hitbox-tool: per-frame collision boxes from sprite sheet alpha -> JSON

- parses the spritec _WxH_CxR naming convention incl. auto-detection of
  integer-upscaled sheets (256x64 named 8x8_4x1 -> 64x64 cells)
- tight alpha bbox per frame with threshold/shrink/pad tuning, row-major
  indices, empty-frame flags; boxes relative to frame origin
- show command draws frames + box outline in the terminal for verification
- go tests: scanning, threshold, shrink/pad clamping, name parsing,
  scale inference, JSON roundtrip

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

hitbox-tool/README.md

@@ -0,0 +1,66 @@
+# hitbox-tool (`hitbox`)
+
+Scans **sprite sheet PNGs** and writes per-frame **collision boxes as
+JSON**, computed from the alpha channel. Closes the loop with
+[`pixel-sprite-maker`](../pixel-sprite-maker/): render a sheet with
+`spritec`, scan it with `hitbox`, and your game gets both graphics and
+hitboxes without a human drawing rectangles. Go, zero dependencies,
+single static binary.
+
+## Build
+
+```bash
+# Arch/Garuda: sudo pacman -S go
+cd hitbox-tool
+go build -o build/hitbox .        # or the VS Code task "build hitbox-tool"
+go test ./...
+```
+
+## Usage
+
+```bash
+# spritec-named sheets need no flags — layout is parsed from the name,
+# and integer upscales are auto-detected (a _8x8_4x1 sheet that is
+# 256x64 px was rendered at --scale 8, so cells are 64x64):
+hitbox scan walk_8x8_4x1.png -o walk.hitbox.json
+
+hitbox scan boss.png --cell 32x32 -o boss.json   # explicit frame size
+hitbox scan portrait.png                          # whole image = one frame, JSON to stdout
+
+hitbox show walk_8x8_4x1.png --frame 2            # verify visually in the terminal
+
+# tuning:
+--threshold 128     # ignore faint pixels (alpha < 128)
+--shrink 1          # tighter, more forgiving hitboxes (n px per side)
+--pad 2             # generous hitboxes (e.g. pickups)
+```
+
+## Output
+
+```json
+{
+  "image": "walk_8x8_4x1.png",
+  "cellW": 8, "cellH": 8, "cols": 4, "rows": 1,
+  "alphaThreshold": 1,
+  "frames": [
+    { "index": 0, "col": 0, "row": 0, "empty": false,
+      "box": { "x": 1, "y": 0, "w": 5, "h": 8 } },
+    { "index": 1, "col": 1, "row": 0, "empty": true, "box": null }
+  ]
+}
+```
+
+- Boxes are **relative to each frame's top-left corner**.
+- Frame order is row-major (`index = row * cols + col`), matching
+  spritec sheets.
+- Cells with no solid pixels get `"empty": true`.
+
+In game code:
+
+```
+hit = px >= frameX + box.x && px < frameX + box.x + box.w
+   && py >= frameY + box.y && py < frameY + box.y + box.h
+```
+
+`hitbox show` draws each frame with `#` for solid pixels and `+` for the
+box outline, so an agent can verify the result without an image viewer.

hitbox-tool/examples/walk_8x8_4x1.hitbox.json

@@ -0,0 +1,58 @@
+{
+  "image": "examples/walk_8x8_4x1.png",
+  "cellW": 64,
+  "cellH": 64,
+  "cols": 4,
+  "rows": 1,
+  "alphaThreshold": 1,
+  "frames": [
+    {
+      "index": 0,
+      "col": 0,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 1,
+      "col": 1,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 2,
+      "col": 2,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 3,
+      "col": 3,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    }
+  ]
+}

hitbox-tool/go.mod

@@ -0,0 +1,3 @@
+module gitea.brasse-pc.eu/brasse/agent-tools/hitbox-tool
+
+go 1.24

