convert normal map to blender's format (swap Red-Green channels)

rgswap.go

package main

import (
	"flag"
	"fmt"
	"image"
	"image/color"
	"image/png"
	"os"
)

func main() {

	var fname string

	flag.StringVar(&fname, "f", "", "A .png image (normal map)")
	flag.Parse()

	file, err := os.Open(fname) // opening the file
	if err != nil {
		fmt.Println(err)
		return
	}
	defer file.Close()

	pic, err := png.Decode(file)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%s: %v\n", fname, err)
		return
	}

	rgba, ok := pic.(*image.RGBA)
	if !ok {
		fmt.Println("Only RGBA .png files are supported.")
		return
	}

	b := pic.Bounds()
	fmt.Println("Swapping channels...")
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			col := rgba.At(x, y).(color.RGBA)
			col.G, col.R = col.R, col.G
			rgba.Set(x, y, col)
		}
	}
	fmt.Println("Writing...")
	out_file, err := os.Create(fname)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer out_file.Close()

	err = png.Encode(out_file, pic)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println("Done!")

}