Could you tell me why LSTM_MAX_LENGTH is 1 in HM-16.5_TEST_LDP?

[Liam-zzZ]

Hi,
in train mode, LSTM_MAX_LENGTH is 20. But in HM-16.5_TEST_LDP, LSTM_MAX_LENGTH is 1 . Could you tell me why LSTM_MAX_LENGTH is different?

[tianyili2017]

Hi,
Sorry for late reply.
In train mode, the 20 residual CTUs on all time steps together, are fed into the LSTM structure at one time. So, LSTM_MAX_LENGTH = 20.
However, in test mode, the frames are encoded one by one, that is to say, only if the current frame is encoded, the residual CTUs for the next frames can be available. The 20 residual CTUs from 20 frames can not be obtained together at once, and thus the LSTM should be run step-by-step. Here, on each step, only one LSTM cell needs to be run forward, whose input contains one residual CTU and the LSTM state for the last frame, and the output contains the splitting probability of 21 CUs (HCPM in paper) and the updated LSTM state for the current frame. The above procedure explains why LSTM_MAX_LENGTH = 1 in test mode.