def get_gaze_thres(gaze_df, frame_count):
    """Get the lowest threshold of confidence user can set so that there is at
    least one gaze point in each frame
    
    :param gaze_df: dataframe containing gaze info
    :param frame_count: number of video's frames

    :return threshold: lowest threshold
    :rtype: float
    """

    threshold_list = gaze_df['confidence'].drop_duplicates().sort_values()
    for threshold in threshold_list:
        good_gaze_df = gaze_df.loc[gaze_df['confidence'] > threshold] 
        if good_gaze_df['world_index'].nunique() < frame_count:
            return threshold