NWPU dataset data format --- luminance label

[ykn96]

In the NWPU dataset readme, there is a section on the dataformat

## Data Format

1. The dataset provides ```*.jpeg``` images, and two types of labels(```jsons``` and ```mats```). The points order is ```x, y```. To be specific, the contents of ```.mat``` files are consistent with the UCF-QNRF. The boxes labels is ```xmin, ymin, xmax, ymax```.
2. Each line in ```train.txt``` and ```val.txt``` has three items, image id, luminance label, and scene level. 
3. Each line in ```test.txt``` has only one item, image id.

short example from train.txt

0001 1 1
0002 1 0
0003 2 0
0004 1 2
0005 1 1
0006 1 3

Correct my understanding if it is wrong, please. If the luminanace label is the 0 or 1 or 2 in the second column, may i know what do each of the 0 1 2 mean?

I would like to select the low illumination and night photos.

期待您的回复！谢谢！