static BWImage gazelle22_borderImage(IBWIntegralImage unposterized, BWImage posterized) {
  assertSameSize(unposterized, posterized);
  int w = posterized.getWidth(), h = posterized.getHeight();
  BWImage out = new(w*2+1, h*2+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));
        if (posterizedContrast != 0) {
          int realContrast = absDiff(unposterized.getPixelBrightness(x, y),
            unposterized.getPixelBrightness(x+1, y));
          double ratio = doubleRatio(realContrast, posterizedContrast);
          out.setPixel(x*2+1, y*2,   (float) ratio);
          out.setPixel(x*2+1, y*2+1, (float) ratio);
        }
      }
      
      if (y < h-1) {
        // check border with bottom neighbor
        
        int posterizedContrast = absDiff(posterized.getInt(x, y),
          posterized.getInt(x, y+1));
        if (posterizedContrast != 0) {
          int realContrast = absDiff(unposterized.getPixelBrightness(x, y),
            unposterized.getPixelBrightness(x, y+1));
          double ratio = doubleRatio(realContrast, posterizedContrast);
          out.setPixel(x*2,   y*2+1,   (float) ratio);
          out.setPixel(x*2+1, y*2+1, (float) ratio);
        }
      }
    }
    
  ret out;
}