.4g)\n", mx.X, mx.Y, mx.Z)
+	fmt.Printf("size:     %.4g x %.4g x %.4g (X Y Z)\n", size.X, size.Y, size.Z)
+	fmt.Printf("center:   (%.4g, %.4g, %.4g)\n", center.X, center.Y, center.Z)
+	fmt.Printf("area:     %.6g\n", merged.SurfaceArea())
+	if b, nm := merged.EdgeStats(); b == 0 && nm == 0 {
+		fmt.Printf("volume:   %.6g (closed mesh)\n", merged.Volume())
+	} else {
+		fmt.Printf("volume:   n/a (open mesh: %d boundary edges, %d non-manifold edges)\n", b, nm)
+	}
+	if len(scene.Meshes) > 1 {
+		fmt.Println("per object:")
+		for _, m := range scene.Meshes {
+			closed := "open"
+			if m.Closed() {
+				closed = "closed"
+			}
+			fmt.Printf("  %-24s %7d verts %7d tris  %s\n", m.Name, len(m.Verts), len(m.Tris), closed)
+		}
+	}
+}
+
+func cmdView(args []string) {
+	fs := flag.NewFlagSet("view", flag.ExitOnError)
+	views := fs.String("views", "front,side,top", "comma-separated view list")
+	width := fs.Int("width", 64, "characters per view")
+	object := fs.String("object", "", "only draw this object")
+	parseInterspersed(fs, args)
+	if fs.NArg() != 1 {
+		die("view takes exactly one file")
+	}
+	scene, err := mesh.ReadFile(fs.Arg(0))
+	if err != nil {
+		die("%v", err)
+	}
+	m := scene.Merged()
+	if *object != "" {
+		if m = scene.Mesh(*object); m == nil {
+			die("no object named %q in %s (use 'mesht info' to list)", *object, fs.Arg(0))
+		}
+	}
+	for _, name := range strings.Split(*views, ",") {
+		name = strings.TrimSpace(strings.ToLower(name))
+		v, ok := mesh.Views[name]
+		if !ok {
+			die("unknown view %q (available: %s)", name, strings.Join(mesh.ViewOrder, ", "))
+		}
+		fmt.Println(mesh.RenderASCII(m, v, *width))
+	}
+}
+
+func cmdCreate(args []string) {
+	if len(args) < 1 {
+		die("create needs a primitive: box, sphere, cylinder, cone, plane, torus")
+	}
+	prim := args[0]
+	fs := flag.NewFlagSet("create", flag.ExitOnError)
+	size := fs.String("size", "1,1,1", "box/plane size")
+	radius := fs.Float64("radius", 0, "radius")
+	tube := fs.Float64("tube", 0.25, "torus tube radius")
+	height := fs.Float64("height", 1, "height")
+	segments := fs.Int("segments", 24, "segments around")
+	rings := fs.Int("rings", 12, "rings (sphere/torus)")
+	name := fs.String("name", "", "object name")
+	out := fs.String("o", "", "output file (.obj or .stl)")
+	ascii := fs.Bool("ascii", false, "write text STL")
+	parseInterspersed(fs, args[1:])
+	if *out == "" {
+		die("create needs -o <out.obj|out.stl>")
+	}
+	var m *mesh.Mesh
+	switch prim {
+	case "box":
+		sz, err := parseVec(*size, true)
+		if err != nil {
+			die("--size: %v", err)
+		}
+		m = mesh.Box(sz)
+	case "sphere":
+		m = mesh.Sphere(defRadius(*radius, 1), *segments, *rings)
+	case "cylinder":
+		m = mesh.Cylinder(defRadius(*radius, 0.5), *height, *segments)
+	case "cone":
+		m = mesh.Cone(defRadius(*radius, 0.5), *height, *segments)
+	case "plane":
+		sz, err := parseVec(*size, true)
+		if err != nil {
+			die("--size: %v", err)
+		}
+		m = mesh.Plane(sz.X, sz.Z)
+	case "torus":
+		m = mesh.Torus(defRadius(*radius, 1), *tube, *segments, *rings)
+	default:
+		die("unknown primitive %q", prim)
+	}
+	if *name != "" {
+		m.Name = *name
+	}
+	scene := &mesh.Scene{Meshes: []*mesh.Mesh{m}}
+	if err := mesh.WriteFile(*out, scene, *ascii); err != nil {
+		die("%v", err)
+	}
+	fmt.Printf("%s  (%s: %d verts, %d tris)\n", *out, m.Name, len(m.Verts), len(m.Tris))
+}
+
+func defRadius(r, def float64) float64 {
+	if r <= 0 {
+		return def
+	}
+	return r
+}
+
+func cmdTransform(args []string) {
+	fs := flag.NewFlagSet("transform", flag.ExitOnError)
+	center := fs.Bool("center", false, "move bbox center to origin")
+	mirror := fs.String("mirror", "", "x|y|z")
+	scale := fs.String("scale", "", "s or x,y,z")
+	rotate := fs.String("rotate", "", "degrees x,y,z")