hitbox-tool/hitbox/hitbox.go

@@ -0,0 +1,230 @@
+// Package hitbox computes per-frame collision boxes from sprite sheet
+// images by scanning the alpha channel.
+package hitbox
+
+import (
+	"encoding/json"
+	"fmt"
+	"image"
+	"image/png"
+	"io"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+)
+
+// Box is a rectangle relative to its frame's top-left corner.
+type Box struct {
+	X int `json:"x"`
+	Y int `json:"y"`
+	W int `json:"w"`
+	H int `json:"h"`
+}
+
+// Frame is the scan result for one sheet cell.
+type Frame struct {
+	Index int  `json:"index"`
+	Col   int  `json:"col"`
+	Row   int  `json:"row"`
+	Empty bool `json:"empty"`
+	Box   *Box `json:"box"` // nil when Empty
+}
+
+// Sheet is the full scan result; the JSON deliverable.
+type Sheet struct {
+	Image     string  `json:"image"`
+	CellW     int     `json:"cellW"`
+	CellH     int     `json:"cellH"`
+	Cols      int     `json:"cols"`
+	Rows      int     `json:"rows"`
+	Threshold int     `json:"alphaThreshold"`
+	Frames    []Frame `json:"frames"`
+}
+
+// layoutRe matches the spritec sheet naming convention:
+// <base>_<cellW>x<cellH>_<cols>x<rows>.<ext>
+var layoutRe = regexp.MustCompile(`_(\d+)x(\d+)_(\d+)x(\d+)\.[A-Za-z]+$`)
+
+// LayoutFromName extracts cell size and grid from a spritec-style file
+// name. ok is false when the name doesn't follow the convention.
+func LayoutFromName(path string) (cellW, cellH, cols, rows int, ok bool) {
+	m := layoutRe.FindStringSubmatch(filepath.Base(path))
+	if m == nil {
+		return 0, 0, 0, 0, false
+	}
+	cellW, _ = strconv.Atoi(m[1])
+	cellH, _ = strconv.Atoi(m[2])
+	cols, _ = strconv.Atoi(m[3])
+	rows, _ = strconv.Atoi(m[4])
+	return cellW, cellH, cols, rows, true
+}
+
+// InferScale detects integer-upscaled sheets: when the image is exactly
+// s times bigger than the name-declared layout (both axes, s >= 1), the
+// real cell size is cell*s. Returns 0 when the layout doesn't fit.
+func InferScale(imgW, imgH, cellW, cellH, cols, rows int) int {
+	baseW, baseH := cellW*cols, cellH*rows
+	if baseW <= 0 || baseH <= 0 || imgW%baseW != 0 || imgH%baseH != 0 {
+		return 0
+	}
+	s := imgW / baseW
+	if s < 1 || imgH/baseH != s {
+		return 0
+	}
+	return s
+}
+
+// LoadPNG reads a PNG image from disk.
+func LoadPNG(path string) (image.Image, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	img, err := png.Decode(f)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w (only PNG is supported — sheets need an alpha channel)", path, err)
+	}
+	return img, nil
+}
+
+// Scan computes a tight box per frame: the smallest rectangle covering
+// every pixel with alpha >= threshold. shrink/pad (in pixels) contract or
+// expand each box afterwards, clamped to the cell.
+func Scan(img image.Image, name string, cellW, cellH, threshold, shrink, pad int) (*Sheet, error) {
+	b := img.Bounds()
+	if cellW <= 0 || cellH <= 0 {
+		cellW, cellH = b.Dx(), b.Dy() // whole image = one frame
+	}
+	if b.Dx()%cellW != 0 || b.Dy()%cellH != 0 {
+		return nil, fmt.Errorf("image is %dx%d which is not divisible by cell %dx%d",
+			b.Dx(), b.Dy(), cellW, cellH)
