Why detect people in the cloud named "filtered"?

Hi, @koide3 
In apps/hdl_people_detection_nodelet.cpp, there are codes as:
```cpp
    auto filtered = backsub->filter(cloud);
    auto clusters = detector->detect(filtered);
```
And it seems to detect people in the filtered cloud that only contains voxels with less than "occupy_thresh "(default value is 2).
May I ask what assumptions they are based on？