4-way JPEG

#!/bin/bash
# This creates an JPEG file with four different appearances depending on how the image is decoded.
# The dct* tools are available at https://gitgud.io/dcttransform/dcttransform

if (($# != 5)); then
  echo "Usage: $0 input1 input2 input3 input4 output"
  exit
fi
for i in {1..4}; do
  if [ ! -f "${!i}" ]; then
    echo "input file does not exist: $1"
    exit
  fi
done
set -x -e

# Working directory.
work=$(mktemp -d)
for i in {1..4}; do
  cp -- "${!i}" "$work/input$i"
done
pushd "$work" || exit

# The quantization table used in the files created.
# All values are set to 32.
yes 32 | head -n 64 > qtable.txt

# Create two color palettes.
# 8colors.jpg contains the colors obtained when pushing Y, Cb, Cr outside the valid ranges.
#   These are the intended colors for the final image.
# 8legal.jpg contains the colors obtained by setting Y, Cb, Cr to 128 +/- 32.
#   Images are mapped to this palette before JPEG compression;
#   then the DCT coefficients are upscaled to send them out of the valid range.
convert -size 64x8 xc: ppm:- \
  | cjpeg -qtables qtable.txt -qslots 0 -sample 1x1 > 8blank.jpg
dcttransform '
    a = new Uint16Array(3*8*64);
    i = 0;
    for (c=0;c<3;c++) for(z=-1;z<=1;z+=2) for(y=-1;y<=1;y+=2) for(x=-1;x<=1;x+=2) {
      a[64*i] = 64*[x,y,z][c];
      i++;
    }
    a
  ' 8blank.jpg > 8colors.jpg
dcttransform 'dct.map(x => x/8)' 8colors.jpg > 8legal.jpg

# Create two more color palettes.
# These are similar to the above except that an additional value for Y, Cb, Cr has been added in the middle.
# This middle color will be created in the final image by using one of the AC components for dithering.
convert -size 216x8 xc: ppm:- \
  | cjpeg -qtables qtable.txt -qslots 0 -sample 1x1 > 27blank.jpg
dcttransform '
    a = new Uint16Array(3*27*64);
    i = 0;
    for(c=0;c<3;c++) for(z=-1;z<=1;z++) for(y=-1;y<=1;y++) for(x=-1;x<=1;x++) {
      a[64*i] = 64*[x,y,z][c];
      a[64*i+4] = 64*([x,y,z][c]==0);
      i++;
    }
    a
  ' 27blank.jpg > 27colors.jpg
convert 27colors.jpg -colorspace LAB -resize 12.5% -scale 800% 27colors.png
dcttransform 'dct.map((x, i) => (i%64==0) ? x/8 : 0)' 27colors.jpg > 27legal.jpg
composite -blend 85% 27colors.png 27legal.jpg 27intermediate1.png
composite -blend 35% 27colors.png 27legal.jpg 27intermediate2.png

# Quantize all images to the colors in the palette.
# Pixelate into 8x8 blocks.
# The images are desaturated a bit before dithering as it seems to improve results.
for i in 1 3 4; do
  convert "input$i" -colorspace sRGB -resize 12.5% +level 20% -remap 27colors.png +dither -remap 27intermediate1.png -remap 27intermediate2.png -remap 27legal.jpg -scale 800% "quant$i-27.ppm"
done
for i in 2 4; do
  convert "input$i" -colorspace sRGB -resize 12.5% +level 20% -remap 8colors.jpg +dither -remap 8legal.jpg -scale 800% "quant$i-8.ppm"
done

# JPEG compress the images using our quantization table.
for f in *.ppm; do
  convert "$f" ppm:- \
    | cjpeg -qtables qtable.txt -qslots 0 -sample 1x1 "$f" > "${f%.*}.jpg"
  dctextract < "${f%.*}.jpg" > "${f%.*}.dct"
done

# Combine the images.
cat quant{1-27,2-8,3-27,4-27,4-8}.dct \
  | dcteval '
      n = dct.length/5;
      a = new Uint16Array(n);
      for (i = 0; i < n; i += 64) {
        // DC component
        a[i] = (
            512*Math.sign(dct[i]) +
          + 128*Math.sign(dct[i]) * (dct[i+2*n]==0) +
          + 256*Math.sign(dct[i+n]) * (dct[i]==0 ? +1 : -1)
          - 128*Math.sign(dct[i+n]) * (dct[i]==0 && dct[i+2*n]==0)
          - 128*Math.sign(dct[i+2*n])
          - 64*Math.sign(dct[i+3*n])
          - 32*Math.sign(dct[i+4*n])
        );
        // AC component 4 in x direction
        a[i+4] = (
            512*(dct[i]==0 && dct[i+2*n]!=0)
          + 128*(dct[i+2*n]==0)
          + 64*(dct[i+3*n]==0)
        );
        // AC component 4 in y direction
        a[i+32] = (
            128*(dct[i+2*n]==0)
          + 512*(dct[i]==0 && dct[i+2*n]==0)
        );
      }
      a
    ' > output.dct
dctreplace output.dct < quant1-27.jpg > output.jpg

# Clean up.
popd
cp -- "$work/output.jpg" "$5"
rm -r -- "$work"