// should return outer outline first and then outline of holes

// (without a separator)

scope regionBorder_innerPoints.

static Pt[] #directions = {

  pt(1, 0), pt(0, 1), pt(-1, 0), pt(0, -1) // r, d, l, u

};

static L<Pt> regionBorder_innerPoints(BWImage_FastRegions regions, int iRegion) {

  new PtBuffer out;

  int w = regions.w;

  var it = regions.regionIterator(iRegion);

  new BitSet seen;

  while (it.next()) {

    int x = it.x(), y = it.y();

    // is it a 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))

      continue;

    //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;

        }

      }

      // if we get here, the region is just 1 pixel (so we're done)

      break;

    }

  }

  ret out;

}