sclass ULetterbox1 implements IIIRecognizer<DoubleRect> {
  public void run(
    IProbabilisticScheduler scheduler,
    URecognizer.Cell cell,
    IVF1<DoubleRect> valueToAction) {
    
    // We inquire 2 pixels from the image.
    
    var whole  = cell;
    var middle = cell.slice(1, 1.0/3.0, 2.0/3.0);
    
    double wholeColor = whole!;
    double middleColor = middle!;
    
    // Inquiry done. In ULetterbox1, we assume the border is black.
    // (We could verify this by doing a third pixel query,
    // but surely that's too slow.!)
    // Here's the logic.
    // If the picture fills the whole screen, it will tend
    // to equal brightness everywhere. Thus whole = middle*3
    // (because it's 3 times as large).
    // If only "middle" is not black, we have whole = middle.
    // Thus the border position can be estimated as:
    double ratio = doubleRatio(wholeColor, middleColor);
    double borderPos = transformBetweenDoubleRanges_verbose(ratio, 1, 1/3.0, 0, 1/3.0);
    printVars(+wholeColor, +middleColor, +ratio, +borderPos);
      
    DoubleRect rect = cell.slice(1, borderPos, 1-borderPos).rawDoubleRect();
    
    // No probabilities used.
    
    scheduler.schedule(1.0, () -> valueToAction.get(rect));
  }
}