I found that field_value_factor modifier could be none, log, log1p, log2p, ln, ln1p,ln2p, square, sqrt, or reciprocal.
I am not sure about the definition of these modifiers:
- log1p = log(1+p)
- log2p = log(2p) -> from wolfram alpha
- ln1p = ln(1+p)
- ln2p = ln(2p)
In wolfram alpha, defintion of #1 is ln(1+x).