Question about the query_lbs and inv_mats

[HaiLin545]

Hello authors， thank you for such an excellent work！
In the code， I find that the numver of SMPL vertex is extended to >20w from its original number 6890 by introducing the UV pose map.
So, the author use a "query_lbs" and "inv_mats" to proceed the LBS. I am confused with:

1. How to generate the "query_lbs" and "inv_mats" ?
2. What's the meaning of matmul( smpl.A, inv_mats) ? and why we need inv_mats?

I would be great appreciate if anyone could answer my question or support some reference. Thanks.