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How to convert Colorful PNG image to Gray-scale?

The Decode function decodes an image that has been encoded in a registered format. RGBA is an in-memory image whose At method returns color.RGBA values.
package main

import (
	"image"
	"image/color"
	"image/png"
	"math"
	"log"
	"os"
)

func main() {
	filename := "pokeball.png"
	infile, err := os.Open(filename)
	
	if err != nil {	
		log.Printf("failed opening %s: %s", filename, err)
		panic(err.Error())
	}
	defer infile.Close()

	imgSrc, _, err := image.Decode(infile)
	if err != nil {
		panic(err.Error())
	}

	// Create a new grayscale image
	bounds := imgSrc.Bounds()
	w, h := bounds.Max.X, bounds.Max.Y
	grayScale := image.NewGray(image.Rectangle{image.Point{0, 0}, image.Point{w, h}})
	for x := 0; x < w; x++ {
		for y := 0; y < h; y++ {
			imageColor := imgSrc.At(x, y)              
			rr, gg, bb, _ := imageColor.RGBA()
			r := math.Pow(float64(rr), 2.2)
			g := math.Pow(float64(gg), 2.2)
			b := math.Pow(float64(bb), 2.2)
			m := math.Pow(0.2125*r+0.7154*g+0.0721*b, 1/2.2)
			Y := uint16(m + 0.5)
			grayColor := color.Gray{uint8(Y >> 8)}
			grayScale.Set(x, y, grayColor)
		}
	}

	// Encode the grayscale image to the new file
	newFileName := "grayscale.png"
	newfile, err := os.Create(newFileName)
	if err != nil {
		log.Printf("failed creating %s: %s", newfile, err)
		panic(err.Error())
	}
	defer newfile.Close()
	png.Encode(newfile,grayScale)
}
C:\golang\codes>go run example.go

C:\golang\codes>
Input :

Output: