Thumbs: Enhance embedding of ICC profiles based on InteropIndex #5178

Signed-off-by: Michael Mayer <michael@photoprism.app>

internal/thumb/vips_icc.go

@@ -0,0 +1,78 @@
+package thumb
+
+import (
+	"fmt"
+
+	"github.com/davidbyttow/govips/v2/vips"
+)
+
+// InteroperabilityIndex EXIF codes used by some cameras to hint at the color space.
+// Sources: EXIF TagNames (R03=Adobe RGB, R98=sRGB, THM=thumbnail)
+// https://unpkg.com/exiftool-vendored.pl@10.50.0/bin/html/TagNames/EXIF.html
+// Additional context: https://regex.info/blog/photo-tech/color-spaces-page7
+const (
+	// InteropIndexAdobeRGB is the EXIF code for Adobe RGB (1998) ("R03").
+	InteropIndexAdobeRGB = "R03"
+	// InteropIndexSRGB is the EXIF code for sRGB ("R98").
+	InteropIndexSRGB = "R98"
+	// InteropIndexThumb marks a thumbnail image; treated as sRGB ("THM").
+	InteropIndexThumb = "THM"
+)
+
+// vipsSetIccProfileForInteropIndex embeds an ICC profile when a JPEG declares
+// its color space via the EXIF InteroperabilityIndex tag (e.g., "R03"/Adobe RGB)
+// but lacks an embedded profile. If an ICC profile is already present, it
+// leaves the image untouched.
+func vipsSetIccProfileForInteropIndex(img *vips.ImageRef, logName string) (err error) {
+	// Some cameras signal color space via EXIF InteroperabilityIndex instead of
+	// embedding an ICC profile. Browsers and libvips ignore this tag, so we
+	// inject a matching ICC profile to produce correct thumbnails.
+	iiFull := img.GetString("exif-ifd4-InteroperabilityIndex")
+
+	if iiFull == "" {
+		return nil
+	}
+
+	// EXIF InteroperabilityIndex is 4 bytes including null; libvips returns
+	// a string with a trailing space. Using the first three bytes covers the
+	// meaningful code (e.g., "R03", "R98").
+	if len(iiFull) < 3 {
+		log.Debugf("interopindex: %s has unexpected interop index %q", logName, iiFull)
+		return nil
+	}
+
+	ii := iiFull[:3]
+	log.Tracef("interopindex: %s read exif and got interopindex %s, %s", logName, ii, iiFull)
+
+	if img.HasICCProfile() {
+		log.Debugf("interopindex: %s already has an embedded ICC profile; skipping fallback.", logName)
+		return nil
+	}
+
+	profilePath := ""
+
+	switch ii {
+	case InteropIndexAdobeRGB:
+		// Use Adobe RGB 1998 compatible profile.
+		profilePath, err = GetIccProfile(IccAdobeRGBCompat, IccAdobeRGBCompatV2, IccAdobeRGBCompatV4)
+
+		if err != nil {
+			return fmt.Errorf("interopindex %s: %w", ii, err)
+		}
+	case InteropIndexSRGB:
+		// sRGB: browsers and libvips assume sRGB by default, so no embed needed.
+	case InteropIndexThumb:
+		// Thumbnail file; specification unclear—treat as sRGB and do nothing.
+	default:
+		log.Debugf("interopindex: %s has unknown interop index %s", logName, ii)
+	}
+
+	if profilePath == "" {
+		return nil
+	}
+
+	// Embed ICC profile. govips expects both an input (fallback) and output
+	// profile; using the same path injects the chosen profile when none is
+	// embedded and keeps colors consistent otherwise.
+	return img.TransformICCProfileWithFallback(profilePath, profilePath)
+}