+	translate := fs.String("translate", "", "x,y,z")
+	fit := fs.Float64("fit", 0, "scale so the largest dimension equals this")
+	object := fs.String("object", "", "only transform this object")
+	out := fs.String("o", "", "output file (default: overwrite input)")
+	ascii := fs.Bool("ascii", false, "write text STL")
+	parseInterspersed(fs, args)
+	if fs.NArg() != 1 {
+		die("transform takes exactly one file")
+	}
+	in := fs.Arg(0)
+	scene, err := mesh.ReadFile(in)
+	if err != nil {
+		die("%v", err)
+	}
+	targets := scene.Meshes
+	if *object != "" {
+		m := scene.Mesh(*object)
+		if m == nil {
+			die("no object named %q in %s (use 'mesht info' to list)", *object, in)
+		}
+		targets = []*mesh.Mesh{m}
+	}
+
+	if *center {
+		mn, mx := mesh.SceneBBox(targets)
+		c := mn.Add(mx.Sub(mn).Mul(0.5))
+		mesh.ApplyAll(targets, mesh.Translate(c.Mul(-1)))
+	}
+	if *mirror != "" {
+		s := mesh.Vec3{X: 1, Y: 1, Z: 1}
+		switch strings.ToLower(*mirror) {
+		case "x":
+			s.X = -1
+		case "y":
+			s.Y = -1
+		case "z":
+			s.Z = -1
+		default:
+			die("--mirror must be x, y or z")
+		}
+		mesh.ApplyAll(targets, mesh.ScaleXYZ(s))
+	}
+	if *scale != "" {
+		v, err := parseVec(*scale, true)
+		if err != nil {
+			die("--scale: %v", err)
+		}
+		mesh.ApplyAll(targets, mesh.ScaleXYZ(v))
+	}
+	if *rotate != "" {
+		v, err := parseVec(*rotate, false)
+		if err != nil {
+			die("--rotate: %v", err)
+		}
+		rot := mesh.RotateZ(v.Z).Mul(mesh.RotateY(v.Y)).Mul(mesh.RotateX(v.X))
+		mesh.ApplyAll(targets, rot)
+	}
+	if *translate != "" {
+		v, err := parseVec(*translate, false)
+		if err != nil {
+			die("--translate: %v", err)
+		}
+		mesh.ApplyAll(targets, mesh.Translate(v))
+	}
+	if *fit > 0 {
+		mn, mx := mesh.SceneBBox(targets)
+		size := mx.Sub(mn)
+		longest := math.Max(size.X, math.Max(size.Y, size.Z))
+		if longest > 0 {
+			f := *fit / longest
+			mesh.ApplyAll(targets, mesh.ScaleXYZ(mesh.Vec3{X: f, Y: f, Z: f}))
+		}
+	}
+
+	if *out == "" {
+		*out = in
+	}
+	if err := mesh.WriteFile(*out, scene, *ascii); err != nil {
+		die("%v", err)
+	}
+	fmt.Printf("%s  (%d objects, %d verts, %d tris)\n", *out, len(scene.Meshes), scene.TotalVerts(), scene.TotalTris())
+}
+
+func cmdMerge(args []string) {
+	fs := flag.NewFlagSet("merge", flag.ExitOnError)
+	out := fs.String("o", "", "output file")
+	flatten := fs.Bool("flatten", false, "merge everything into a single object")
+	ascii := fs.Bool("ascii", false, "write text STL")
+	parseInterspersed(fs, args)
+	if fs.NArg() < 2 {
+		die("merge needs at least two input files")
+	}
+	if *out == "" {
+		die("merge needs -o <out>")
+	}
+	scene := &mesh.Scene{}
+	for _, path := range fs.Args() {
+		s, err := mesh.ReadFile(path)
+		if err != nil {
+			die("%v", err)
+		}
+		scene.Append(s)
+	}
+	if *flatten {
+		scene = &mesh.Scene{Meshes: []*mesh.Mesh{scene.Merged()}}
+	}
+	if err := mesh.WriteFile(*out, scene, *ascii); err != nil {
+		die("%v", err)
+	}
+	fmt.Printf("%s  (%d objects, %d verts, %d tris)\n", *out, len(scene.Meshes), scene.TotalVerts(), scene.TotalTris())
+}
+
+func cmdConvert(args []string) {
+	fs := flag.NewFlagSet("convert", flag.ExitOnError)
+	out := fs.String("o", "", "output file")
+	ascii := fs.Bool("ascii", false, "write text STL")
+	parseInterspersed(fs, args)
+	if fs.NArg() != 1 || *out == "" {
+		die("convert takes one input file and -o <out>")
+	}
+	scene, err := mesh.ReadFile(fs.Arg(0))
+	if err != nil {
+		die("%v", err)
+	}
+	if err := mesh.WriteFile(*out, scene, *ascii); err != nil {
+		die("%v", err)
+	}
+	fmt.Printf("%s  (%d objects, %d verts, %d tris)\n", *out, len(scene.Meshes), scene.TotalVerts(), scene.TotalTris())
+}
+
+func die(format string, a ...any) {
+	fmt.Fprintf(os.Stderr, "mesht: "+format+"\n", a...)
+	os.Exit(1)
+}

