static L<Pt> regionOutline(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 outline pixel with background above
  out.add(pt(x, y));
  
  ret out;
}