create auto contrast filter

This filter improve contrast
and make the comics more
readable

internal/converter/converter.go

@@ -116,6 +116,7 @@ func (c *Converter) InitParse() {
 	c.AddIntParam(&c.Options.CropRatioBottom, "crop-ratio-bottom", c.Options.CropRatioBottom, "Crop ratio bottom: ratio of pixels allow to be non blank while cutting on the bottom.")
 	c.AddIntParam(&c.Options.Brightness, "brightness", c.Options.Brightness, "Brightness readjustement: between -100 and 100, > 0 lighter, < 0 darker")
 	c.AddIntParam(&c.Options.Contrast, "contrast", c.Options.Contrast, "Contrast readjustement: between -100 and 100, > 0 more contrast, < 0 less contrast")
+	c.AddBoolParam(&c.Options.AutoContrast, "autocontrast", c.Options.AutoContrast, "Improve contrast automatically")
 	c.AddBoolParam(&c.Options.AutoRotate, "autorotate", c.Options.AutoRotate, "Auto Rotate page when width > height")
 	c.AddBoolParam(&c.Options.AutoSplitDoublePage, "autosplitdoublepage", c.Options.AutoSplitDoublePage, "Auto Split double page when width > height")
 	c.AddBoolParam(&c.Options.NoBlankImage, "noblankimage", c.Options.NoBlankImage, "Remove blank image")
@@ -227,6 +228,7 @@ func (c *Converter) Parse() {
 	}
 
 	if c.Options.Auto {
+		c.Options.AutoContrast = true
 		c.Options.AutoRotate = true
 		c.Options.AutoSplitDoublePage = true
 	}
@@ -256,6 +258,7 @@ func (c *Converter) Parse() {
 		c.Options.Crop = false
 		c.Options.Brightness = 0
 		c.Options.Contrast = 0
+		c.Options.AutoContrast = false
 		c.Options.AutoRotate = false
 		c.Options.NoBlankImage = false
 		c.Options.NoResize = true

internal/converter/options/converter_options.go

@@ -32,6 +32,7 @@ type Options struct {
 	CropRatioBottom            int     `yaml:"crop_ratio_bottom"`
 	Brightness                 int     `yaml:"brightness"`
 	Contrast                   int     `yaml:"contrast"`
+	AutoContrast               bool    `yaml:"auto_contrast"`
 	AutoRotate                 bool    `yaml:"auto_rotate"`
 	AutoSplitDoublePage        bool    `yaml:"auto_split_double_page"`
 	NoBlankImage               bool    `yaml:"no_blank_image"`
@@ -203,6 +204,7 @@ func (o *Options) ShowConfig() string {
 		{"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},
 		{"Contrast", o.Contrast, o.Contrast != 0},
+		{"AutoContrast", o.AutoContrast, true},
 		{"AutoRotate", o.AutoRotate, true},
 		{"AutoSplitDoublePage", o.AutoSplitDoublePage, true},
 		{"NoBlankImage", o.NoBlankImage, true},

internal/epub/imagefilters/epub_image_filters_autocontrast.go

@@ -0,0 +1,90 @@
+package epubimagefilters
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+// Automatically improve contrast
+func AutoContrast() *autocontrast {
+	return &autocontrast{}
+}
+
+type autocontrast struct {
+}
+
+// compute the color number between 0 and 1 that hold half of the pixel
+func (f *autocontrast) mean(src image.Image) float32 {
+	bucket := map[uint32]int{}
+	for x := src.Bounds().Min.X; x < src.Bounds().Max.X; x++ {
+		for y := src.Bounds().Min.Y; y < src.Bounds().Max.Y; y++ {
+			v, _, _, _ := color.GrayModel.Convert(src.At(x, y)).RGBA()
+			bucket[v]++
+		}
+	}
+
+	// calculate color idx
+	var colorIdx uint32
+	{
+		// limit to half of the pixel
+		limit := src.Bounds().Dx() * src.Bounds().Dy() / 2
+		// loop on all color from 0 to 65536
+		for colorIdx = 0; colorIdx < 1<<16; colorIdx++ {
+			if limit-bucket[colorIdx] < 0 {
+				break
+			}
+			limit -= bucket[colorIdx]
+		}
+	}
+	// return the color idx between 0 and 1
+	return float32(colorIdx) / (1 << 16)
+}
+
+// ensure value stay into 0 to 1 bound
+func (f *autocontrast) cap(v float32) float32 {
+	if v < 0 {
+		return 0
+	}
+	if v > 1 {
+		return 1
+	}
+	return v
+}
+
+// power of 2 for float32
+func (f *autocontrast) pow2(v float32) float32 {
+	return v * v
+}
+
+// Draw into the dst after applying the filter
+func (f *autocontrast) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	// half of the pixel has this color idx
+	colorMean := f.mean(src)
+
+	// if colorMean > 0.5, it means the color is mostly clear
+	// in that case we will add a lot more darkness other light
+	// compute dark factor
+	d := f.pow2(colorMean)
+	// compute light factor
+	l := f.pow2(1 - colorMean)
+
+	gift.ColorFunc(func(r0, g0, b0, a0 float32) (r float32, g float32, b float32, a float32) {
+		// convert to gray color the source RGB
+		y := 0.299*r0 + 0.587*g0 + 0.114*b0
+
+		// compute a curve from dark and light factor applying to the color
+		c := (1 - d) + (d+l)*y
+
+		// applying the coef
+		return f.cap(r0 * c), f.cap(g0 * c), f.cap(b0 * c), a0
+	}).Draw(dst, src, options)
+}
+
+// Bounds calculates the appropriate bounds of an image after applying the filter.
+func (*autocontrast) Bounds(srcBounds image.Rectangle) (dstBounds image.Rectangle) {
+	dstBounds = srcBounds
+	return
+}

