srecord noeq G22RegionThinner_v2<Img extends WidthAndHeight>(IImageRegion<Img> originalRegion) is Steppable {

  Rect bounds;

  IImageRegion<Img> region;

  // 0 = outside of region

  // 1 = inner pixel

  // 2 = border pixel

  // index is in bounds coordinates

  byte[] pixels;

  int idx(int x, int y) {

    ret (y-bounds.y)*bounds.w+x-bounds.x;

  }

  int idx(Pt p) { ret idx(p.x, p.y); }

  Pt idxToPt(int idx) {

    ret pt(bounds.x+(idx % bounds.w), bounds.y+idx/bounds.w);

  }

  byte getPixel(Pt p) {

    ret !containsPt(bounds, p) ? 0 : pixels[idx(p)];

  }

  void init {

    if (bounds != null) ret;

    bounds = originalRegion.bounds();

    pixels = new byte[area(bounds)];

    for (Pt p : originalRegion.pixelIterator())

      pixels[idx(p.x, p.y)] = 1;

    region = new ThinnedRegion;

  }

  class ThinnedRegion is IImageRegion<Img> {

    public Img image() { ret originalRegion.image(); }

    public Rect bounds() { ret bounds; }

    public bool contains(int x, int y) {

      ret containsPt(bounds, x, y) && pixels[idx(x, y)] > 0;

    }

    public ItIt<Pt> pixelIterator() {

      ret iff_null(new IF0<Pt> {

        int idx = 0;

        public Pt get() {

          for (; idx < pixels.length; idx++)

            if (pixels[idx] > 0)

              ret idxToPt(idx++);

          null;

        }

      });

    }

  }

  public bool step() {

    init();

    L<PtBuffer> traces = g22_allBorderTraces_withDiagonals(region);

    for (points : traces)

      for (p : points)

        pixels[idx(p)] = 2;

    new PtBuffer toDelete;

    for (points : traces)

      for ping (p : points) {

        if (deletableBorderPoint(p))

          toDelete.add(p);

      }

    for (p : toDelete)

      pixels[idx(p)] = 0;

    ret nempty(toDelete);

  }

  bool deletableBorderPoint(Pt p) {

    int surroundingBorderPixels = 0, surroundingInnerPixels = 0;

    for (int dir = 1; dir <= 8; dir++) {

      Pt p2 = ptPlus(p, onePathDirection(dir));

      byte value = getPixel(p2);

      if (value == 2)

        surroundingBorderPixels++;

      else if (value == 1)

        surroundingInnerPixels++;

    }

    bool deletable = surroundingInnerPixels > 0 && surroundingBorderPixels <= 2;

    printVars ifdef G22RegionThinner_debug(+p, +surroundingInnerPixels, +surroundingBorderPixels, +deletable);

    ret deletable;

  }

  IImageRegion region aka get() { ret or(region, originalRegion); }

}