+	}
+	if threshold < 1 || threshold > 255 {
+		return nil, fmt.Errorf("alpha threshold %d out of range 1-255", threshold)
+	}
+	cols, rows := b.Dx()/cellW, b.Dy()/cellH
+	sheet := &Sheet{
+		Image: name, CellW: cellW, CellH: cellH,
+		Cols: cols, Rows: rows, Threshold: threshold,
+	}
+	for row := 0; row < rows; row++ {
+		for col := 0; col < cols; col++ {
+			fr := Frame{Index: row*cols + col, Col: col, Row: row}
+			minX, minY := cellW, cellH
+			maxX, maxY := -1, -1
+			for y := 0; y < cellH; y++ {
+				for x := 0; x < cellW; x++ {
+					_, _, _, a := img.At(b.Min.X+col*cellW+x, b.Min.Y+row*cellH+y).RGBA()
+					if int(a>>8) >= threshold {
+						if x < minX {
+							minX = x
+						}
+						if y < minY {
+							minY = y
+						}
+						if x > maxX {
+							maxX = x
+						}
+						if y > maxY {
+							maxY = y
+						}
+					}
+				}
+			}
+			if maxX < 0 {
+				fr.Empty = true
+			} else {
+				box := Box{X: minX, Y: minY, W: maxX - minX + 1, H: maxY - minY + 1}
+				box = adjust(box, shrink-pad, cellW, cellH)
+				fr.Box = &box
+			}
+			sheet.Frames = append(sheet.Frames, fr)
+		}
+	}
+	return sheet, nil
+}
+
+// adjust contracts the box by delta px on every side (negative delta
+// expands), clamped to the cell and to a minimum size of 1x1.
+func adjust(b Box, delta, cellW, cellH int) Box {
+	b.X += delta
+	b.Y += delta
+	b.W -= 2 * delta
+	b.H -= 2 * delta
+	if b.X < 0 {
+		b.W += b.X
+		b.X = 0
+	}
+	if b.Y < 0 {
+		b.H += b.Y
+		b.Y = 0
+	}
+	if b.X+b.W > cellW {
+		b.W = cellW - b.X
+	}
+	if b.Y+b.H > cellH {
+		b.H = cellH - b.Y
+	}
+	if b.W < 1 {
+		b.W = 1
+		if b.X > cellW-1 {
+			b.X = cellW - 1
+		}
+	}
+	if b.H < 1 {
+		b.H = 1
+		if b.Y > cellH-1 {
+			b.Y = cellH - 1
+		}
+	}
+	return b
+}
+
+// WriteJSON encodes the sheet as indented JSON.
+func (s *Sheet) WriteJSON(w io.Writer) error {
+	enc := json.NewEncoder(w)
+	enc.SetIndent("", "  ")
+	return enc.Encode(s)
+}
+
+// Ascii draws one frame with '#' for solid pixels and '+' for the box
+// outline, so results can be verified in a terminal.
+func Ascii(img image.Image, s *Sheet, index, threshold int) string {
+	if index < 0 || index >= len(s.Frames) {
+		return "(no such frame)\n"
+	}
+	fr := s.Frames[index]
+	b := img.Bounds()
+	out := make([]rune, 0, (s.CellW+1)*s.CellH)
+	onBoxEdge := func(x, y int) bool {
+		if fr.Box == nil {
+			return false
+		}
+		bx := fr.Box
+		inX := x >= bx.X && x < bx.X+bx.W
+		inY := y >= bx.Y && y < bx.Y+bx.H
+		edgeX := x == bx.X || x == bx.X+bx.W-1
+		edgeY := y == bx.Y || y == bx.Y+bx.H-1
+		return (inX && inY) && (edgeX || edgeY)
+	}
+	for y := 0; y < s.CellH; y++ {
+		for x := 0; x < s.CellW; x++ {
+			_, _, _, a := img.At(b.Min.X+fr.Col*s.CellW+x, b.Min.Y+fr.Row*s.CellH+y).RGBA()
+			solid := int(a>>8) >= threshold
+			switch {
+			case solid && onBoxEdge(x, y):
+				out = append(out, '#')
+			case solid:
+				out = append(out, '#')
+			case onBoxEdge(x, y):
+				out = append(out, '+')
+			default:
+				out = append(out, '.')
+			}
+		}
+		out = append(out, '\n')
+	}
+	return string(out)
+}

