// background must be white
static Pair<Pt> bwii_detectSingleLine(BWIntegralImage ii) {
  int whiteValue = 255;
  int imgW = ii.getWidth(), imgH = ii.getHeight();
  IIntIntPred predX = (x1, x2) -> ii.getPixelAverage(x1, 0, x2, imgH) != whiteValue;
  IIntIntPred predY = (y1, y2) -> ii.getPixelAverage(0, y1, imgW, y2) != whiteValue;
  
  // find bounding box
  
  Pair<Pt> endpoints = pair(
    pt(indexOfRangePredicate_IIntIntPred(imgW, predX),
       indexOfRangePredicate_IIntIntPred(imgH, predY)),
    pt(lastIndexOfRangePredicate_IIntIntPred(imgW, predX),
       lastIndexOfRangePredicate_IIntIntPred(imgH, predY)));
       
  // find out which diagonal it is
  
  if (ii.getPixel(endpoints.a) != whiteValue) {
    endpoint.s.x
    ? pair(pt(endpoints.b.x, endpoints.a.y),
           pt(endpoints.a.x, endpoints.b.y)) : endpoints;
}