[GRASS-user] face to DEM?

[Adam Dershowitz]

I have some 3-d vector faces.  They were defined in a text file like  
this:
F 10
1 2 1
2 2 2
...
...
etc.

I imported them into grass like this:
v.in.ascii -zn input=faces.txt out=faces format=standard
and all seems fine.

I can see the faces, and if I click on points around the edges I can  
get a height as well.
I can't figure out how to generate a raster DEM from these faces.   
Essentially I would like to know the approximate elevation at each  
raster point inside the faces.
I tried v.to.rast but then I get "Column parameter missing"
For v.to.rast3 I get  "Unable to get layer info for vector map"
I also tried v.extrude but I see all 0s (ie Number of areas: 0 etc.)

Did I do something wrong on my import?  Did I miss some other way to  
do the conversion?
I want to be able to do some mapcalcs with the heights of an these  
faces compared to another raster map, but I can't figure out how to  
convert these faces to something that I can work with.

Thanks much,

--Adam