Why use additional one frame when training?

[applezy8866]

Dear,
I found that in dataset.py:
" if not self.test_mode:
self.num_frames_per_sample += 1"
and in tracker.py:
"num_frame = img.size(1) - 1"
so what‘s the usage of the additional frame?

[a1600012888]

It is not used for training more frames.
Since I am adding velocity supervisions, the velocity groud truth is from the box position at next frame. That's why I need to load one more frame.