mesh-tool/mesh/ascii.go

@@ -0,0 +1,169 @@
+package mesh
+
+import (
+	"fmt"
+	"math"
+	"strings"
+)
+
+// View is an orthographic camera basis: Right/Up span the screen plane,
+// Toward points from the scene toward the viewer (bigger depth = closer).
+type View struct {
+	Name              string
+	Axes              string // human-readable axis legend
+	Right, Up, Toward Vec3
+}
+
+var Views = map[string]View{
+	"front": {"front", "X→right Y↑up (seen from +Z)", Vec3{1, 0, 0}, Vec3{0, 1, 0}, Vec3{0, 0, 1}},
+	"back":  {"back", "-X→right Y↑up (seen from -Z)", Vec3{-1, 0, 0}, Vec3{0, 1, 0}, Vec3{0, 0, -1}},
+	"side":  {"side", "-Z→right Y↑up (seen from +X)", Vec3{0, 0, -1}, Vec3{0, 1, 0}, Vec3{1, 0, 0}},
+	"top":   {"top", "X→right Z↓down-screen (seen from above, +Y)", Vec3{1, 0, 0}, Vec3{0, 0, -1}, Vec3{0, 1, 0}},
+	"iso": {"iso", "isometric from (+X +Y +Z)",
+		Vec3{1, 0, -1}.Norm(), Vec3{-1, 2, -1}.Norm(), Vec3{1, 1, 1}.Norm()},
+}
+
+// ViewOrder is the canonical ordering for multi-view output.
+var ViewOrder = []string{"front", "side", "top", "iso", "back"}
+
+const shadeRamp = " .:-=+*#%@"
+
+// charAspect compensates terminal cells being ~2x taller than wide.
+const charAspect = 0.5
+
+// RenderASCII draws the mesh from the given view into a text block of
+// the given character width. Triangles are z-buffer rasterized and
+// shaded by how much each face points toward the light (over the
+// viewer's shoulder), so curvature and depth read as brightness.
+func RenderASCII(m *Mesh, v View, width int) string {
+	if width < 8 {
+		width = 8
+	}
+	if len(m.Tris) == 0 {
+		return "(empty mesh)\n"
+	}
+
+	// project all vertices into view space
+	type pv struct{ x, y, z float64 }
+	pts := make([]pv, len(m.Verts))
+	minX, minY := math.Inf(1), math.Inf(1)
+	maxX, maxY := math.Inf(-1), math.Inf(-1)
+	for i, w := range m.Verts {
+		p := pv{w.Dot(v.Right), w.Dot(v.Up), w.Dot(v.Toward)}
+		pts[i] = p
+		minX, maxX = math.Min(minX, p.x), math.Max(maxX, p.x)
+		minY, maxY = math.Min(minY, p.y), math.Max(maxY, p.y)
+	}
+	spanX, spanY := maxX-minX, maxY-minY
+	if spanX == 0 {
+		spanX = 1e-9
+	}
+	if spanY == 0 {
+		spanY = 1e-9
+	}
+	height := int(float64(width) * (spanY / spanX) * charAspect)
+	if height < 1 {
+		height = 1
+	}
+	if height > 4*width {
+		height = 4 * width
+	}
+	sx := float64(width-1) / spanX
+	sy := float64(height-1) / spanY
+
+	depth := make([]float64, width*height)
+	for i := range depth {
+		depth[i] = math.Inf(-1)
+	}
+	shade := make([]float64, width*height)
+	for i := range shade {
+		shade[i] = -1
+	}
+
+	light := v.Toward.Mul(0.8).Add(v.Up.Mul(0.5)).Add(v.Right.Mul(0.3)).Norm()
+
+	for ti, t := range m.Tris {
+		n := m.FaceNormal(ti)
+		// abs: downloaded models often have mixed winding; treat both
+		// sides as lit so the silhouette never goes black
+		lum := 0.15 + 0.85*math.Abs(n.Dot(light))
+
+		a, b, c := pts[t[0]], pts[t[1]], pts[t[2]]
+		ax, ay := (a.x-minX)*sx, (maxY-a.y)*sy
+		bx, by := (b.x-minX)*sx, (maxY-b.y)*sy
+		cx, cy := (c.x-minX)*sx, (maxY-c.y)*sy
+
+		x0 := int(math.Floor(math.Min(ax, math.Min(bx, cx))))
+		x1 := int(math.Ceil(math.Max(ax, math.Max(bx, cx))))
+		y0 := int(math.Floor(math.Min(ay, math.Min(by, cy))))
+		y1 := int(math.Ceil(math.Max(ay, math.Max(by, cy))))
+		if x0 < 0 {
+			x0 = 0
+		}
+		if y0 < 0 {
+			y0 = 0
+		}
+		if x1 >= width {
+			x1 = width - 1
+		}
+		if y1 >= height {
+			y1 = height - 1
+		}
+
+		area := (bx-ax)*(cy-ay) - (by-ay)*(cx-ax)
+		if area == 0 {
+			continue
+		}
+		for py := y0; py <= y1; py++ {
+			for px := x0; px <= x1; px++ {
+				fx, fy := float64(px), float64(py)
+				w0 := (bx-ax)*(fy-ay) - (by-ay)*(fx-ax)
+				w1 := (cx-bx)*(fy-by) - (cy-by)*(fx-bx)
+				w2 := (ax-cx)*(fy-cy) - (ay-cy)*(fx-cx)
+				if !sameSide(w0, w1, w2, area) {
+					continue
+				}
+				// barycentric depth: w2 tracks b, w0 tracks c
+				l1 := w2 / area
+				l2 := w0 / area
+				l0 := 1 - l1 - l2
+				z := l0*a.z + l1*b.z + l2*c.z
+				idx := py*width + px
+				if z > depth[idx] {
+					depth[idx] = z
+					shade[idx] = lum
+				}
+			}
+		}
+	}
+
+	var sb strings.Builder
+	mn, mx := m.BBox()
+	size := mx.Sub(mn)
+	fmt.Fprintf(&sb, "%s view — %s\nmodel %.3g x %.3g x %.3g (XYZ)\n",
+		v.Name, v.Axes, size.X, size.Y, size.Z)
+	ramp := []rune(shadeRamp)
+	for py := 0; py < height; py++ {
+		for px := 0; px < width; px++ {
+			s := shade[py*width+px]
+			if s < 0 {
+				sb.WriteByte(' ')
+				continue
+			}
+			i := int(s * float64(len(ramp)-1))
+			if i >= len(ramp) {
+				i = len(ramp) - 1
+			}
+			sb.WriteRune(ramp[i])
+		}
+		sb.WriteByte('\n')
+	}
+	return sb.String()
+}
+
+func sameSide(w0, w1, w2, area float64) bool {
+	if area > 0 {
+		return w0 >= 0 && w1 >= 0 && w2 >= 0
+	}
+	return w0 <= 0 && w1 <= 0 && w2 <= 0
+}