hitbox-tool/hitbox/hitbox_test.go

@@ -0,0 +1,137 @@
+package hitbox
+
+import (
+	"bytes"
+	"encoding/json"
+	"image"
+	"image/color"
+	"testing"
+)
+
+// sheet4 builds a 2x1 sheet of 4x4 cells: frame 0 has a 2x2 blob at
+// (1,1); frame 1 is empty.
+func sheet4() image.Image {
+	img := image.NewNRGBA(image.Rect(0, 0, 8, 4))
+	for y := 1; y <= 2; y++ {
+		for x := 1; x <= 2; x++ {
+			img.SetNRGBA(x, y, color.NRGBA{255, 0, 0, 255})
+		}
+	}
+	return img
+}
+
+func TestScanTightBox(t *testing.T) {
+	s, err := Scan(sheet4(), "test.png", 4, 4, 1, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s.Cols != 2 || s.Rows != 1 || len(s.Frames) != 2 {
+		t.Fatalf("layout %dx%d frames=%d", s.Cols, s.Rows, len(s.Frames))
+	}
+	f0 := s.Frames[0]
+	if f0.Empty || f0.Box == nil {
+		t.Fatal("frame 0 should have a box")
+	}
+	if *f0.Box != (Box{X: 1, Y: 1, W: 2, H: 2}) {
+		t.Errorf("frame 0 box = %+v", *f0.Box)
+	}
+	f1 := s.Frames[1]
+	if !f1.Empty || f1.Box != nil {
+		t.Errorf("frame 1 should be empty, got %+v", f1)
+	}
+}
+
+func TestScanWholeImageDefault(t *testing.T) {
+	s, err := Scan(sheet4(), "x.png", 0, 0, 1, 0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(s.Frames) != 1 || s.CellW != 8 || s.CellH != 4 {
+		t.Errorf("whole-image scan: %+v", s)
+	}
+}
+
+func TestScanErrors(t *testing.T) {
+	if _, err := Scan(sheet4(), "x.png", 3, 4, 1, 0, 0); err == nil {
+		t.Error("non-divisible cell size should error")
+	}
+	if _, err := Scan(sheet4(), "x.png", 4, 4, 0, 0, 0); err == nil {
+		t.Error("threshold 0 should error")
+	}
+}
+
+func TestShrinkAndPad(t *testing.T) {
+	img := image.NewNRGBA(image.Rect(0, 0, 4, 4))
+	for y := 0; y < 4; y++ {
+		for x := 0; x < 4; x++ {
+			img.SetNRGBA(x, y, color.NRGBA{0, 0, 0, 255})
+		}
+	}
+	s, _ := Scan(img, "x.png", 4, 4, 1, 1, 0) // shrink 1
+	if *s.Frames[0].Box != (Box{X: 1, Y: 1, W: 2, H: 2}) {
+		t.Errorf("shrunk box = %+v", *s.Frames[0].Box)
+	}
+	s, _ = Scan(img, "x.png", 4, 4, 1, 0, 3) // pad clamps to cell
+	if *s.Frames[0].Box != (Box{X: 0, Y: 0, W: 4, H: 4}) {
+		t.Errorf("padded box = %+v", *s.Frames[0].Box)
+	}
+	s, _ = Scan(img, "x.png", 4, 4, 1, 10, 0) // over-shrink -> min 1x1
+	if s.Frames[0].Box.W < 1 || s.Frames[0].Box.H < 1 {
+		t.Errorf("over-shrunk box = %+v", *s.Frames[0].Box)
+	}
+}
+
+func TestThreshold(t *testing.T) {
+	img := image.NewNRGBA(image.Rect(0, 0, 2, 1))
+	img.SetNRGBA(0, 0, color.NRGBA{0, 0, 0, 100})
+	img.SetNRGBA(1, 0, color.NRGBA{0, 0, 0, 200})
+	s, _ := Scan(img, "x.png", 2, 1, 150, 0, 0)
+	if *s.Frames[0].Box != (Box{X: 1, Y: 0, W: 1, H: 1}) {
+		t.Errorf("threshold box = %+v", *s.Frames[0].Box)
+	}
+	s, _ = Scan(img, "x.png", 2, 1, 50, 0, 0)
+	if *s.Frames[0].Box != (Box{X: 0, Y: 0, W: 2, H: 1}) {
+		t.Errorf("low-threshold box = %+v", *s.Frames[0].Box)
+	}
+}
+
+func TestLayoutFromName(t *testing.T) {
+	w, h, c, r, ok := LayoutFromName("/tmp/walk_8x8_4x1.png")
+	if !ok || w != 8 || h != 8 || c != 4 || r != 1 {
+		t.Errorf("parsed %d %d %d %d ok=%v", w, h, c, r, ok)
+	}
+	if _, _, _, _, ok := LayoutFromName("plain.png"); ok {
+		t.Error("plain.png should not match")
+	}
+}
+
+func TestInferScale(t *testing.T) {
+	// walk_8x8_4x1.png rendered at scale 8 -> 256x64
+	if s := InferScale(256, 64, 8, 8, 4, 1); s != 8 {
+		t.Errorf("scale = %d, want 8", s)
+	}
+	if s := InferScale(32, 8, 8, 8, 4, 1); s != 1 {
+		t.Errorf("unscaled = %d, want 1", s)
+	}
+	if s := InferScale(250, 64, 8, 8, 4, 1); s != 0 {
+		t.Errorf("non-divisible = %d, want 0", s)
+	}
+	if s := InferScale(256, 32, 8, 8, 4, 1); s != 0 {
+		t.Errorf("axis mismatch = %d, want 0", s)
+	}
+}
+
+func TestJSONShape(t *testing.T) {
+	s, _ := Scan(sheet4(), "test.png", 4, 4, 1, 0, 0)
+	var buf bytes.Buffer
+	if err := s.WriteJSON(&buf); err != nil {
+		t.Fatal(err)
+	}
+	var back Sheet
+	if err := json.Unmarshal(buf.Bytes(), &back); err != nil {
+		t.Fatalf("invalid json: %v", err)
+	}
+	if back.Frames[0].Box.W != 2 {
+		t.Errorf("json roundtrip box = %+v", back.Frames[0].Box)
+	}
+}

