static BWImage gazelle22_borderImage(BWImage posterizedWithMeta) {
  PosterizeBWImage op = cast getMetaSrc(posterizedWithMeta);
  ret gazelle22_borderImage(op.img, posterizedWithMeta, op.brightnessLevels);
}

static BWImage gazelle22_borderImage(BWImage unposterized, BWImage posterized, int brightnessLevels) {
  assertSameSize(unposterized, posterized);
  int w = posterized.getWidth(), h = posterized.getHeight();
  BWImage out = new(w*2+1, h*2+1);
  int singleStep = iceil(doubleRatio(255, brightnessLevels-1));
  
  for y to h:
    for x to w: {
      if (x < w-1) {
        // check border with right neighbor
        
        int posterizedContrast = absDiff(posterized.getInt(x, y),
          posterized.getInt(x+1, y));
          
        float result;
        if (posterizedContrast != 0) {
          if (posterizedContrast > singleStep) result = 1f;
          else {
            int realContrast = absDiff(unposterized.getInt(x, y),
              unposterized.getInt(x+1, y));
            result = floatRatio(realContrast, singleStep);
          }
          out.setPixel(x*2+1, y*2,   result);
          out.setPixel(x*2+1, y*2+1, result);
        }
      }
      
      if (y < h-1) {
        // check border with bottom neighbor
        
        int posterizedContrast = absDiff(posterized.getInt(x, y),
          posterized.getInt(x, y+1));
          
        float result;
        if (posterizedContrast != 0) {
          if (posterizedContrast > singleStep) result = 1f;
          else {
            int realContrast = absDiff(unposterized.getInt(x, y),
              unposterized.getInt(x+1, y));
            result = floatRatio(realContrast, singleStep);
          }
          out.setPixel(x*2,   y*2+1, result);
          out.setPixel(x*2+1, y*2+1, result);
        }
      }
    }
    
  ret out;
}