mesh-tool/mesh/measure.go

@@ -0,0 +1,59 @@
+package mesh
+
+import "math"
+
+// BBox returns the axis-aligned bounding box of the mesh.
+func (m *Mesh) BBox() (min, max Vec3) {
+	if len(m.Verts) == 0 {
+		return Vec3{}, Vec3{}
+	}
+	min, max = m.Verts[0], m.Verts[0]
+	for _, v := range m.Verts[1:] {
+		min = min.Min(v)
+		max = max.Max(v)
+	}
+	return
+}
+
+// SurfaceArea sums the area of all triangles.
+func (m *Mesh) SurfaceArea() float64 {
+	sum := 0.0
+	for _, t := range m.Tris {
+		a, b, c := m.Verts[t[0]], m.Verts[t[1]], m.Verts[t[2]]
+		sum += b.Sub(a).Cross(c.Sub(a)).Len() / 2
+	}
+	return sum
+}
+
+// SignedVolume computes the enclosed volume via the divergence theorem.
+// Only meaningful for closed meshes; positive when windings face outward.
+func (m *Mesh) SignedVolume() float64 {
+	sum := 0.0
+	for _, t := range m.Tris {
+		a, b, c := m.Verts[t[0]], m.Verts[t[1]], m.Verts[t[2]]
+		sum += a.Dot(b.Cross(c))
+	}
+	return sum / 6
+}
+
+// Volume is the absolute enclosed volume.
+func (m *Mesh) Volume() float64 { return math.Abs(m.SignedVolume()) }
+
+// SceneBBox returns the bounding box over the given meshes.
+func SceneBBox(meshes []*Mesh) (min, max Vec3) {
+	first := true
+	for _, m := range meshes {
+		if len(m.Verts) == 0 {
+			continue
+		}
+		mn, mx := m.BBox()
+		if first {
+			min, max = mn, mx
+			first = false
+		} else {
+			min = min.Min(mn)
+			max = max.Max(mx)
+		}
+	}
+	return
+}

