// background must be white
static Pair<Pt> bwii_detectSingleLine(BWIntegralImage ii) {
  int imgW = ii.getWidth(), imgH = ii.getHeight();
  IPred<Int, Int> predX = (x1, x2) -> ii.getPixelAverage(x1, 0, x2, imgH) < 255;
  IPred<Int, Int> predY = (y1, y2) -> ii.getPixelAverage(0, y1, imgW, y2) < 255;
  ret pair(
    pt(indexOfRangePredicate    (imgW, predX),
       indexOfRangePredicate    (imgH, predY)),
    pt(lastIndexOfRangePredicate(imgW, predX),
       lastIndexOfRangePredicate(imgH, predY)));
}