def get_param3():
    """
    Return the default dictionary to start CaCo software
    ====================================================
    param3 defines all the CaCo parameters
    """
    keys = ('image_px', 'foliage_cover', 'canopy_cover', 'crown_porosity',
        'total_gap_fraction', 'total_gap_px', 'total_gap_mean',
        'total_gap_std_dev', 'total_gap_std_err', 'normal_gap_px',
        'normal_gap_mean', 'normal_gap_std_dev', 'normal_gap_std_err',
        'normal_gap_number', 'large_gap_fraction', 'big_gap_px',
        'big_gap_mean', 'big_gap_std_dev', 'big_gap_std_err', 'big_gap_number')
    band_choices = ['red', 'green', 'blue', 'greeness', 'grey']
    th_choices = ['otzu', 'isodata', 'minimum']
    param3 = {'input_dir': 'select directory', 'output_dir':
        'select directory', 'output_xls': 'result_cc.xls', 'raw_processing':
        True, 'band': 'grey', 'band_choices': band_choices, 'keys': keys,
        'th_choices': th_choices, 'threshold': 'minimum', 'th_switch': True,
        'th_dir': 'th_img', 'get_tree_id': False}
    """
    notation example to get tree id
    -------------------------------

              abies01_001
              ^       ^
    "tree identifier"_"photo_id"
    """
    return param3
