EXIF orientation: bake it in when normalizing images (#103)

imagenorm decoded and re-encoded to JPEG but ignored the EXIF Orientation
tag. Phone cameras store the sensor pixels one way and set an EXIF flag to
rotate on display, so a "portrait" JPEG is really a landscape bitmap tagged
"rotate 90°" — and our re-encode strips EXIF, so without baking the rotation
in, a packet photographed in portrait reaches the vision model sideways
(bad OCR) and any future thumbnail is wrong.

- exifOrientation: a small pure-Go parser that walks the JPEG APP1/Exif
  segment for tag 0x0112, returning 1 (normal) for non-JPEG or unparseable
  input — never guess a rotation onto a correct image. No cgo, no new dep.
- applyOrientation: bakes in all 8 orientations (the 4 rotations + mirrors)
  after downscale (cheaper to rotate the small image; a 90° turn swaps the
  sides but not the longest edge, so the downscale bound still holds).
- Non-JPEG paths (HEIC/webp/png) are untouched — their decoders own
  orientation and they carry no JPEG EXIF.

Tests build oriented JPEGs (a corner marker + a spliced Exif APP1) and
assert the marker lands where each orientation says, dims swapping for the
quarter-turns; plus parser defaults for non-JPEG / no-EXIF / garbage.
Stays CGO_ENABLED=0.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/imagenorm/imagenorm_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/binary"
 	"hash/crc32"
 	"image"
+	"image/color"
 	"image/jpeg"
 	"image/png"
 	"os"
@@ -200,3 +201,120 @@ func TestNormalizeRejectsPixelBomb(t *testing.T) {
 		})
 	}
 }
+
+// orientedJPEG builds a JPEG whose top-left quadrant is white and the rest black
+// — a marker to track through a rotation — tagged with the given EXIF orientation
+// (1..8). The marker lets a test assert the pixels actually moved to where that
+// orientation says they should.
+func orientedJPEG(t *testing.T, orient int) []byte {
+	t.Helper()
+	const w, h = 40, 24
+	m := image.NewRGBA(image.Rect(0, 0, w, h))
+	for y := range h {
+		for x := range w {
+			c := color.RGBA{0, 0, 0, 255}
+			if x < w/2 && y < h/2 {
+				c = color.RGBA{255, 255, 255, 255}
+			}
+			m.Set(x, y, c)
+		}
+	}
+	var jb bytes.Buffer
+	if err := jpeg.Encode(&jb, m, &jpeg.Options{Quality: 95}); err != nil {
+		t.Fatalf("encode jpeg: %v", err)
+	}
+	if orient == 0 {
+		return jb.Bytes() // caller wants a plain JPEG with no EXIF
+	}
+	return spliceExifOrientation(t, jb.Bytes(), orient)
+}
+
+// spliceExifOrientation inserts a minimal little-endian Exif APP1 segment
+// carrying just the Orientation tag right after the JPEG SOI marker.
+func spliceExifOrientation(t *testing.T, jpg []byte, orient int) []byte {
+	t.Helper()
+	var tiff bytes.Buffer
+	tiff.WriteString("II") // little-endian
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(0x2A))
+	_ = binary.Write(&tiff, binary.LittleEndian, uint32(8)) // IFD0 offset
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(1)) // one entry
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(0x0112))
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(3))      // SHORT
+	_ = binary.Write(&tiff, binary.LittleEndian, uint32(1))      // count
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(orient)) // value
+	_ = binary.Write(&tiff, binary.LittleEndian, uint16(0))      // value pad
+	_ = binary.Write(&tiff, binary.LittleEndian, uint32(0))      // next IFD
+	payload := append([]byte("Exif\x00\x00"), tiff.Bytes()...)
+
+	segLen := len(payload) + 2
+	seg := []byte{0xFF, 0xE1, byte(segLen >> 8), byte(segLen)}
+	seg = append(seg, payload...)
+
+	out := make([]byte, 0, len(jpg)+len(seg))
+	out = append(out, jpg[:2]...) // SOI
+	out = append(out, seg...)
+	return append(out, jpg[2:]...)
+}
+
+func bright(c color.Color) bool {
+	r, g, b, _ := c.RGBA() // 16-bit
+	return (r+g+b)/3 > 0x8000
+}
+
+// TestNormalizeAppliesExifOrientation is the #103 regression: a phone photo tagged
+// "rotate 90°" must come out of Normalize with the pixels upright, not sideways —
+// the re-encode strips EXIF, so the rotation has to be baked into the bitmap.
+func TestNormalizeAppliesExifOrientation(t *testing.T) {
+	// The white marker starts centred at (10,6) in the 40x24 source. For each
+	// orientation, wantW/H is the corrected canvas and (mx,my) is where that
+	// marker must land — derived from the same transform Normalize applies.
+	cases := []struct {
+		orient       int
+		wantW, wantH int
+		mx, my       int
+	}{
+		{0, 40, 24, 10, 6},  // no EXIF → unchanged, marker top-left
+		{1, 40, 24, 10, 6},  // normal → unchanged
+		{3, 40, 24, 29, 17}, // 180 → bottom-right
+		{6, 24, 40, 17, 10}, // 90° CW → top-right (dims swap)
+		{8, 24, 40, 6, 29},  // 90° CCW → bottom-left (dims swap)
+	}
+	for _, tc := range cases {
+		out, format, err := Normalize(bytes.NewReader(orientedJPEG(t, tc.orient)), Options{})
+		if err != nil {
+			t.Fatalf("orient %d: Normalize err = %v", tc.orient, err)
+		}
+		if format != "jpeg" {
+			t.Errorf("orient %d: format = %q, want jpeg", tc.orient, format)
+		}
+		m, _, err := image.Decode(bytes.NewReader(out))
+		if err != nil {
+			t.Fatalf("orient %d: decode output: %v", tc.orient, err)
+		}
+		if m.Bounds().Dx() != tc.wantW || m.Bounds().Dy() != tc.wantH {
+			t.Errorf("orient %d: output %dx%d, want %dx%d",
+				tc.orient, m.Bounds().Dx(), m.Bounds().Dy(), tc.wantW, tc.wantH)
+		}
+		if !bright(m.At(m.Bounds().Min.X+tc.mx, m.Bounds().Min.Y+tc.my)) {
+			t.Errorf("orient %d: white marker not at (%d,%d) — orientation not applied",
+				tc.orient, tc.mx, tc.my)
+		}
+	}
+}
+
+// TestExifOrientationParsing pins the parser against non-JPEG and no-EXIF inputs,
+// which must default to 1 (never guess a rotation onto a correct image).
+func TestExifOrientationParsing(t *testing.T) {
+	if o := exifOrientation(pngBytes(t, 8, 8)); o != 1 {
+		t.Errorf("PNG orientation = %d, want 1 (no JPEG EXIF path)", o)
+	}
+	if o := exifOrientation(orientedJPEG(t, 0)); o != 1 {
+		t.Errorf("JPEG without EXIF orientation = %d, want 1", o)
+	}
+	if o := exifOrientation(orientedJPEG(t, 6)); o != 6 {
+		t.Errorf("JPEG tagged 6 → %d, want 6", o)
+	}
+	if o := exifOrientation([]byte("not an image")); o != 1 {
+		t.Errorf("garbage bytes → %d, want 1", o)
+	}
+}