mesh-tool/mesh/mesh.go

@@ -0,0 +1,129 @@
+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
+}

mesh-tool/mesh/mesh_test.go

@@ -0,0 +1,184 @@
+package mesh
+
+import (
+	"bytes"
+	"math"
+	"strings"
+	"testing"
+)
+
+func almost(t *testing.T, name string, got, want, tol float64) {
+	t.Helper()
+	if math.Abs(got-want) > tol {
+		t.Errorf("%s = %g, want %g (±%g)", name, got, want, tol)
+	}
+}
+
+func TestPrimitiveVolumes(t *testing.T) {
+	box := Box(Vec3{1, 2, 3})
+	almost(t, "box volume", box.SignedVolume(), 6, 1e-9)
+	almost(t, "box area", box.SurfaceArea(), 22, 1e-9)
+	if !box.Closed() {
+		t.Error("box should be watertight")
+	}
+
+	sph := Sphere(1, 64, 32)
+	almost(t, "sphere volume", sph.SignedVolume(), 4*math.Pi/3, 0.07)
+	almost(t, "sphere area", sph.SurfaceArea(), 4*math.Pi, 0.15)
+	if !sph.Closed() {
+		t.Error("sphere should be watertight")
+	}
+
+	cyl := Cylinder(0.5, 2, 64)
+	almost(t, "cylinder volume", cyl.SignedVolume(), math.Pi*0.25*2, 0.01)
+	if !cyl.Closed() {
+		t.Error("cylinder should be watertight")
+	}
+
+	cone := Cone(1, 3, 64)
+	almost(t, "cone volume", cone.SignedVolume(), math.Pi/3*3, 0.02)
+	if !cone.Closed() {
+		t.Error("cone should be watertight")
+	}
+
+	tor := Torus(2, 0.5, 64, 32)
+	almost(t, "torus volume", tor.SignedVolume(), 2*math.Pi*math.Pi*2*0.25, 0.25)
+	if !tor.Closed() {
+		t.Error("torus should be watertight")
+	}
+}
+
+func TestBBoxAndMeasure(t *testing.T) {
+	box := Box(Vec3{2, 4, 6})
+	mn, mx := box.BBox()
+	if mn != (Vec3{-1, -2, -3}) || mx != (Vec3{1, 2, 3}) {
+		t.Errorf("bbox = %v..%v", mn, mx)
+	}
+}
+
+func TestTransformMirrorKeepsVolumePositive(t *testing.T) {
+	box := Box(Vec3{1, 1, 1})
+	box.Apply(ScaleXYZ(Vec3{-1, 1, 1}))
+	almost(t, "mirrored box volume", box.SignedVolume(), 1, 1e-9)
+	box.Apply(RotateY(45).Mul(RotateX(30)))
+	almost(t, "rotated box volume", box.SignedVolume(), 1, 1e-9)
+	box.Apply(Translate(Vec3{10, -5, 3}))
+	almost(t, "translated box volume", box.SignedVolume(), 1, 1e-6)
+}
+
+func TestOBJRoundTrip(t *testing.T) {
+	scene := &Scene{Meshes: []*Mesh{Box(Vec3{1, 2, 3}), Sphere(1, 8, 4)}}
+	scene.Meshes[0].Name = "crate"
+	scene.Meshes[1].Name = "ball"
+	var buf bytes.Buffer
+	if err := WriteOBJ(&buf, scene); err != nil {
+		t.Fatal(err)
+	}
+	back, err := ReadOBJ(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(back.Meshes) != 2 {
+		t.Fatalf("got %d meshes, want 2", len(back.Meshes))
+	}
+	if back.Meshes[0].Name != "crate" || back.Meshes[1].Name != "ball" {
+		t.Errorf("names = %q, %q", back.Meshes[0].Name, back.Meshes[1].Name)
+	}
+	almost(t, "roundtrip crate volume", back.Meshes[0].SignedVolume(), 6, 1e-9)
+	if !back.Meshes[1].Closed() {
+		t.Error("roundtripped sphere should stay watertight")
+	}
+}
+
+func TestOBJQuadsAndNegativeIndices(t *testing.T) {
+	src := `
+v 0 0 0
+v 1 0 0
+v 1 1 0
+v 0 1 0
+f 1 2 3 4
+f -4 -3 -2
+`
+	s, err := ReadOBJ(strings.NewReader(src))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := s.TotalTris(); got != 3 {
+		t.Errorf("tris = %d, want 3 (quad fan + one negative-index tri)", got)
+	}
+}
+
+func TestSTLRoundTrips(t *testing.T) {
+	box := Box(Vec3{1, 2, 3})
+	scene := &Scene{Meshes: []*Mesh{box}}
+
+	var bin bytes.Buffer
+	if err := WriteSTLBinary(&bin, scene); err != nil {
+		t.Fatal(err)
+	}
+	back, err := ReadSTL(bytes.NewReader(bin.Bytes()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := back.Meshes[0]
+	if len(m.Verts) != 8 {
+		t.Errorf("binary stl weld: %d verts, want 8", len(m.Verts))
+	}
+	almost(t, "binary stl volume", m.SignedVolume(), 6, 1e-6)
+
+	var asc bytes.Buffer
+	if err := WriteSTLAscii(&asc, scene); err != nil {
+		t.Fatal(err)
+	}
+	back2, err := ReadSTL(bytes.NewReader(asc.Bytes()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	almost(t, "ascii stl volume", back2.Meshes[0].SignedVolume(), 6, 1e-6)
+	if !back2.Meshes[0].Closed() {
+		t.Error("ascii stl roundtrip should stay watertight")
+	}
+}
+
+func TestSceneAppendRenames(t *testing.T) {
+	a := &Scene{Meshes: []*Mesh{Box(Vec3{1, 1, 1})}}
+	b := &Scene{Meshes: []*Mesh{Box(Vec3{2, 2, 2})}}
+	a.Append(b)
+	if a.Meshes[0].Name == a.Meshes[1].Name {
+		t.Errorf("duplicate names after append: %q", a.Meshes[0].Name)
+	}
+}
+
+func TestRenderASCII(t *testing.T) {
+	sph := Sphere(1, 32, 16)
+	out := RenderASCII(sph, Views["front"], 40)
+	if !strings.Contains(out, "front view") {
+		t.Errorf("missing header:\n%s", out)
+	}
+	ink := 0
+	for _, r := range out {
+		if strings.ContainsRune(shadeRamp[1:], r) {
+			ink++
+		}
+	}
+	if ink < 100 {
+		t.Errorf("sphere render suspiciously empty (%d shaded cells):\n%s", ink, out)
+	}
+	// a sphere should be roughly as tall as wide after aspect correction
+	lines := strings.Split(strings.TrimRight(out, "\n"), "\n")
+	rows := len(lines) - 2 // minus the two header lines
+	if rows < 15 || rows > 25 {
+		t.Errorf("40-wide sphere should be ~20 rows, got %d", rows)
+	}
+}
+
+func TestEdgeStatsOpenMesh(t *testing.T) {
+	p := Plane(1, 1)
+	if p.Closed() {
+		t.Error("plane must not be watertight")
+	}
+	b, nm := p.EdgeStats()
+	if b != 4 || nm != 0 {
+		t.Errorf("plane edge stats = %d boundary, %d non-manifold; want 4, 0", b, nm)
+	}
+}

