Nat,
As Matt mentions, pre-processing may be a good route here.
But, to answer your questions specifically:
1) No, Filters do not (yet) work for rasters in the same way as vectors, but it
is something we could consider adding the future.
2) If you are using Mapnik2/trunk then you can request the reading of just one
band via the gdal plugin like:
<Parameter name="band">1</Parameter>
3) And if you are using Mapnik2/trunk you can also colorize the output to
effectively make transparent values less than 0.
See an example of colorizing a raster here:
http://trac.mapnik.org/browser/trunk/tests/data/good_maps/raster_colorizer.xml
Dane
On Jun 2, 2011, at 5:15 PM, Nat Wilson wrote:
> I think this is an easy question, but having completed the tutorial, I don't
> yet feel up to tackling it.
>
> I have an ASCII raster layer that I read with the Gdal plugin. I would like
> to prevent certain cells from being drawn (for example, only show cells with
> a value greater than 0). I had a look at the <Filter> rule, but couldn't tell
> what the variable same should be (gdal info calls it "Band 1", but that
> doesn't work), or if <Filter> is even meant to be used with raster layers.
>
> I also didn't see any way to do this with a RasterSymbolizer directly - is
> there an undocumented technique?
>
> Could someone give me a clue?
>
> Thanks,
>
> njw
>
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