first working thingy

.gitignore

+/ImageModulator
+/output
+/output.png
+/.idea

demodulate.go

+package main
+
+import (
+	"image"
+	"os"
+)
+
+var inFile *os.File
+var inImage image.Image
+var format string
+var outSlice []byte
+
+func de() {
+	inFile, err = os.Open(inFilePath)
+	if err != nil {
+		println("Error while opening input,", err)
+		os.Exit(1)
+	}
+	inImage, format, err = image.Decode(inFile)
+	if err != nil {
+		println("Error while decoding input,", err)
+		os.Exit(1)
+	}
+
+	if format != "png" {
+		println("Expected png, got", format)
+		os.Exit(1)
+	}
+
+	for y := 0; y < inImage.Bounds().Dy(); y++ {
+		for x := 0; x < inImage.Bounds().Dx(); x++ {
+			r, g, b, a := inImage.At(x, y).RGBA()
+			if r == g && g == b && b == 0 {
+				outSlice = append(outSlice, byte(a))
+			}
+		}
+	}
+	println("Output", len(outSlice), "bytes")
+	err = os.WriteFile("output", outSlice, 0644)
+	if err != nil {
+		println("Error while writing output,", err)
+		os.Exit(1)
+	}
+}

go.mod

+module git.randomchars.net/RandomChars/ImageModulator
+
+go 1.16

main.go

+package main
+
+import (
+	"flag"
+	"image"
+	"image/color"
+	"image/png"
+	"io"
+	"math"
+	"os"
+)
+
+var err error
+
+var (
+	inFilePath string
+	demodulate bool
+)
+
+var (
+	in []byte
+	out io.Writer
+)
+
+func init() {
+	flag.BoolVar(&demodulate, "d", false, "De-modulate data from a modulated image.")
+}
+
+func main() {
+	flag.Parse()
+
+	if flag.NArg() != 1 {
+		println("Expected argument: infile.")
+		os.Exit(1)
+	}
+
+	inFilePath = flag.Arg(0)
+
+	if demodulate {
+		de()
+		os.Exit(0)
+	}
+
+	in, err = os.ReadFile(inFilePath)
+	if err != nil {
+		println("Error while reading input,", err)
+		os.Exit(1)
+	}
+
+	l := int(math.Ceil(math.Sqrt(float64(len(in)))))
+	h := int(math.Ceil(float64(len(in)) / float64(l)))
+	println("Image length", l, "height", h)
+
+	img := image.NewRGBA64(image.Rect(0, 0, l, h))
+	c := 0
+	for y := 0; y < h; y++ {
+		for x := 0; x < l; x++ {
+			if c >= len(in) {
+				img.Set(x, y, color.RGBA{
+					R: 255,
+					G: 255,
+					B: 255,
+					A: 255,
+				})
+				continue
+			}
+			img.Set(x, y, color.RGBA{
+				R: 0,
+				G: 0,
+				B: 0,
+				A: in[c],
+			})
+			c++
+		}
+	}
+	out, err = os.OpenFile("output.png", os.O_WRONLY|os.O_CREATE, 0644)
+	if err != nil {
+		println("Error while opening output,", err)
+		os.Exit(1)
+	}
+	err = png.Encode(out, img)
+	if err != nil {
+		println("Error while encoding output,", err)
+		os.Exit(1)
+	}
+}