def map_rois_to_fpn_levels(rois, k_min, k_max):
    """Determine which FPN level each RoI in a set of RoIs should map to based
    on the heuristic in the FPN paper.
    """
    # Compute level ids
    s = np.sqrt(box_utils.boxes_area(rois))
    s0 = cfg.FPN.ROI_CANONICAL_SCALE  # default: 224
    lvl0 = cfg.FPN.ROI_CANONICAL_LEVEL  # default: 4

    # Eqn.(1) in FPN paper
    target_lvls = np.floor(lvl0 + np.log2(s / s0 + 1e-6))
    target_lvls = np.clip(target_lvls, k_min, k_max)
    return target_lvls
Determine which FPN level each RoI in a set of RoIs should map to based
    on the heuristic in the FPN paper.
    
detectron/modeling/FPN.py