About descriptors in NetVLAD

[wenching33]

Hi @antoine77340
In the paper "Learnable pooling with Context Gating for video classification".
At page 4, NetVLAD aggregation section. Would please tell me where the descriptor xi comes from?
In original VLAD, they are descriptors produced by SIFT on each frame.
Here, Does the xi mean each frame-level feature(dimension=1024) ?
Thanks !