mesh-tool/mesh/obj.go

@@ -0,0 +1,203 @@
+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:])
+}

mesh-tool/mesh/primitives.go

@@ -0,0 +1,179 @@
+package mesh
+
+import "math"
+
+// All primitives are centered at the origin with +Y up and get outward
+// (counter-clockwise) winding; ensureOutward fixes the global
+// orientation via the signed volume as a safety net.
+
+func ensureOutward(m *Mesh) *Mesh {
+	if m.SignedVolume() < 0 {
+		m.FlipWinding()
+	}
+	return m
+}
+
+// Box builds an axis-aligned box of the given size.
+func Box(size Vec3) *Mesh {
+	x, y, z := size.X/2, size.Y/2, size.Z/2
+	m := &Mesh{
+		Name: "box",
+		Verts: []Vec3{
+			{-x, -y, -z}, {x, -y, -z}, {x, y, -z}, {-x, y, -z}, // back  (z-)
+			{-x, -y, z}, {x, -y, z}, {x, y, z}, {-x, y, z}, // front (z+)
+		},
+	}
+	quads := [][4]int{
+		{0, 3, 2, 1}, // back
+		{4, 5, 6, 7}, // front
+		{0, 1, 5, 4}, // bottom
+		{2, 3, 7, 6}, // top
+		{1, 2, 6, 5}, // right
+		{0, 4, 7, 3}, // left
+	}
+	for _, q := range quads {
+		m.Tris = append(m.Tris, Triangle{q[0], q[1], q[2]}, Triangle{q[0], q[2], q[3]})
+	}
+	return ensureOutward(m)
+}
+
+// Plane builds a flat rectangle in the XZ plane (an open mesh).
+func Plane(w, d float64) *Mesh {
+	x, z := w/2, d/2
+	return &Mesh{
+		Name:  "plane",
+		Verts: []Vec3{{-x, 0, -z}, {x, 0, -z}, {x, 0, z}, {-x, 0, z}},
+		Tris:  []Triangle{{0, 2, 1}, {0, 3, 2}}, // +Y facing
+	}
+}
+
+// Sphere builds a UV sphere. segments = around the equator (>= 3),
+// rings = from pole to pole (>= 2).
+func Sphere(r float64, segments, rings int) *Mesh {
+	if segments < 3 {
+		segments = 3
+	}
+	if rings < 2 {
+		rings = 2
+	}
+	m := &Mesh{Name: "sphere"}
+	top := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, r, 0})
+	// interior rings, top to bottom
+	ringStart := make([]int, rings)
+	for i := 1; i < rings; i++ {
+		theta := math.Pi * float64(i) / float64(rings)
+		y := r * math.Cos(theta)
+		rad := r * math.Sin(theta)
+		ringStart[i] = len(m.Verts)
+		for j := 0; j < segments; j++ {
+			phi := 2 * math.Pi * float64(j) / float64(segments)
+			m.Verts = append(m.Verts, Vec3{rad * math.Cos(phi), y, rad * math.Sin(phi)})
+		}
+	}
+	bottom := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, -r, 0})
+
+	at := func(ring, seg int) int { return ringStart[ring] + seg%segments }
+	for j := 0; j < segments; j++ {
+		m.Tris = append(m.Tris, Triangle{top, at(1, j), at(1, j+1)}) // top cap
+		m.Tris = append(m.Tris, Triangle{bottom, at(rings-1, j+1), at(rings-1, j)})
+	}
+	for i := 1; i < rings-1; i++ {
+		for j := 0; j < segments; j++ {
+			a, b := at(i, j), at(i, j+1)
+			c, d := at(i+1, j+1), at(i+1, j)
+			m.Tris = append(m.Tris, Triangle{a, b, c}, Triangle{a, c, d})
+		}
+	}
+	return ensureOutward(m)
+}
+
+// Cylinder builds a closed cylinder of height h around the Y axis.
+func Cylinder(r, h float64, segments int) *Mesh {
+	if segments < 3 {
+		segments = 3
+	}
+	m := &Mesh{Name: "cylinder"}
+	y := h / 2
+	topC := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, y, 0})
+	botC := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, -y, 0})
+	topStart := len(m.Verts)
+	for j := 0; j < segments; j++ {
+		phi := 2 * math.Pi * float64(j) / float64(segments)
+		m.Verts = append(m.Verts, Vec3{r * math.Cos(phi), y, r * math.Sin(phi)})
+	}
+	botStart := len(m.Verts)
+	for j := 0; j < segments; j++ {
+		phi := 2 * math.Pi * float64(j) / float64(segments)
+		m.Verts = append(m.Verts, Vec3{r * math.Cos(phi), -y, r * math.Sin(phi)})
+	}
+	t := func(j int) int { return topStart + j%segments }
+	b := func(j int) int { return botStart + j%segments }
+	for j := 0; j < segments; j++ {
+		m.Tris = append(m.Tris,
+			Triangle{topC, t(j + 1), t(j)},     // top cap
+			Triangle{botC, b(j), b(j + 1)},     // bottom cap
+			Triangle{t(j), t(j + 1), b(j + 1)}, // side
+			Triangle{t(j), b(j + 1), b(j)},     // side
+		)
+	}
+	return ensureOutward(m)
+}
+
+// Cone builds a closed cone with its base at -h/2 and apex at +h/2.
+func Cone(r, h float64, segments int) *Mesh {
+	if segments < 3 {
+		segments = 3
+	}
+	m := &Mesh{Name: "cone"}
+	apex := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, h / 2, 0})
+	baseC := len(m.Verts)
+	m.Verts = append(m.Verts, Vec3{0, -h / 2, 0})
+	start := len(m.Verts)
+	for j := 0; j < segments; j++ {
+		phi := 2 * math.Pi * float64(j) / float64(segments)
+		m.Verts = append(m.Verts, Vec3{r * math.Cos(phi), -h / 2, r * math.Sin(phi)})
+	}
+	at := func(j int) int { return start + j%segments }
+	for j := 0; j < segments; j++ {
+		m.Tris = append(m.Tris,
+			Triangle{apex, at(j + 1), at(j)},
+			Triangle{baseC, at(j), at(j + 1)},
+		)
+	}
+	return ensureOutward(m)
+}
+
+// Torus builds a torus around the Y axis: ring radius R (center of tube
+// to center of torus) and tube radius r.
+func Torus(R, r float64, segments, rings int) *Mesh {
+	if segments < 3 {
+		segments = 3
+	}
+	if rings < 3 {
+		rings = 3
+	}
+	m := &Mesh{Name: "torus"}
+	for i := 0; i < segments; i++ { // around the main ring
+		phi := 2 * math.Pi * float64(i) / float64(segments)
+		cx, cz := math.Cos(phi), math.Sin(phi)
+		for j := 0; j < rings; j++ { // around the tube
+			theta := 2 * math.Pi * float64(j) / float64(rings)
+			rad := R + r*math.Cos(theta)
+			m.Verts = append(m.Verts, Vec3{rad * cx, r * math.Sin(theta), rad * cz})
+		}
+	}
+	at := func(i, j int) int { return (i%segments)*rings + j%rings }
+	for i := 0; i < segments; i++ {
+		for j := 0; j < rings; j++ {
+			a, b := at(i, j), at(i+1, j)
+			c, d := at(i+1, j+1), at(i, j+1)
+			m.Tris = append(m.Tris, Triangle{a, b, c}, Triangle{a, c, d})
+		}
+	}
+	return ensureOutward(m)
+}

