fix(writer): transparent takes no effect on halftone image

example/with-halftone/README.md

@@ -4,16 +4,33 @@
 > the gap between qr blocks could mislead the recognizer.
 
 ```go
+
+var (
+	transparent = flag.Bool("transparent", false, "set background to transparent")
+)
+
 func main() {
 	qrc, err := qrcode.New("https://github.com/yeqown/go-qrcode")
 	if err != nil {
 		panic(err)
 	}
-
-	w0, err := standard.New("./repository_qrcode.png",
+	
+	options := []standard.ImageOption{
 		standard.WithHalftone("./test.jpeg"),
 		standard.WithQRWidth(21),
-	)
+	}
+	filename := "./halftone-qr.png"
+
+	if *transparent {
+		options = append(
+			options,
+			standard.WithBuiltinImageEncoder(standard.PNG_FORMAT),
+			standard.WithBgTransparent(),
+		)
+		filename = "./halftone-qr-transparent.png"
+	}
+
+	w0, err := standard.New(filename, options...)
 	handleErr(err)
 	err = qrc.Save(w0)
 	handleErr(err)
@@ -26,4 +43,5 @@ func main() {
 
 #### output image
 
-<img src="./halftone-qr.jpeg">
+<img src="./halftone-qr.png">
+<img src="./halftone-qr-transparent.png">

example/with-halftone/main.go

@@ -1,20 +1,40 @@
 package main
 
 import (
+	"flag"
+
 	"github.com/yeqown/go-qrcode/v2"
 	"github.com/yeqown/go-qrcode/writer/standard"
 )
 
+var (
+	transparent = flag.Bool("transparent", false, "set background to transparent")
+)
+
 func main() {
+	flag.Parse()
+
 	qrc, err := qrcode.New("https://github.com/yeqown/go-qrcode")
 	if err != nil {
 		panic(err)
 	}
 
-	w0, err := standard.New("./repository_qrcode.png",
+	options := []standard.ImageOption{
 		standard.WithHalftone("./test.jpeg"),
 		standard.WithQRWidth(21),
-	)
+	}
+	filename := "./halftone-qr.png"
+
+	if *transparent {
+		options = append(
+			options,
+			standard.WithBuiltinImageEncoder(standard.PNG_FORMAT),
+			standard.WithBgTransparent(),
+		)
+		filename = "./halftone-qr-transparent.png"
+	}
+
+	w0, err := standard.New(filename, options...)
 	handleErr(err)
 	err = qrc.Save(w0)
 	handleErr(err)

writer/standard/writer.go

@@ -139,7 +139,7 @@ func draw(mat qrcode.Matrix, opt *outputImageOptions) image.Image {
 			60,
 		)
 
-		//_ = imgkit.Save(halftoneImg, "mask.jpeg")
+		// _ = imgkit.Save(halftoneImg, "mask.jpeg")
 	}
 
 	// iterate the matrix to Draw each pixel
@@ -168,10 +168,10 @@ func draw(mat qrcode.Matrix, opt *outputImageOptions) image.Image {
 			for i := 0; i < 3; i++ {
 				for j := 0; j < 3; j++ {
 					ctx2.x, ctx2.y = ctx.x+float64(i)*halftoneW, ctx.y+float64(j)*halftoneW
-					ctx2.color = halftoneImg.At(x*3+i, y*3+j)
 					if i == 1 && j == 1 {
 						ctx2.color = ctx.color
-						// only center block keep the origin color.
+					} else {
+						ctx2.color = halftoneColor(halftoneImg, opt.bgTransparent, x*3+i, y*3+j)
 					}
 					shape.Draw(ctx2)
 				}
@@ -197,13 +197,34 @@ func draw(mat qrcode.Matrix, opt *outputImageOptions) image.Image {
 		}
 
 		// DONE(@yeqown): calculate the xOffset and yOffset which point(xOffset, yOffset)
-		//should icon upper-left to start
+		// should icon upper-left to start
 		dc.DrawImage(opt.logoImage(), (w-logoWidth)/2, (h-logoHeight)/2)
 	}
 done:
 	return dc.Image()
 }
 
+// halftoneImage is an image.Gray type image, which At(x, y) return color.Gray.
+// black equals to color.Gray{0}, white equals to color.Gray{255}.
+func halftoneColor(halftoneImage image.Image, transparent bool, x, y int) color.Color {
+
+	c0 := halftoneImage.At(x, y)
+	c1, ok := halftoneImage.At(x, y).(color.Gray)
+	if !ok {
+		log.Printf("halftoneColor: not a gray image, got: %T\n", c0)
+		return c0
+	}
+
+	if c1.Y == 255 {
+		if transparent {
+			return color.RGBA{}
+		}
+		return color.RGBA{R: 255, G: 255, B: 255, A: 255}
+	}
+
+	return color.RGBA{A: 255}
+}
+
 func validLogoImage(qrWidth, qrHeight, logoWidth, logoHeight, logoSizeMultiplier int) bool {
 	return qrWidth >= logoSizeMultiplier*logoWidth && qrHeight >= logoSizeMultiplier*logoHeight
 }