### Input parameters for Farneback optical flow

I'm going through Farneback optical flow estimation and, in particular, through the following line

```
cuda::FarnebackOpticalFlow::create(int numLevels=5, double pyrScale=0.5, bool fastPyramids=false, int winSize=13, int numIters=10, int polyN=5, double polySigma=1.1, int flags=0)
```

creating the Farneback estimator.
It seem I could not find a comprehensive documentation for the input parameters. Although I understand most of them, two are still not clear to me:

```
bool fastPyramids
```

and

```
double polySigma
```

Concerning the former, what is the fast pyramids approach?
Concerning the latter, I have found that

polySigma is the standard deviation of
the Gaussian that is used to smooth
derivatives used as a basis for the
polynomial expansion; for polyN=5, you
can set polySigma=1.1, for polyN=7, a
good value would be polySigma=1.5.

In which way are we smoothing derivatives?

Thank you for any help.