mesh-tool/mesh/stl.go

@@ -0,0 +1,182 @@
+package mesh
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+	"strconv"
+	"strings"
+)
+
+// ReadSTL reads binary or ASCII STL (auto-detected). STL stores loose
+// triangles, so identical vertices are welded back together to recover
+// connectivity (needed for watertight checks and sane OBJ export).
+func ReadSTL(r io.Reader) (*Scene, error) {
+	data, err := io.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+	if len(data) >= 84 {
+		n := binary.LittleEndian.Uint32(data[80:84])
+		if int(84+50*n) == len(data) {
+			return readSTLBinary(data)
+		}
+	}
+	if bytes.HasPrefix(bytes.TrimLeft(data, " \t\r\n"), []byte("solid")) {
+		return readSTLAscii(data)
+	}
+	return nil, fmt.Errorf("not a valid STL file (neither binary layout nor 'solid ...' text)")
+}
+
+type welder struct {
+	mesh  *Mesh
+	index map[Vec3]int
+}
+
+func newWelder(name string) *welder {
+	return &welder{mesh: &Mesh{Name: name}, index: map[Vec3]int{}}
+}
+
+func (w *welder) add(a, b, c Vec3) {
+	var t Triangle
+	for i, v := range [3]Vec3{a, b, c} {
+		idx, ok := w.index[v]
+		if !ok {
+			idx = len(w.mesh.Verts)
+			w.mesh.Verts = append(w.mesh.Verts, v)
+			w.index[v] = idx
+		}
+		t[i] = idx
+	}
+	if t[0] == t[1] || t[1] == t[2] || t[2] == t[0] {
+		return // degenerate
+	}
+	w.mesh.Tris = append(w.mesh.Tris, t)
+}
+
+func readSTLBinary(data []byte) (*Scene, error) {
+	n := int(binary.LittleEndian.Uint32(data[80:84]))
+	w := newWelder("stl")
+	off := 84
+	f32 := func(o int) float64 {
+		return float64(math.Float32frombits(binary.LittleEndian.Uint32(data[o : o+4])))
+	}
+	for i := 0; i < n; i++ {
+		// 12 bytes normal (ignored), 3 * 12 bytes vertices, 2 bytes attrs
+		var v [3]Vec3
+		for k := 0; k < 3; k++ {
+			base := off + 12 + k*12
+			v[k] = Vec3{f32(base), f32(base + 4), f32(base + 8)}
+		}
+		w.add(v[0], v[1], v[2])
+		off += 50
+	}
+	return &Scene{Meshes: []*Mesh{w.mesh}}, nil
+}
+
+func readSTLAscii(data []byte) (*Scene, error) {
+	name := "stl"
+	w := newWelder(name)
+	var cur []Vec3
+	sc := bufio.NewScanner(bytes.NewReader(data))
+	sc.Buffer(make([]byte, 0, 64*1024), 16*1024*1024)
+	lineNo := 0
+	for sc.Scan() {
+		lineNo++
+		fields := strings.Fields(sc.Text())
+		if len(fields) == 0 {
+			continue
+		}
+		switch fields[0] {
+		case "solid":
+			if len(fields) > 1 {
+				w.mesh.Name = fields[1]
+			}
+		case "vertex":
+			if len(fields) < 4 {
+				return nil, fmt.Errorf("stl 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("stl line %d: bad vertex: %v", lineNo, err)
+			}
+			cur = append(cur, v)
+		case "endfacet":
+			if len(cur) != 3 {
+				return nil, fmt.Errorf("stl line %d: facet has %d vertices, want 3", lineNo, len(cur))
+			}
+			w.add(cur[0], cur[1], cur[2])
+			cur = cur[:0]
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return nil, err
+	}
+	if len(w.mesh.Tris) == 0 {
+		return nil, fmt.Errorf("stl contains no triangles")
+	}
+	return &Scene{Meshes: []*Mesh{w.mesh}}, nil
+}
+
+// WriteSTLBinary writes the whole scene as one binary STL solid
+// (STL has no concept of multiple named objects).
+func WriteSTLBinary(w io.Writer, s *Scene) error {
+	m := s.Merged()
+	bw := bufio.NewWriter(w)
+	header := make([]byte, 80)
+	copy(header, []byte("exported by mesht (agent-tools)"))
+	bw.Write(header)
+	binary.Write(bw, binary.LittleEndian, uint32(len(m.Tris)))
+	buf := make([]byte, 50)
+	for i, t := range m.Tris {
+		n := m.FaceNormal(i)
+		le := binary.LittleEndian
+		le.PutUint32(buf[0:], math.Float32bits(float32(n.X)))
+		le.PutUint32(buf[4:], math.Float32bits(float32(n.Y)))
+		le.PutUint32(buf[8:], math.Float32bits(float32(n.Z)))
+		for k := 0; k < 3; k++ {
+			v := m.Verts[t[k]]
+			le.PutUint32(buf[12+k*12:], math.Float32bits(float32(v.X)))
+			le.PutUint32(buf[16+k*12:], math.Float32bits(float32(v.Y)))
+			le.PutUint32(buf[20+k*12:], math.Float32bits(float32(v.Z)))
+		}
+		buf[48], buf[49] = 0, 0
+		bw.Write(buf)
+	}
+	return bw.Flush()
+}
+
+// WriteSTLAscii writes the scene as a text STL solid.
+func WriteSTLAscii(w io.Writer, s *Scene) error {
+	m := s.Merged()
+	bw := bufio.NewWriter(w)
+	fmt.Fprintf(bw, "solid %s\n", sanitizeToken(m.Name))
+	for i, t := range m.Tris {
+		n := m.FaceNormal(i)
+		fmt.Fprintf(bw, "  facet normal %g %g %g\n    outer loop\n", n.X, n.Y, n.Z)
+		for k := 0; k < 3; k++ {
+			v := m.Verts[t[k]]
+			fmt.Fprintf(bw, "      vertex %g %g %g\n", v.X, v.Y, v.Z)
+		}
+		fmt.Fprintf(bw, "    endloop\n  endfacet\n")
+	}
+	fmt.Fprintf(bw, "endsolid %s\n", sanitizeToken(m.Name))
+	return bw.Flush()
+}
+
+func sanitizeToken(s string) string {
+	s = strings.ReplaceAll(strings.TrimSpace(s), " ", "_")
+	if s == "" {
+		return "mesh"
+	}
+	return s
+}

