Serve and accept JPEG XL as an image format (part of #222)

A JPEG XL source is accepted, and orig of one is JPEG XL. The format
jxl works in plain and encrypted URLs and on the generator page,
served as image/jxl; auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance, which overrides the quality,
so q is turned into a distance, keeping 100 lossy. govips cannot have
libvips strip metadata from JPEG XL, so it is removed from the image
before saving; libvips 8.16 and later still add an EXIF block of their
own, with nothing from the source. The conversion to sRGB moved ahead
of the format switch, and a CMYK image with no ICC profile is
converted to sRGB too, as libvips cannot save CMYK as JPEG XL.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -65,6 +65,7 @@ const (
 	FormatPNG      Format = "png"
 	FormatWebP     Format = "webp"
 	FormatAVIF     Format = "avif"
+	FormatJXL      Format = "jxl"
 	FormatGIF      Format = "gif"
 )
 
@@ -305,6 +306,7 @@ const (
 	mimeGIF  = "image/gif"
 	mimeWebP = "image/webp"
 	mimeAVIF = "image/avif"
+	mimeJXL  = "image/jxl"
 )
 
 // SupportedInputFormats returns MIME types this processor can read.
@@ -315,6 +317,7 @@ func (p *ImageProcessor) SupportedInputFormats() []string {
 		mimeGIF,
 		mimeWebP,
 		mimeAVIF,
+		mimeJXL,
 	}
 }
 
@@ -326,6 +329,7 @@ func (p *ImageProcessor) SupportedOutputFormats() []Format {
 		FormatGIF,
 		FormatWebP,
 		FormatAVIF,
+		FormatJXL,
 	}
 }
 
@@ -342,6 +346,8 @@ func FormatToMIME(format Format) string {
 		return mimeGIF
 	case FormatAVIF:
 		return mimeAVIF
+	case FormatJXL:
+		return mimeJXL
 	case FormatOriginal:
 		return "application/octet-stream"
 	default:
@@ -411,9 +417,11 @@ func (p *ImageProcessor) detectFormat(img *vips.ImageRef) string {
 		return "webp"
 	case vips.ImageTypeAVIF, vips.ImageTypeHEIF:
 		return string(FormatAVIF)
+	case vips.ImageTypeJXL:
+		return string(FormatJXL)
 	case vips.ImageTypeUnknown, vips.ImageTypeMagick, vips.ImageTypePDF,
 		vips.ImageTypeSVG, vips.ImageTypeTIFF, vips.ImageTypeBMP,
-		vips.ImageTypeJP2K, vips.ImageTypeJXL:
+		vips.ImageTypeJP2K:
 		return "unknown"
 	default:
 		return "unknown"
@@ -485,6 +493,17 @@ func (p *ImageProcessor) encode(
 		quality = defaultQuality
 	}
 
+	// Stripping drops the ICC profile as well, and clients show an image
+	// with no profile as sRGB, so convert to sRGB first. "srgb" names
+	// libvips' built-in profile; govips' own sRGB path variable is set on
+	// first use but read without a lock, so concurrent requests race on it.
+	if img.HasICCProfile() {
+		err := img.TransformICCProfileWithFallback("srgb", "srgb")
+		if err != nil {
+			return nil, fmt.Errorf("failed to convert to sRGB: %w", err)
+		}
+	}
+
 	var params vips.ExportParams
 
 	switch format {
@@ -516,6 +535,9 @@ func (p *ImageProcessor) encode(
 			Quality: quality,
 		}
 
+	case FormatJXL:
+		return exportJXL(img, quality)
+
 	case FormatOriginal:
 		return nil, fmt.Errorf("%w: %s", ErrUnsupportedOutputFormat, format)
 
@@ -523,17 +545,6 @@ func (p *ImageProcessor) encode(
 		return nil, fmt.Errorf("%w: %s", ErrUnsupportedOutputFormat, format)
 	}
 
-	// Stripping drops the ICC profile as well, and clients show an image
-	// with no profile as sRGB, so convert to sRGB first. "srgb" names
-	// libvips' built-in profile; govips' own sRGB path variable is set on
-	// first use but read without a lock, so concurrent requests race on it.
-	if img.HasICCProfile() {
-		err := img.TransformICCProfileWithFallback("srgb", "srgb")
-		if err != nil {
-			return nil, fmt.Errorf("failed to convert to sRGB: %w", err)
-		}
-	}
-
 	// Drop EXIF, XMP, IPTC and the ICC profile. govips ignores this for
 	// GIF, which carries none of them.
 	params.StripMetadata = true
@@ -546,6 +557,64 @@ func (p *ImageProcessor) encode(
 	return output, nil
 }
 
+// exportJXL encodes img as JPEG XL at quality, with libvips' default effort
+// and without metadata. govips sends libvips a distance, the JPEG XL
+// encoder's own measure of quality, along with the quality, and libvips then
+// uses the distance alone, so the quality is also given as a distance.
+func exportJXL(img *vips.ImageRef, quality int) ([]byte, error) {
+	// libvips converts a CMYK image to sRGB before it saves WebP, AVIF or
+	// PNG, but cannot save one as JPEG XL. encode has already converted
+	// any image with an ICC profile to sRGB, so this is CMYK with none.
+	if img.Interpretation() == vips.InterpretationCMYK {
+		err := img.ToColorSpace(vips.InterpretationSRGB)
+		if err != nil {
+			return nil, fmt.Errorf("failed to convert CMYK to sRGB: %w", err)
+		}
+	}
+
+	// govips cannot make libvips strip metadata from JPEG XL, so it is
+	// removed from the image itself. RemoveMetadata removes EXIF, XMP and
+	// IPTC but keeps the ICC profile. libvips 8.16 and later still write an
+	// EXIF block of their own: orientation, resolution, size, colour space
+	// and fixed defaults.
+	err := img.RemoveMetadata()
+	if err != nil {
+		return nil, err
+	}
+
+	err = img.RemoveICCProfile()
+	if err != nil {
+		return nil, err
+	}
+
+	params := vips.NewJxlExportParams()
+	params.Quality = quality
+	params.Distance = jxlDistance(quality)
+
+	output, _, err := img.ExportJxl(params)
+	if err != nil {
+		return nil, err
+	}
+
+	return output, nil
+}
+
+// jxlDistance turns a quality from 1 to 100 into the JPEG XL encoder's
+// distance, with the formula libvips and libjxl use for their own quality
+// setting, except that 100 stays lossy (distance 0.1) where libjxl makes it
+// lossless.
+//
+//nolint:mnd // the constants of that formula
+func jxlDistance(quality int) float64 {
+	q := float64(quality)
+
+	if quality >= 30 {
+		return 0.1 + (100-q)*0.09
+	}
+
+	return 53.0/3000.0*q*q - 23.0/20.0*q + 25.0
+}
+
 // formatFromString converts a format string to Format.
 func (p *ImageProcessor) formatFromString(format string) Format {
 	switch format {
@@ -559,6 +628,8 @@ func (p *ImageProcessor) formatFromString(format string) Format {
 		return FormatWebP
 	case string(FormatAVIF):
 		return FormatAVIF
+	case string(FormatJXL):
+		return FormatJXL
 	default:
 		return FormatJPEG
 	}