[GRASSLIST:5139] Raster crop/mask.

[DzZero]

I have setup GRASS 5.0 and all seems to be working perectly.  I have 
determined how to create a new project and import vector (dxf) and 
raster (8bit tiled TIFFs) layers. Now I am trying to determine how I 
might be able to draw a polygon and import one dxf (known) then use that 
polygon as a mask or cropping line then in turn right out the area still 
visible of the TIFF. I am also curious about rotation and transformation 
(2d) of the rasters.

Can anyone direct me to a good tutorial on this. I have been searching 
for awhile but can not seem to find anything specific.  I have tinkered 
with creating masks but I am not sure if I am doing it correctly nor how 
I would use one once I have it created.

Also I have not looked into this much as of yet but does anyone have a 
suggestion on a good 3D tutorial?  ie: Drapping Orthorectified imagery 
over a point surface (dem/dtm)?

Thanks,
Jason Nielsen

PS. I am a total newbie to GRASS and GIS is not my field of expertise.