// background must be white
static Rect bwii_detectSingleLine(BWIntegralImage ii) {
  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 rectFromPoints(
    indexOfRangePredicate    (imgW, predX),
    indexOfRangePredicate    (imgH, predY),
    lastIndexOfRangePredicate(imgW, predX),
    lastIndexOfRangePredicate(imgH, predY));
}