internal/epub/imageprocessor/epub_image_processor.go

@@ -197,6 +197,12 @@ func (e *EPUBImageProcessor) transformImage(src image.Image, srcId int) []image.
 		filters = append(filters, gift.Rotate90())
 	}
 
+	if e.Image.AutoContrast {
+		f := epubimagefilters.AutoContrast()
+		filters = append(filters, f)
+		splitFilters = append(splitFilters, f)
+	}
+
 	if e.Image.Contrast != 0 {
 		f := gift.Contrast(float32(e.Image.Contrast))
 		filters = append(filters, f)

internal/epub/options/epub_options.go

@@ -26,6 +26,7 @@ type Image struct {
 	Quality             int
 	Brightness          int
 	Contrast            int
+	AutoContrast        bool
 	AutoRotate          bool
 	AutoSplitDoublePage bool
 	NoBlankImage        bool

main.go

@@ -114,33 +114,19 @@ $ go install github.com/celogeek/go-comic-converter/v%d@%s
 		DryVerbose:                 cmd.Options.DryVerbose,
 		Quiet:                      cmd.Options.Quiet,
 		Image: &epuboptions.Image{
+			Crop:                &epuboptions.Crop{Enabled: cmd.Options.Crop, Left: cmd.Options.CropRatioLeft, Up: cmd.Options.CropRatioUp, Right: cmd.Options.CropRatioRight, Bottom: cmd.Options.CropRatioBottom},
 			Quality:             cmd.Options.Quality,
-			GrayScale:     cmd.Options.Grayscale,
-			GrayScaleMode: cmd.Options.GrayscaleMode,
-			Crop: &epuboptions.Crop{
-				Enabled: cmd.Options.Crop,
-				Left:    cmd.Options.CropRatioLeft,
-				Up:      cmd.Options.CropRatioUp,
-				Right:   cmd.Options.CropRatioRight,
-				Bottom:  cmd.Options.CropRatioBottom,
-			},
 			Brightness:          cmd.Options.Brightness,
 			Contrast:            cmd.Options.Contrast,
+			AutoContrast:        cmd.Options.AutoContrast,
 			AutoRotate:          cmd.Options.AutoRotate,
 			AutoSplitDoublePage: cmd.Options.AutoSplitDoublePage,
 			NoBlankImage:        cmd.Options.NoBlankImage,
 			Manga:               cmd.Options.Manga,
 			HasCover:            cmd.Options.HasCover,
-			View: &epuboptions.View{
-				Width:        profile.Width,
-				Height:       profile.Height,
-				AspectRatio:  cmd.Options.AspectRatio,
-				PortraitOnly: cmd.Options.PortraitOnly,
-				Color: epuboptions.Color{
-					Foreground: cmd.Options.ForegroundColor,
-					Background: cmd.Options.BackgroundColor,
-				},
-			},
+			View:                &epuboptions.View{Width: profile.Width, Height: profile.Height, AspectRatio: cmd.Options.AspectRatio, PortraitOnly: cmd.Options.PortraitOnly, Color: epuboptions.Color{Foreground: cmd.Options.ForegroundColor, Background: cmd.Options.BackgroundColor}},
+			GrayScale:           cmd.Options.Grayscale,
+			GrayScaleMode:       cmd.Options.GrayscaleMode,
 			Resize:              !cmd.Options.NoResize,
 			Format:              cmd.Options.Format,
 		},