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));

  }

}