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;
  Pt p = regions.samplePixel(iRegion);
  int x = p.x, y = p.y;
  
  // move up until we're at the edge of the region
  // probably we don't actually need to do this because of the way
  // the image was scanned, but we'll do it anyway
  while (y > 0 && regions.inRegion(iRegion, x, y-1))
    y--;
  
  // found our first border pixel with background above
  int dir = 3; // direction up
  
  // remember first point
  int firstX = x, firstY = y;
  
  int safety = regions.size();
  loop: do {
    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(x2, y2, iRegion)) {
        x = x2; y = y2; dir = newDir;
        continue loop;
      }
    }
    
    // if we get here, the region is just 1 pixel (so we're done)
    break;
  } while ((x != firstX || y != firstY) && --safety >= 0);
  
  ret out;
}