[GRASS-dev] Re: [GRASS-user] low radiance values after i.atcor and i.topo.corr

[Dylan Beaudette]

On Monday 23 June 2008, Markus Neteler wrote:
> On Mon, Jun 23, 2008 at 1:52 PM, Glynn Clements
>
> <glynn at gclements.plus.com> wrote:
> > Markus Neteler wrote:
> >> > And i.smap is gibberish; e.g. write_img() calls G_is_c_null_value() on
> >> > "char"s, so that module probably hasn't worked since 4.x.
> >>
> >> i.smap works, I have tested it recently and also others use it
> >> regularly.

This is very troubling, as I have been using i.smap for sometime now 
(GRASS64-svn), and the output has thus far been convincing. However, I have 
only been using it with CELL input maps...


> > Oh, it may produce output, but not necessarily correct output.
> >
> > Specifically, write_img() does:
> >
> >           if(G_is_c_null_value((CELL *)&img[row][col]))
> >                   G_set_c_null_value(&files->cellbuf[col], 1);

Just words, but this again points to the dire need for a comprehensive test 
suite. 

> > But "img" is a "unsigned char **", i.e. a 2D array of bytes. The test
> > will only succeed if the array contains the sequence 00,00,00,80 (for
> > little-endian systems) or 80,00,00,00 (big-endian systems). I suspect
> > that the test was originally against zero, but it was incorrectly
> > changed when null support was added.
> >
> > If that's the case, the net result will be that it writes garbage
> > where it should be writing nulls, and occasionally writes nulls where
> > it shouldn't.

Is there any way to evaluate this? Would this bug only affect input maps that 
are FP?


> That sounds quite ugly. But from looking at the code: is a major
> rewrite needed to fix that? Since we "just" write out classes?
>
> > Beyond that, it seems fairly straightforward. I have committed an
> > update which compiles without warning, but hasn't been tested.
>
> Backported. But...
>
> >> Any opinions on
> >> i.gensig
> >> i.gensiset
> >> ? Both are used for supervised classification where you have training
> >> areas instead of just looking at the pixels as does i.cluster.
> >
> > Looking briefly at i.gensig, the algorithm seems to be quite heavily
> > based upon categories (e.g. using the Cell_stats stuff), so it would
> > need to be substantially re-written to support FP in any meaningful
> > sense.

I have never been able to use the GRASS classification routines on FP maps- 
usually just results in a map of NULL data or 0.


Dylan

> ... without an update to i.gensigset I cannot test the FP capabilities of
> i.smap. It seems that it was cloned from i.gensig.
>
> Markus




-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341