hitbox-tool/main.go

@@ -0,0 +1,204 @@
+// hitbox scans sprite sheet PNGs and writes per-frame collision boxes
+// as JSON, using the alpha channel to find each frame's solid pixels.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"gitea.brasse-pc.eu/brasse/agent-tools/hitbox-tool/hitbox"
+)
+
+var version = "dev"
+
+const usage = `hitbox - collision box annotator for sprite sheets
+
+Usage:
+  hitbox scan <sheet.png> [flags]     compute per-frame boxes -> JSON
+  hitbox show <sheet.png> [flags]     draw frames + boxes in the terminal
+  hitbox version
+
+Scan flags:
+  --cell <WxH>       frame size, e.g. 8x8. Default: parsed from the
+                     spritec naming convention <name>_<W>x<H>_<C>x<R>.png;
+                     if neither is given the whole image is one frame.
+  --threshold <n>    alpha 1-255 that counts as solid (default 1)
+  --shrink <n>       contract every box by n px per side (forgiving hits)
+  --pad <n>          expand every box by n px per side
+  -o <path>          write JSON here (default: stdout)
+
+Show flags: --cell, --threshold, plus
+  --frame <n>        only this frame index (default: all)
+
+Boxes are relative to each frame's top-left corner. Frame indices are
+row-major (index = row * cols + col), matching spritec sheets.
+In game code: hit if (px,py) inside (frameX + box.x, frameY + box.y,
+box.w, box.h).
+`
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Print(usage)
+		os.Exit(2)
+	}
+	switch os.Args[1] {
+	case "scan":
+		cmdScan(os.Args[2:])
+	case "show":
+		cmdShow(os.Args[2:])
+	case "version", "--version", "-v":
+		fmt.Println("hitbox", version)
+	case "help", "--help", "-h":
+		fmt.Print(usage)
+	default:
+		die("unknown command %q — run 'hitbox 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...))
+}
+
+// parseCell resolves the frame size from --cell, or from the spritec
+// naming convention (auto-detecting integer upscales: a sheet named
+// _8x8_4x1 that is 256x64 px was rendered at scale 8, so cells are 64x64).
+func parseCell(spec, path string, imgW, imgH int) (int, int, error) {
+	if spec != "" {
+		parts := strings.SplitN(strings.ToLower(spec), "x", 2)
+		if len(parts) == 2 {
+			w, err1 := strconv.Atoi(parts[0])
+			h, err2 := strconv.Atoi(parts[1])
+			if err1 == nil && err2 == nil && w > 0 && h > 0 {
+				return w, h, nil
+			}
+		}
+		return 0, 0, fmt.Errorf("--cell %q must look like 8x8", spec)
+	}
+	if w, h, cols, rows, ok := hitbox.LayoutFromName(path); ok {
+		if s := hitbox.InferScale(imgW, imgH, w, h, cols, rows); s > 1 {
+			fmt.Fprintf(os.Stderr, "hitbox: image is %dx the named layout — using %dx%d cells\n", s, w*s, h*s)
