A simple way of grayscaling an image using standard libraries in golang

grayscale.go

package main

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

func main() {

	// open and load the image file
	imgfile, err := os.Open("image.png")
	if err != nil {
		fmt.Println(err)
	}
	defer imgfile.Close()

	// decode the image file to image.Image type
	img, _, err := image.Decode(imgfile)
	if err != nil {
		fmt.Println(err)
	}

	// target to store the grayscaled image, having the same dimensions as the image file
	target := image.NewRGBA64(img.Bounds())

	// change the colour of each and every pixels of the image file and set to the target
	i := 0
	for i < img.Bounds().Max.Y {
		j := 0
		for j < img.Bounds().Max.X {
			r, g, b, a := img.At(j, i).RGBA()

			// set r,g & b values of the pixel to average of r,g,b to make it grayscale
			average := (r + g + b) / 3
			target.Set(j, i, color.NRGBA64{R: uint16(average), G: uint16(average), B: uint16(average), A: uint16(a)})
			j++
		}
		i++
	}

	// create a new file for the grayscale image
	res, err := os.Create("result.png")
	if err != nil {
		fmt.Println(err)
	}

	// encode the grayscaled image.Image to result file
	if err := png.Encode(res, target); err != nil {
		fmt.Println(err)
	}

}