# LSB 圖片隱寫（使用 R 通道）

package main

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

func main() {
    addMark("original.png", "mark.png", "marked.png")
    decodeMark("marked.png", "decoded.png")
}

func addMark(original, mark, marked string) {
    // open original image
    originalFile, err := os.Open(original)
    if err != nil {
        log.Fatalln(err)
    }
    defer originalFile.Close()
    originalImage, _, err := image.Decode(originalFile)
    if err != nil {
        log.Fatalln(err)
    }

    // open mark image
    markFile, err := os.Open(mark)
    if err != nil {
        log.Fatalln(err)
    }
    defer markFile.Close()
    markImage, _, err := image.Decode(markFile)
    if err != nil {
        log.Fatalln(err)
    }

    // prepare marked image
    bounds := originalImage.Bounds()
    w, h := bounds.Max.X, bounds.Max.Y
    markedImage := image.NewRGBA(image.Rect(0, 0, w, h))
    for x := 0; x < w; x++ {
        for y := 0; y < h; y++ {
            // mark
            oldColor := originalImage.At(x, y)
            r, g, b, a := oldColor.RGBA()

            markR, _, _, _ := markImage.At(x, y).RGBA()
            var mark uint32 = 1
            if markR < 128 {
                mark = 0
            }

            r = (r & 0b11111110) + mark
            markedImage.SetRGBA(x, y, color.RGBA{uint8(r), uint8(g), uint8(b), uint8(a)})
        }
    }

    outfile, err := os.Create(marked)
    if err != nil {
        log.Fatalln(err)
    }
    defer outfile.Close()
    png.Encode(outfile, markedImage)
}

func decodeMark(marked, decoded string) {
    // open original image
    originalFile, err := os.Open(marked)
    if err != nil {
        log.Fatalln(err)
    }
    defer originalFile.Close()
    originalImage, _, err := image.Decode(originalFile)
    if err != nil {
        log.Fatalln(err)
    }

    bounds := originalImage.Bounds()
    w, h := bounds.Max.X, bounds.Max.Y

    newImage := image.NewRGBA(image.Rect(0, 0, w, h))
    for x := 0; x < w; x++ {
        for y := 0; y < h; y++ {

            // decode
            oldColor := originalImage.At(x, y)
            originalImage.At(x, y)
            r, _, _, _ := oldColor.RGBA()
            if r&1 == 1 {
                newImage.Set(x, y, color.RGBA{255, 255, 255, 255})
            } else {
                newImage.Set(x, y, color.RGBA{0, 0, 0, 255})
            }
        }
    }

    outfile, err := os.Create(decoded)
    if err != nil {
        log.Fatalln(err)
    }
    defer outfile.Close()
    png.Encode(outfile, newImage)
}