Program to convert RGBA to grayscale Golang

Question

I'm currently working on writing a program to convert an RGBA image to grayscale. I know there are much simpler ways of doing this however I'm trying to do it this way do to this method being more suitable for the work I'm doing in the future.

I'm trying to step through each pixel one by one using two for loops. Then convert the RGBA value of the pixel to gray and then save the new image.

Below is my code:

package main

import (
    "image/color"
    "image/jpeg"
    "log"
    "os"
)

type ImageSet interface {
    Set(x, y int, c color.Color)
}

func main() {
    file, err := os.Open("biden.jpg")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    img, err := jpeg.Decode(file)
    if err != nil {
        log.Fatal(os.Stderr, "%s: %v\n", "biden.jpg", err)
    }

    b := img.Bounds()

  imgSet = img.(ImageSet)
    for y := b.Min.Y; y < b.Max.Y; y++ {
        for x := b.Min.X; x < b.Max.X; x++ {
            oldPixel := img.At(x, y)
            r, g, b, a := oldPixel.RGBA()
            //fmt.Println(r, g, b, a)
            r = (r+g+b)/3
        //  g = (r+g+b)/3
            //b = (r+g+b)/3
            pixel := color.RGBA{uint8(r), uint8(r), uint8(r), uint8(a)}
            imgSet.Set(x, y, pixel)
        }
    }

    fd, err := os.Create("gray.jpg")
    if err != nil {
        log.Fatal(err)
    }

    err = jpeg.Encode(fd, img)
    if err != nil {
        log.Fatal(err)
    }

    err = fd.Close()
    if err != nil {
        log.Fatal(err)
    }

}

I get the following errors when trying to run this code.

   # command-line-arguments
.\rgbtogray.go:28: undefined: imgSet
.\rgbtogray.go:38: undefined: imgSet in imgSet.Set
.\rgbtogray.go:47: not enough arguments in call to jpeg.Encode
    have (*os.File, image.Image)
    want (io.Writer, image.Image, *jpeg.Options)
[Finished in 4.002s]

Can anyone see where these errors are coming from and why the code does not work?

Edit:

Made a couple of changes -

    package main

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

type ImageSet interface {
    Set(x, y int, c color.Color)
}

func main() {
    file, err := os.Open("biden.jpg")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    img, err := jpeg.Decode(file)
    if err != nil {
        log.Fatal(os.Stderr, "%s: %v\n", "biden.jpg", err)
    }

    b := img.Bounds()

    imgSet := img.(ImageSet)
    for y := b.Min.Y; y < b.Max.Y; y++ {
        for x := b.Min.X; x < b.Max.X; x++ {
      oldPixel := img.At(x, y)
           r, g, b, a := oldPixel.RGBA()
           //fmt.Println(r, g, b, a)
           g = (r+g+b)/3
           pixel := color.RGBA{uint8(g), uint8(g), uint8(g), uint8(a)}
           imgSet.Set(x, y, pixel)
        }
    }

    fd, err := os.Create("gray.jpg")
  defer fd.Close()
    png.Encode(fd, img)

}

The error I now get is -

 panic: interface conversion: *image.YCbCr is not main.ImageSet: missing method Set

I know using png.Encode at the bottom is strange but this is how someone I know fixed their issues.