/// <summary>
/// Calculate rows
/// </summary>
/// <param name="rects">A collection of the bounding boxes of each object in NA</param>
/// <returns>A collection of the bounding boxes of each row</returns>
private List<FS_RECTF> NormalizeRects(IEnumerable<FS_RECTF> rects)
{
    List<FS_RECTF> rows = new List<FS_RECTF>();

    float left, right, top, bottom;
    left = bottom = float.MaxValue;
    right = top = float.MinValue;
    float lowestBottom = float.NaN;
    float highestTop = float.NaN;

    foreach (var rc in rects)
    {
        float h = (highestTop - lowestBottom);
        //check if new row is required
        if (float.IsNaN(lowestBottom))
        {
            lowestBottom = rc.bottom;
            highestTop = rc.top;
        }
        else if (rc.top < lowestBottom + h / 2 || rc.bottom > highestTop - h / 2)
        {
            rows.Add(new FS_RECTF(left, top, right, bottom));
            lowestBottom = rc.bottom;
            highestTop = rc.top;
            left = bottom = float.MaxValue;
            right = top = float.MinValue;
        }
        else
        {
            if (lowestBottom > rc.bottom)
                lowestBottom = rc.bottom;
            if (highestTop < rc.top)
                highestTop = rc.top;
        }

        //concatenate previous and current rectangle
        if (left > rc.left)
            left = rc.left;
        if (right < rc.right)
            right = rc.right;
        if (top < rc.top)
            top = rc.top;
        if (bottom > rc.bottom)
            bottom = rc.bottom;
    }
    rows.Add(new FS_RECTF(left, top, right, bottom));

    return rows;
}