add grayscale mode

it will apply different formula to convert image into grayscale.

it works only if the source is not already in grayscale.

0 = normal
1 = average
2 = luminance

internal/converter/converter.go

@@ -108,6 +108,7 @@ func (c *Converter) InitParse() {
 	c.AddStringParam(&c.Options.Profile, "profile", c.Options.Profile, fmt.Sprintf("Profile to use: \n%s", c.Options.AvailableProfiles()))
 	c.AddIntParam(&c.Options.Quality, "quality", c.Options.Quality, "Quality of the image")
 	c.AddBoolParam(&c.Options.Grayscale, "grayscale", c.Options.Grayscale, "Grayscale image. Ideal for eInk devices.")
+	c.AddIntParam(&c.Options.GrayscaleMode, "grayscale-mode", c.Options.GrayscaleMode, "Grayscale Mode\n0 = normal\n1 = average\n2 = luminance")
 	c.AddBoolParam(&c.Options.Crop, "crop", c.Options.Crop, "Crop images")
 	c.AddIntParam(&c.Options.CropRatioLeft, "crop-ratio-left", c.Options.CropRatioLeft, "Crop ratio left: ratio of pixels allow to be non blank while cutting on the left.")
 	c.AddIntParam(&c.Options.CropRatioUp, "crop-ratio-up", c.Options.CropRatioUp, "Crop ratio up: ratio of pixels allow to be non blank while cutting on the top.")
@@ -370,6 +371,11 @@ func (c *Converter) Validate() error {
 		return errors.New("title page should be 0, 1 or 2")
 	}
 
+	// Grayscale Mode
+	if c.Options.GrayscaleMode < 0 || c.Options.GrayscaleMode > 2 {
+		return errors.New("grayscale mode should be 0, 1 or 2")
+	}
+
 	return nil
 }
 

internal/converter/options/converter_options.go

@@ -24,6 +24,7 @@ type Options struct {
 	Profile                    string  `yaml:"profile"`
 	Quality                    int     `yaml:"quality"`
 	Grayscale                  bool    `yaml:"grayscale"`
+	GrayscaleMode              int     `yaml:"grayscale_mode"` // 0 = normal, 1 = average, 2 = luminance
 	Crop                       bool    `yaml:"crop"`
 	CropRatioLeft              int     `yaml:"crop_ratio_left"`
 	CropRatioUp                int     `yaml:"crop_ratio_up"`
@@ -179,6 +180,14 @@ func (o *Options) ShowConfig() string {
 		titlePage = "when epub is splitted"
 	}
 
+	grayscaleMode := "normal"
+	switch o.GrayscaleMode {
+	case 1:
+		grayscaleMode = "average"
+	case 2:
+		grayscaleMode = "luminance"
+	}
+
 	var b strings.Builder
 	for _, v := range []struct {
 		Key       string
@@ -189,6 +198,7 @@ func (o *Options) ShowConfig() string {
 		{"Format", o.Format, true},
 		{"Quality", o.Quality, o.Format == "jpeg"},
 		{"Grayscale", o.Grayscale, true},
+		{"Grayscale Mode", grayscaleMode, o.Grayscale},
 		{"Crop", o.Crop, true},
 		{"CropRatio", fmt.Sprintf("%d Left - %d Up - %d Right - %d Bottom", o.CropRatioLeft, o.CropRatioUp, o.CropRatioRight, o.CropRatioBottom), o.Crop},
 		{"Brightness", o.Brightness, o.Brightness != 0},

internal/epub/imageprocessor/epub_image_processor.go

@@ -215,8 +215,23 @@ func (e *EPUBImageProcessor) transformImage(src image.Image, srcId int) []image.
 	}
 
 	if e.Image.GrayScale {
-		filters = append(filters, gift.Grayscale())
+		var f gift.Filter
-		splitFilters = append(splitFilters, gift.Grayscale())
+		switch e.Image.GrayScaleMode {
+		case 1: // average
+			f = gift.ColorFunc(func(r0, g0, b0, a0 float32) (r float32, g float32, b float32, a float32) {
+				y := (r0 + g0 + b0) / 3
+				return y, y, y, a0
+			})
+		case 2: // luminance
+			f = gift.ColorFunc(func(r0, g0, b0, a0 float32) (r float32, g float32, b float32, a float32) {
+				y := 0.2126*r0 + 0.7152*g0 + 0.0722*b0
+				return y, y, y, a0
+			})
+		default:
+			f = gift.Grayscale()
+		}
+		filters = append(filters, f)
+		splitFilters = append(splitFilters, f)
 	}
 
 	filters = append(filters, epubimagefilters.Pixel())

internal/epub/options/epub_options.go

@@ -33,6 +33,7 @@ type Image struct {
 	HasCover            bool
 	View                *View
 	GrayScale           bool
+	GrayScaleMode       int
 	Resize              bool
 	Format              string
 }

main.go

@@ -116,6 +116,7 @@ $ go install github.com/celogeek/go-comic-converter/v%d@%s
 		Image: &epuboptions.Image{
 			Quality:       cmd.Options.Quality,
 			GrayScale:     cmd.Options.Grayscale,
+			GrayScaleMode: cmd.Options.GrayscaleMode,
 			Crop: &epuboptions.Crop{
 				Enabled: cmd.Options.Crop,
 				Left:    cmd.Options.CropRatioLeft,