diff --git a/internal/comic-converter/core.go b/internal/comic-converter/core.go
index 7ff6392..71f674a 100644
--- a/internal/comic-converter/core.go
+++ b/internal/comic-converter/core.go
@@ -2,26 +2,14 @@ package comicconverter
 
 import (
 	"image"
+	"image/color"
 	"image/jpeg"
 	"os"
 
 	"golang.org/x/image/draw"
 )
 
-type ComicConverter struct {
-	Options ComicConverterOptions
-	img     *image.Gray
-}
-
-type ComicConverterOptions struct {
-	Quality int
-}
-
-func New(opt ComicConverterOptions) *ComicConverter {
-	return &ComicConverter{Options: opt}
-}
-
-func (c *ComicConverter) Load(file string) *ComicConverter {
+func Load(file string) *image.Gray {
 	f, err := os.Open(file)
 	if err != nil {
 		panic(err)
@@ -35,32 +23,26 @@ func (c *ComicConverter) Load(file string) *ComicConverter {
 
 	switch imgt := img.(type) {
 	case *image.Gray:
-		c.img = imgt
+		return imgt
 	default:
 		newImg := image.NewGray(img.Bounds())
 		draw.Draw(newImg, newImg.Bounds(), img, image.Point{}, draw.Src)
-		c.img = newImg
+		return newImg
 	}
-
-	return c
 }
 
-func (c *ComicConverter) isBlank(x, y int) bool {
-	r, g, b, _ := c.img.At(x, y).RGBA()
+func isBlank(c color.Color) bool {
+	r, g, b, _ := c.RGBA()
 	return r > 60000 && g > 60000 && b > 60000
 }
 
-func (c *ComicConverter) CropMarging() *ComicConverter {
-	if c.img == nil {
-		panic("load image first")
-	}
-
-	imgArea := c.img.Bounds()
+func CropMarging(img *image.Gray) *image.Gray {
+	imgArea := img.Bounds()
 
 LEFT:
 	for x := imgArea.Min.X; x < imgArea.Max.X; x++ {
 		for y := imgArea.Min.Y; y < imgArea.Max.Y; y++ {
-			if !c.isBlank(x, y) {
+			if !isBlank(img.At(x, y)) {
 				break LEFT
 			}
 		}
@@ -70,7 +52,7 @@ LEFT:
 UP:
 	for y := imgArea.Min.Y; y < imgArea.Max.Y; y++ {
 		for x := imgArea.Min.X; x < imgArea.Max.X; x++ {
-			if !c.isBlank(x, y) {
+			if !isBlank(img.At(x, y)) {
 				break UP
 			}
 		}
@@ -80,7 +62,7 @@ UP:
 RIGHT:
 	for x := imgArea.Max.X - 1; x >= imgArea.Min.X; x-- {
 		for y := imgArea.Min.Y; y < imgArea.Max.Y; y++ {
-			if !c.isBlank(x, y) {
+			if !isBlank(img.At(x, y)) {
 				break RIGHT
 			}
 		}
@@ -90,40 +72,32 @@ RIGHT:
 BOTTOM:
 	for y := imgArea.Max.Y - 1; y >= imgArea.Min.Y; y-- {
 		for x := imgArea.Min.X; x < imgArea.Max.X; x++ {
-			if !c.isBlank(x, y) {
+			if !isBlank(img.At(x, y)) {
 				break BOTTOM
 			}
 		}
 		imgArea.Max.Y--
 	}
 
-	newImg := image.NewGray(image.Rectangle{
-		Min: image.Point{0, 0},
-		Max: image.Point{imgArea.Dx(), imgArea.Dy()},
-	})
-
-	draw.Draw(newImg, newImg.Bounds(), c.img, imgArea.Min, draw.Src)
-
-	c.img = c.img.SubImage(imgArea).(*image.Gray)
-
-	return c
+	return img.SubImage(imgArea).(*image.Gray)
 }
 
-func (c *ComicConverter) Resize(w, h int) *ComicConverter {
-	if c.img == nil {
-		panic("load image first")
-	}
-
-	dim := c.img.Bounds()
+func Resize(img *image.Gray, w, h int) *image.Gray {
+	dim := img.Bounds()
 	origWidth := dim.Dx()
 	origHeight := dim.Dy()
 
