The library includes cellular (Worley) noise. The F1 channel is the distance to the closest feature point and F2 is the distance to the second closest feature point. There is an implementation that uses 3x3 areas and a faster variant that uses 2x2 area. The faster variant is very similar for the F1 channel, with only a few artifacts:

The faster variant has many artifacts in the F2 channel:

There's also a 3D version, where the slower variant uses 3x3x3 and the faster variant uses 2x2x2:

The faster variant has many artifacts in the F2 channel: