[GRASS5] [bug #1956] (grass) r.reclass: re-reclass categories off by one/many

[Glynn Clements]

Request Tracker wrote:

> Subject: r.reclass: re-reclass categories off by one/many 

> r.reclass produces a shift in assigned categories when reclassifying
> raster layers which are themselves products of a reclassification
> (re-reclass). this shift is easily visible when comparing the
> resulting files in MAPSET/cellhd. it appears to me to be a small
> mistake in the compose() function in
> src/raster/r.reclass/cmd/reclass.c which is only called for
> re-reclass.
> 
> if the lowest assigned category number is not 0, the way the category
> table is filled will be inadeqate (i.e. array is not filled starting
> from position 0), as G_put_reclass() will read that array starting
> from 0 later on.
> 
> my C isn't so hot, but the following change made it work for me:
> 
> in src/raster/r.reclass/cmd/reclass.c:
> <start diff output>
> 64c64
> < 	new->table[i] = k;
> ---
> > 	new->table[i - new->min] = k;
> </end>

This looks correct; however, the same needs to be done for the no-data
case, i.e.:

--- src/raster/r.reclass/cmd/reclass.c	28 Mar 2002 17:30:39 -0000	1.4
+++ src/raster/r.reclass/cmd/reclass.c	20 Jun 2003 00:48:30 -0000
@@ -55,13 +55,13 @@
 	j = old->table[i - old->min];
 	if (G_is_c_null_value(&j) || j < mid->min || j > mid->max)
 	{
-	    G_set_c_null_value(&new->table[i], 1);
+	    G_set_c_null_value(&new->table[i - new->min], 1);
 	    continue;
 	}
 
 	k = mid->table[j - mid->min];
 
-	new->table[i] = k;
+	new->table[i - new->min] = k;
     }
 }

I've committed this fix to the CVS HEAD version.

> this also worked for the interactive mode (i expect it calls the same
> function).

The interactive version simply runs the non-interactive version to do
the actual reclass operation.

> maybe this solution is a little too simple, i don't claim to have
> fully (or even partially) understood the design behind the structs
> involved.
> 
> what mystifies me is why i haven't seen anything in the bug reports
> about this. the problem surfaced in a grass course at the university
> of freiburg using the spearfish data set. the task was to reclassify
> the "texture" raster layer, which is a reclassification of the "soils"
> raster layer. the results where quite obviously weird - i would have
> thought this might have happened elsewhere. my search of the bug
> lists, google groups etc didn't produce any hits, though.

I suspect it's because reclassing of reclass maps isn't particularly
common; it wasn't even possible until the "pre4" release (March
2002).

-- 
Glynn Clements <glynn.clements at virgin.net>