clarification AMOTA about ab3dmot

[xinshuoweng]

if you can only reach recall = 0.6, I.e., not detecting the other 40% objects, you still have MOTA values at all 40 recall values, which are just 0s at the other 16 recall values. So the overall AMOTA score will be low because it is averaged with many 0s, which means that the metric is penalizing systems with a low recall.

Instead, if you divide by 24 recall values (values that are larger than 0, ignore the other 16 0s), you are not penalizing an MOT system with a lower recall.