[GRASS-dev] [GRASS GIS] #1405: [PATCH] Direct floating point values comparison in r.terraflow

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1

#1405: [PATCH] Direct floating point values comparison in r.terraflow
-------------------------+--------------------------------------------------
Reporter: dron | Owner: grass-dev@…
     Type: defect | Status: new
Priority: normal | Milestone:
Component: Raster | Version: svn-trunk
Keywords: r.terraflow | Platform: Unspecified
      Cpu: Unspecified |
-------------------------+--------------------------------------------------
r.terraflow module does direct comparison of two floating point values.
This is wrong and results in emergency exit with "cell file resolution
differs from current region" message. There is a trivial patch fixing
that:

{{{
--- main.cc (revision 47154)
+++ main.cc (working copy)
@@ -216,8 +216,8 @@
      G_fatal_error(_("Cannot read header of [%s]"), cellname);

    /* check compatibility with module region */
- if (!((region->ew_res == cell_hd.ew_res)
- && (region->ns_res == cell_hd.ns_res))) {
+ if (!(fabs(region->ew_res - cell_hd.ew_res) < 0.0000001
+ && fabs(region->ns_res - cell_hd.ns_res) < 0.0000001)) {
      G_fatal_error(_("cell file %s resolution differs from current
region"),
                                   cellname);
    } else {
}}}

Applicable for 6.4, 6.5 and 7.0 branches.

Best regards,[[BR]]
Andrey

--
Ticket URL: <http://trac.osgeo.org/grass/ticket/1405&gt;
GRASS GIS <http://grass.osgeo.org>

2

#1405: [PATCH] Direct floating point values comparison in r.terraflow
-------------------------+--------------------------------------------------
Reporter: dron | Owner: grass-dev@…
     Type: defect | Status: new
Priority: normal | Milestone:
Component: Raster | Version: svn-trunk
Keywords: r.terraflow | Platform: Unspecified
      Cpu: Unspecified |
-------------------------+--------------------------------------------------

Comment(by glynn):

Replying to [ticket:1405 dron]:
{{{
+ if (!(fabs(region->ew_res - cell_hd.ew_res) < 0.0000001
+ && fabs(region->ns_res - cell_hd.ns_res) < 0.0000001)) {
}}}

The tolerance is probably too small.

I'd suggest checking that the difference over the entire region is less
than some fraction of a cell, e.g.
{{{
fabs(region->ns_res - cell_hd.ns_res) * region->rows < region->ns_res *
0.1
}}}

The ideal check is probably a 1:1 correspondence between raster cells and
region cells, but that isn't easy to implement. Note that this depends on
both the resolution and the alignment. In the worst case, if the raster is
offset by half a cell relative to the region, even a miniscule difference
in resolution could result in cells being either duplicated or omitted.

--
Ticket URL: <http://trac.osgeo.org/grass/ticket/1405#comment:1&gt;
GRASS GIS <http://grass.osgeo.org>