Added tool to change alpha layer of image.

cmd/imadapt/imadapt.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"image"
+	"image/color"
+	"image/png"
+	"log"
+	"os"
+)
+
+func run() error {
+	flag.Parse()
+	args := flag.Args()
+	if len(args) < 1 {
+		return errors.New("no command specified")
+	}
+	switch args[0] {
+	case "setalpha":
+		flags := flag.NewFlagSet("setalpha", flag.ContinueOnError)
+		var alpha int
+		flags.IntVar(&alpha, "alpha", 127, "sets the target alpha")
+		flags.Parse(args[1:])
+		for _, path := range flags.Args() {
+			err := setAlpha(path, alpha)
+			if err != nil {
+				return fmt.Errorf("couldn't set alpha for '%s'; error: %v", path, err)
+			}
+		}
+	}
+	return nil
+}
+
+func setAlpha(path string, alpha int) error {
+	src, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer src.Close()
+	im, _, err := image.Decode(src)
+	if err != nil {
+		return err
+	}
+	bounds := im.Bounds()
+	dst := image.NewNRGBA(bounds)
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		for x := bounds.Min.X; x < bounds.Max.X; x++ {
+			c := color.NRGBAModel.Convert(im.At(x, y)).(color.NRGBA)
+			if c.A > 0 {
+				c.A = uint8(alpha)
+			}
+			dst.Set(x, y, c)
+		}
+	}
+	return encodePNG(path, dst)
+}
+
+func encodePNG(path string, im image.Image) error {
+	dst, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer dst.Close()
+	return png.Encode(dst, im)
+}
+
+func main() {
+	if err := run(); err != nil {
+		log.Fatal(err)
+	}
+}