+			return w * s, h * s, nil
+		}
+		return w, h, nil
+	}
+	return 0, 0, nil // whole image = one frame
+}
+
+func cmdScan(args []string) {
+	fs := flag.NewFlagSet("scan", flag.ExitOnError)
+	cell := fs.String("cell", "", "frame size WxH")
+	threshold := fs.Int("threshold", 1, "solid alpha 1-255")
+	shrink := fs.Int("shrink", 0, "contract boxes n px per side")
+	pad := fs.Int("pad", 0, "expand boxes n px per side")
+	out := fs.String("o", "", "output JSON path (default stdout)")
+	parseInterspersed(fs, args)
+	if fs.NArg() != 1 {
+		die("scan takes exactly one PNG file")
+	}
+	path := fs.Arg(0)
+	img, err := hitbox.LoadPNG(path)
+	if err != nil {
+		die("%v", err)
+	}
+	b := img.Bounds()
+	cw, ch, err := parseCell(*cell, path, b.Dx(), b.Dy())
+	if err != nil {
+		die("%v", err)
+	}
+	sheet, err := hitbox.Scan(img, path, cw, ch, *threshold, *shrink, *pad)
+	if err != nil {
+		die("%v", err)
+	}
+	if *out == "" {
+		if err := sheet.WriteJSON(os.Stdout); err != nil {
+			die("%v", err)
+		}
+		return
+	}
+	f, err := os.Create(*out)
+	if err != nil {
+		die("%v", err)
+	}
+	defer f.Close()
+	if err := sheet.WriteJSON(f); err != nil {
+		die("%v", err)
+	}
+	solid := 0
+	for _, fr := range sheet.Frames {
+		if !fr.Empty {
+			solid++
+		}
+	}
+	fmt.Printf("%s  (%d frames of %dx%d, %d with pixels)\n",
+		*out, len(sheet.Frames), sheet.CellW, sheet.CellH, solid)
+}
+
+func cmdShow(args []string) {
+	fs := flag.NewFlagSet("show", flag.ExitOnError)
+	cell := fs.String("cell", "", "frame size WxH")
+	threshold := fs.Int("threshold", 1, "solid alpha 1-255")
+	frame := fs.Int("frame", -1, "frame index (default: all)")
+	parseInterspersed(fs, args)
+	if fs.NArg() != 1 {
+		die("show takes exactly one PNG file")
+	}
+	path := fs.Arg(0)
+	img, err := hitbox.LoadPNG(path)
+	if err != nil {
+		die("%v", err)
+	}
+	b := img.Bounds()
+	cw, ch, err := parseCell(*cell, path, b.Dx(), b.Dy())
+	if err != nil {
+		die("%v", err)
+	}
+	sheet, err := hitbox.Scan(img, path, cw, ch, *threshold, 0, 0)
+	if err != nil {
+		die("%v", err)
+	}
+	for _, fr := range sheet.Frames {
+		if *frame >= 0 && fr.Index != *frame {
+			continue
+		}
+		if fr.Empty {
+			fmt.Printf("frame %d (col %d, row %d): empty\n\n", fr.Index, fr.Col, fr.Row)
+			continue
+		}
+		fmt.Printf("frame %d (col %d, row %d): box x=%d y=%d w=%d h=%d\n",
+			fr.Index, fr.Col, fr.Row, fr.Box.X, fr.Box.Y, fr.Box.W, fr.Box.H)
+		fmt.Print(hitbox.Ascii(img, sheet, fr.Index, *threshold))
+		fmt.Println()
+	}
+}
+
+func die(format string, a ...any) {
+	fmt.Fprintf(os.Stderr, "hitbox: "+format+"\n", a...)
+	os.Exit(1)
+}