sclass Gazelle22_GradientImage {
  // input
  BWImage posterized;
  double gradientThreshold = .25;
  
  // output
  BWImage out;
  
  // internal
  PosterizeBWImage op;
  BWImage unposterized;
  int w, h, brightnessLevels, singleStep;
  
  *(BWImage *posterized) {}

  bool init() {
    op = (PosterizeBWImage) getMetaSrc(posterized);
    if (op == null) false;
    unposterized = op.img;
    brightnessLevels = op.brightnessLevels;
    
    assertSameSize(unposterized, posterized);
    w = posterized.getWidth();
    h = posterized.getHeight();
    singleStep = iceil(doubleRatio(255, brightnessLevels-1));
    true;
  }
  
  BWImage get() {
    init();
    
    out = new BWImage(w*2+1, h*2+1);
    for y to h:
      for x to w: {
        if (x < w-1) checkPair(x, y, x+1, y);
        if (y < h-1) checkPair(x, y, x, y+1);
      }
      
    ret out;
  }
  
  // also returns true for inner pixels
  // may not work 100% yet (see gazelle22_highlightRegionOutline)
  bool isGradientPoint(int x, int y) {
    if (op == null) false;
    
    new IntMinMax posterizedMinMax;
    new IntMinMax unposterizedMinMax;
    
    // need to grab point and all 8 neighbors to cover all cases
    for (int y2 = max(0, y-1); y2 <= min(h-1, y+1); y2++)
      for (int x2 = max(0, x-1); x2 <= min(w-1, x+1); x2++) {
        posterizedMinMax.add(posterized.getInt(x2, y2));
        unposterizedMinMax.add(unposterized.getInt(x2, y2));
      }
    
    int posterizedContrast = posterizedMinMax.rangeLength();
    if (posterizedContrast > singleStep) false;
    if (posterizedContrast == 0) true;
    int unposterizedContrast = unposterizedMinMax.rangeLength();
    ret unposterizedContrast <= posterizedContrast*gradientThreshold;
  }

  void checkPair(int x, int y, int x2, int y2) {  
    int posterizedContrast = absDiff(posterized.getInt(x, y),
      posterized.getInt(x2, y2));
            
    if (posterizedContrast == 0) ret;
    if (posterizedContrast > singleStep) ret;
    
    int realContrast = absDiff(unposterized.getInt(x, y),
      unposterized.getInt(x2, y2));
    double ratio = doubleRatio(realContrast, singleStep);
    float result = cast (.5-ratio)*2;
    /*if (ratio < .5)*/ {
      out.setPixel(x*2+1-(y2-y), y*2+1, result);
      out.setPixel(x*2+1, y*2+1-(x2-x), result);
    }
  }
}

Began life as a copy of #1033934