-	width, height := 1, 1
-
-	if origHeight > 0 && origWidth > 0 {
-		width, height = origWidth*h/origHeight, origHeight*w/origWidth
+	if origWidth == 0 || origHeight == 0 {
+		newImg := image.NewGray(image.Rectangle{
+			image.Point{0, 0},
+			image.Point{w, h},
+		})
+		draw.Draw(newImg, newImg.Bounds(), image.NewUniform(color.White), newImg.Bounds().Min, draw.Src)
+		return newImg
 	}
 
+	width, height := origWidth*h/origHeight, origHeight*w/origWidth
+
 	if width > w {
 		width = w
 	}
@@ -136,32 +110,24 @@ func (c *ComicConverter) Resize(w, h int) *ComicConverter {
 		Max: image.Point{width, height},
 	})
 
-	draw.BiLinear.Scale(newImg, newImg.Bounds(), c.img, c.img.Bounds(), draw.Src, nil)
+	draw.BiLinear.Scale(newImg, newImg.Bounds(), img, img.Bounds(), draw.Src, nil)
 
-	c.img = newImg
-
-	return c
+	return newImg
 }
 
-func (c *ComicConverter) Save(output string) *ComicConverter {
-	if c.img == nil {
-		panic("load image first")
-	}
+func Save(img *image.Gray, output string, quality int) {
 	o, err := os.Create(output)
 	if err != nil {
 		panic(err)
 	}
 	defer o.Close()
 
-	quality := 75
-	if c.Options.Quality > 0 {
-		quality = c.Options.Quality
+	if quality == 0 {
+		quality = 75
 	}
 
-	err = jpeg.Encode(o, c.img, &jpeg.Options{Quality: quality})
+	err = jpeg.Encode(o, img, &jpeg.Options{Quality: quality})
 	if err != nil {
 		panic(err)
 	}
-
-	return c
 }
diff --git a/main.go b/main.go
index 85cd7b0..148933e 100644
--- a/main.go
+++ b/main.go
@@ -3,20 +3,56 @@ package main
 import (
 	"fmt"
 	comicconverter "go-comic-converter/internal/comic-converter"
+	"io/fs"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
 )
 
-func main() {
-	cv := comicconverter.
-		New(comicconverter.ComicConverterOptions{
-			Quality: 75,
-		})
+type Todo struct {
+	Input  string
+	Output string
+}
 
-	for i := 1; i < 10; i++ {
-		cv.
-			Load(fmt.Sprintf("/Users/vincent/Downloads/0000%d.jpg", i)).
-			CropMarging().
-			Resize(1860, 2480).
-			Save(fmt.Sprintf("/Users/vincent/Downloads/0000%d_gray.jpg", i))
+func main() {
+	wg := &sync.WaitGroup{}
+	todos := make(chan Todo, runtime.NumCPU())
+
+	wg.Add(runtime.NumCPU())
+	for i := 0; i < runtime.NumCPU(); i++ {
+		go func() {
+			defer wg.Done()
+			for todo := range todos {
+				fmt.Printf("Processing %s\n", todo.Input)
+				comicconverter.Save(
+					comicconverter.Resize(
+						comicconverter.CropMarging(
+							comicconverter.Load(todo.Input),
+						), 1860, 2480), todo.Output, 75,
+				)
+			}
+		}()
 	}
 
+	dirname := "/Users/vincent/Downloads/Bleach T01 (Tite KUBO) [eBook officiel 1920]"
+	filepath.WalkDir(dirname, func(path string, d fs.DirEntry, err error) error {
+		if d.IsDir() {
+			return nil
+		}
+		input := path
+		ext := filepath.Ext(path)
+		if strings.ToLower(ext) != ".jpg" {
+			return nil
+		}
+		output := fmt.Sprintf("%s_gray%s", input[0:len(input)-len(ext)], ext)
+
+		todos <- Todo{input, output}
+
+		return nil
+	})
+
+	close(todos)
+
+	wg.Wait()
 }