mesh-tool/mesh/transform.go

@@ -0,0 +1,20 @@
+package mesh
+
+// Apply transforms every vertex by mat. Mirroring transforms (negative
+// determinant) flip triangle winding, so it is corrected here to keep
+// normals pointing the same way relative to the surface.
+func (m *Mesh) Apply(mat Mat4) {
+	for i := range m.Verts {
+		m.Verts[i] = mat.Apply(m.Verts[i])
+	}
+	if mat.Det3() < 0 {
+		m.FlipWinding()
+	}
+}
+
+// ApplyAll transforms a set of meshes.
+func ApplyAll(meshes []*Mesh, mat Mat4) {
+	for _, m := range meshes {
+		m.Apply(mat)
+	}
+}

mesh-tool/mesh/vec.go

@@ -0,0 +1,122 @@
+package mesh
+
+import "math"
+
+// Vec3 is a point or direction in 3D space.
+type Vec3 struct{ X, Y, Z float64 }
+
+func (a Vec3) Add(b Vec3) Vec3    { return Vec3{a.X + b.X, a.Y + b.Y, a.Z + b.Z} }
+func (a Vec3) Sub(b Vec3) Vec3    { return Vec3{a.X - b.X, a.Y - b.Y, a.Z - b.Z} }
+func (a Vec3) Mul(s float64) Vec3 { return Vec3{a.X * s, a.Y * s, a.Z * s} }
+func (a Vec3) Dot(b Vec3) float64 { return a.X*b.X + a.Y*b.Y + a.Z*b.Z }
+func (a Vec3) Len() float64       { return math.Sqrt(a.Dot(a)) }
+func (a Vec3) Cross(b Vec3) Vec3 {
+	return Vec3{
+		a.Y*b.Z - a.Z*b.Y,
+		a.Z*b.X - a.X*b.Z,
+		a.X*b.Y - a.Y*b.X,
+	}
+}
+
+// Norm returns the unit vector, or the zero vector for zero-length input.
+func (a Vec3) Norm() Vec3 {
+	l := a.Len()
+	if l == 0 {
+		return Vec3{}
+	}
+	return a.Mul(1 / l)
+}
+
+// Min/Max return the component-wise minimum/maximum.
+func (a Vec3) Min(b Vec3) Vec3 {
+	return Vec3{math.Min(a.X, b.X), math.Min(a.Y, b.Y), math.Min(a.Z, b.Z)}
+}
+func (a Vec3) Max(b Vec3) Vec3 {
+	return Vec3{math.Max(a.X, b.X), math.Max(a.Y, b.Y), math.Max(a.Z, b.Z)}
+}
+
+// Mat4 is a row-major 4x4 transform matrix.
+type Mat4 [16]float64
+
+func Identity() Mat4 {
+	return Mat4{
+		1, 0, 0, 0,
+		0, 1, 0, 0,
+		0, 0, 1, 0,
+		0, 0, 0, 1,
+	}
+}
+
+// Mul returns m * n (n is applied first when transforming points).
+func (m Mat4) Mul(n Mat4) Mat4 {
+	var r Mat4
+	for row := 0; row < 4; row++ {
+		for col := 0; col < 4; col++ {
+			sum := 0.0
+			for k := 0; k < 4; k++ {
+				sum += m[row*4+k] * n[k*4+col]
+			}
+			r[row*4+col] = sum
+		}
+	}
+	return r
+}
+
+// Apply transforms a point (w = 1).
+func (m Mat4) Apply(v Vec3) Vec3 {
+	return Vec3{
+		m[0]*v.X + m[1]*v.Y + m[2]*v.Z + m[3],
+		m[4]*v.X + m[5]*v.Y + m[6]*v.Z + m[7],
+		m[8]*v.X + m[9]*v.Y + m[10]*v.Z + m[11],
+	}
+}
+
+// Det3 is the determinant of the upper-left 3x3. Negative means the
+// transform mirrors space, which flips triangle winding.
+func (m Mat4) Det3() float64 {
+	return m[0]*(m[5]*m[10]-m[6]*m[9]) -
+		m[1]*(m[4]*m[10]-m[6]*m[8]) +
+		m[2]*(m[4]*m[9]-m[5]*m[8])
+}
+
+func Translate(t Vec3) Mat4 {
+	m := Identity()
+	m[3], m[7], m[11] = t.X, t.Y, t.Z
+	return m
+}
+
+func ScaleXYZ(s Vec3) Mat4 {
+	m := Identity()
+	m[0], m[5], m[10] = s.X, s.Y, s.Z
+	return m
+}
+
+func RotateX(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		1, 0, 0, 0,
+		0, c, -s, 0,
+		0, s, c, 0,
+		0, 0, 0, 1,
+	}
+}
+
+func RotateY(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		c, 0, s, 0,
+		0, 1, 0, 0,
+		-s, 0, c, 0,
+		0, 0, 0, 1,
+	}
+}
+
+func RotateZ(deg float64) Mat4 {
+	s, c := math.Sincos(deg * math.Pi / 180)
+	return Mat4{
+		c, -s, 0, 0,
+		s, c, 0, 0,
+		0, 0, 1, 0,
+		0, 0, 0, 1,
+	}
+}