John:
Admittedly I work with GRASS solely on the text-based side - rarely if ever
touching the GUI - but hopefully I can help:
1) To be honest, it's probably easiest to continue to use d.his and then
display the resulting map using the GRASS plugin - QGIS doesn't really have
many (if any?) raster tools, while GRASS was created primarily to deal with
raster features (and added vectors later).
2) I believe r.mapcalc is the way to do this on the fly - not sure what you are
asking, because I'm sure d.rast calls this "on the fly" when you go to display
the image?
You can always do something like
r.mapcalc "{$output_map} = if({$input_map[0,0]} >= 3000, {$input_map[0,0]},
null())"
so it doesn't stop the processing.
3. You CAN do raster reprojection on the fly. However, your results won't be
anywhere near as "clean" as a vector reprojection as a result of the different
format type. Also, there are some rules - I believe the projection has to be in
the current region of the location you're reprojecting to, and also the
resolution must be sufficient in order to handle the map.
The r.proj part of the manual has two good procedures for doing so:
http://grass.osgeo.org/grass64/manuals/html64_user/r.proj.html
(old version but should still be okay)
4) I think it's an actual limitation - I am assuming, for a categorical map,
you would like say all cats < 10 to have a transparency but all cats >= 10 to
not have a transparency? I'm not sure how to do this, if this exists - I'd just
use r.mapcalc and display two different layers.
I hope I didn't misinterpret what you're writing, and hopefully that was of
some help.
Cheers
John
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