// should return outer outline first and then outline of holes
// (without a separator)

srecord noeq RegionBorder_innerPoints(BWImage_FastRegions regions, int iRegion) is Steppable {
  static Pt[] #directions = {
    pt(1, 0), pt(0, 1), pt(-1, 0), pt(0, -1) // r, d, l, u
  };
  
  int w, x, y;
  BWImage_FastRegions.RegionIterator it;
  BitSet seen;
  bool tracing;

  void init {
    w = regions.w;
    seen = new BitSet;
    it = regions.regionIterator(iRegion);
  }

  public bool step() {
    if (seen == null) init();
    
    if (tracing) {
    } else {
      if (!it.next()) false; // done
      x = it.x(); y = it.y();
        
      // not a border pixel? continue search for border pixel
      if (regions.inRegion(iRegion, x, y-1)
        && regions.inRegion(iRegion, x-1, y)
        && regions.inRegion(iRegion, x+1, y)
        && regions.inRegion(iRegion, x, y+1))
        true;
    
      //print("border pixel: " + x + "/" + y);
      int dir = 3; // start with direction up (any direction should do)
    
      loop: while (true) {
        int pos = y*w+x;
        if (seen.get(pos))
          break;
        seen.set(pos);
          
        out.add(x, y);
        
        for (int turn = 3; turn <= 6; turn++) { // try left, straight, right, back
          int newDir = (dir+turn) & 3;
          Pt d = directions[newDir];
          int x2 = x+d.x, y2 = y+d.y;
          if (regions.inRegion(iRegion, x2, y2)) {
            x = x2; y = y2; dir = newDir;
            continue loop;
          }
        }
      }
    }
  }
}