Go言語でグレースケールの画像に変換 ref: https://qiita.com/tenntenn/items/0471e5f494df82c3e825

file0.txt

package main

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

func main() {
    img, _ := png.Decode(os.Stdin)
    bounds := img.Bounds()
    dest := image.NewGray16(bounds)
    for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
        for x := bounds.Min.X; x < bounds.Max.X; x++ {
            c := color.Gray16Model.Convert(img.At(x, y))
            gray, _ := c.(color.Gray16)
            dest.Set(x, y, gray)
        }
    }
    png.Encode(os.Stdout, dest)
}

file1.txt

$ go run gray.go < gopher.png > gopher_gray.png