images: Fix WebP quality and hint parameters being ignored

The WASM-based WebP encoder introduced in v0.153.0 ignored per-image
quality and hint parameters, always using the global site configuration.

Root cause:
- codec.go only passed quality when conf.Quality > 0, missing lossless (q0)
- No tracking for whether hint was explicitly set per-image
- webp.go EncodeOptions only handled quality override, not hint
- Validation rejected q0 (valid for lossless), and default override
  defeated explicitly-set q0

Fix:
- Add hintSetForImage field to ImageConfig (mirroring qualitySetForImage)
- Pass quality/hint to encoder only when explicitly set per-image
- Handle both quality and hint overrides in webp.go EncodeOptions
- Allow q0 in validation (change "< 1" to "< 0")
- Only override with default when quality wasn't explicitly set

Test strategy:
- Golden tests validate quality by file size variance:
  q0=60736B (lossless), q1=752B, q33=1688B, q75=3188B, q100=17620B
- Golden tests validate hint by distinct MD5 hashes:
  photo=76ebc120..., drawing=bfaaee0d..., icon=55ecc0cd...
- Combined test (q50 drawing) validates both parameters together

AI assistance disclosure: This code was written by Antigravity with
Gemini 3 Pro (High), revised by Claude Code with Opus 4.5 (Thinking),
and reviewed by Antigravity with Gemini 3 Pro (High).

Author: simonheimlicher

internal/warpc/webp.go

@@ -202,6 +202,10 @@ func (d *WebpCodec) DecodeConfig(r io.Reader) (image.Config, error) {
 }
 
 func (d *WebpCodec) Encode(w io.Writer, img image.Image) error {
+	return d.EncodeOptions(w, img, nil)
+}
+
+func (d *WebpCodec) EncodeOptions(w io.Writer, img image.Image, options map[string]any) error {
 	b := img.Bounds()
 	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
 		return errors.New("webp: image is too large to encode")
@@ -299,6 +303,20 @@ func (d *WebpCodec) Encode(w io.Writer, img image.Image) error {
 	// encodeGray
 	// decode
 	// config
+
+	opts := map[string]any{
+		"quality":     d.quality, // a number between 0 and 100. Set to 0 for lossless.
+		"hint":        d.hint,    // drawing, icon, photo, picture, or text
+		"useSharpYuv": true,      // Use sharp (and slow) RGB->YUV conversion.
+	}
+
+	// Override with per-image options if provided.
+	for _, key := range []string{"quality", "hint"} {
+		if v, ok := options[key]; ok {
+			opts[key] = v
+		}
+	}
+
 	message := Message[WebpInput]{
 		Header: Header{
 			Version:       1,
@@ -310,11 +328,7 @@ func (d *WebpCodec) Encode(w io.Writer, img image.Image) error {
 		Data: WebpInput{
 			Source:      bytes.NewReader(imageBytes),
 			Destination: w,
-			Options: map[string]any{
-				"quality":     d.quality, // a number between 0 and 100. Set to 0 for lossless.
-				"hint":        d.hint,    // drawing, icon, photo, picture, or text
-				"useSharpYuv": true,      // Use sharp (and slow) RGB->YUV conversion.
-			},
+			Options:     opts,
 			Params: map[string]any{
 				"width":          bounds.Max.X,
 				"height":         bounds.Max.Y,

resources/images/codec.go

@@ -46,6 +46,14 @@ type ToEncoder interface {
 	EncodeTo(conf ImageConfig, w io.Writer, src image.Image) error
 }
 
+// EncoderWithOptions defines the encoding of an image format with options.
+// This is currently only used for WebP and the options are passed as a map
+// to match the internal WASM API. The options map may be nil when no options
+// need to be overridden.
+type EncoderWithOptions interface {
+	EncodeOptions(w io.Writer, src image.Image, options map[string]any) error
+}
+
 // CodecStdlib defines both decoding and encoding of an image format as defined by the standard library.
 type CodecStdlib interface {
 	Decoder
@@ -123,6 +131,19 @@ func (d *Codec) EncodeTo(conf ImageConfig, w io.Writer, img image.Image) error {
 	case BMP:
 		return bmp.Encode(w, img)
 	case WEBP:
+		if enc, ok := d.webp.(EncoderWithOptions); ok {
+			var opts map[string]any
+			if conf.qualitySetForImage || conf.hintSetForImage {
+				opts = make(map[string]any)
+				if conf.qualitySetForImage {
+					opts["quality"] = conf.Quality
+				}
+				if conf.hintSetForImage {
+					opts["hint"] = conf.Hint
+				}
+			}
+			return enc.EncodeOptions(w, img, opts)
+		}
 		return d.webp.Encode(w, img)
 	default:
 		return errors.New("format not supported")

resources/images/config.go

@@ -226,6 +226,7 @@ func DecodeImageConfig(options []string, defaults *config.ConfigNamespace[Imagin
 			c.Filter = filter
 		} else if _, ok := hints[part]; ok {
 			c.Hint = part
+			c.hintSetForImage = true
 		} else if part[0] == '#' {
 			c.BgColor, err = hexStringToColorGo(part[1:])
 			if err != nil {
@@ -236,8 +237,8 @@ func DecodeImageConfig(options []string, defaults *config.ConfigNamespace[Imagin
 			if err != nil {
 				return c, err
 			}
-			if c.Quality < 1 || c.Quality > 100 {
-				return c, errors.New("quality ranges from 1 to 100 inclusive")
+			if c.Quality < 0 || c.Quality > 100 {
+				return c, errors.New("quality ranges from 0 to 100 inclusive")
 			}
 			c.qualitySetForImage = true
 		} else if part[0] == 'r' {
@@ -305,8 +306,9 @@ func DecodeImageConfig(options []string, defaults *config.ConfigNamespace[Imagin
 		c.TargetFormat = sourceFormat
 	}
 
-	if c.Quality <= 0 && c.TargetFormat.RequiresDefaultQuality() {
-		// We need a quality setting for all JPEGs and WEBPs.
+	if !c.qualitySetForImage && c.Quality <= 0 && c.TargetFormat.RequiresDefaultQuality() {
+		// We need a quality setting for all JPEGs and WEBPs,
+		// unless the user explicitly set quality (e.g., q0 for lossless WebP).
 		c.Quality = defaults.Config.Imaging.Quality
 	}
 
@@ -358,7 +360,8 @@ type ImageConfig struct {
 
 	// Hint about what type of picture this is. Used to optimize encoding
 	// when target is set to webp.
-	Hint string
+	Hint            string
+	hintSetForImage bool // Whether the above is set for this image.
 
 	Width  int
 	Height int

resources/images/images_golden_integration_test.go

@@ -388,6 +388,14 @@ Home.
 {{ template "process" (dict "spec" "png" "img" $highContrast) }}
 {{ template "process" (dict "spec" "resize 300x300" "img" $giphy) }}
 {{ template "process" (dict "spec" "resize 300x300 webp" "img" $giphy) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q0" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q1" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q33" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q75" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q100" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp drawing" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp icon" "img" $sunset) }}
+{{ template "process" (dict "spec" "resize 300x300 webp q50 drawing" "img" $sunset) }}
 {{ template "process" (dict "spec" "resize 400x" "img" $highContrast) }}
 
 {{ define "process"}}