[GRASS5] ERROR: cell_values_double: xdr_double failed for index 387.

[Glynn Clements]

Hamish wrote:

> > > I'm getting this error when I try to access a DCELL map after setting
> > > the region to cross 180 degrees lon:
> > > 
> > > ERROR: cell_values_double: xdr_double failed for index 387.
> > > (lib/gis/get_row.c line 414)
> > > 
> > > G57:~> g.region -p
> > > projection: 3 (Latitude-Longitude)
> > > zone:       0
> > > datum:      wgs84
> > > ellipsoid:  wgs84
> > > north:      25:10S
> > > south:      56S
> > > west:       153:05E
> > > east:       174:40W
> > > nsres:      0:05
> > > ewres:      0:05
> > > rows:       370
> > > cols:       387
> > > 
> > > 
> > > g.region -d && d.zoom -p
> > > works ok until I hit 180E, then the above error.
> > > 
> > > Works ok with a CELL map; fails on any FCELL or DCELL map.
> > > fails using d.rast, r.mapcalc, etc. with GRASS 5.0.3 -> GRASS 5.7-cvs.
> > 
> > I can't reproduce this (using the "global" sample data set).
> 
> Ok, global data set, try it with:
> G57:~> g.region -d
> G57:~> r.mapcalc nationsF='float(nations)'
> G57:~> d.rast nationsF
> G57:~> d.zoom      # (japan)
> G57:~> d.zoom -p   # to the east
> [...]
> north: 13:50S  south: 27:20S  east: 178W  west: 167:05E
> East limit of default region reached.
> ERROR: cell_values_float: xdr_float failed for index 179.

OK, I can reproduce with:

	g.region -d
	r.mapcalc nationsF='float(nations)'
	g.region n=13:50S s=27:20S e=178W w=167:05E
	d.rast nationsF

[Now I can't understand how I failed to reproduce it before; AFAICT,
this will always occur if you read an FP raster when the region
crosses the 180-degree line.]

The problem is in how the FP versions of the resampling code access
the input data. Essentially, they assume that the column mapping will
always be monotonically increasing, and read the input data
sequentially (from an XDR stream):

    xdr_setpos(xdrs, 0);

    for (i = 0; i < n; i++)
    {
	if (!cmap[i])
	{
	    c[i] = 0;
	    continue;
	}

	if (cmap[i] == cmapold)
	{
	    c[i] = c[i-1];
	    continue;
	}

	while (cmapold++ != cmap[i]) /* skip */
	    if (!xdr_float(xdrs, &c[i]))
	    {
		G_fatal_error("cell_values_float: xdr_float failed for index %d.", i);
		return;
	    } 

	cmapold--;
    }

The loop iterates i over destination columns, with cmap[i] being the
corresponding source column; cmapold contains the previous source
column. If cmap[i] is 0, the source column is beyond the borders of
the map. If cmap[i] == cmapold, the source column is the same as for
the last pass, so the previous value can be reused. Otherwise, it
keeps reading values from the XDR stream until it reaches the desired
source column.

Except, if the source column decreases, i.e. when the data crosses the
180-degree line, it won't reach the desired source column; it will
reach the end of the stream.

It needs to check for this, and rewind the stream to the beginning if
it occurs. Try the attached patch.

FWIW, this has never worked. The only change to
cell_values_{float,double} between the Feb_26_2001 snapshot (the
oldest version of GRASS 5.x which I have) and my re-write a few weeks
back was internationalising an error message:

-	    sprintf(rsbbuf, "cell_values_d: xdr_double failed for index %d.", n);
+	    sprintf(rsbbuf, _("cell_values_d: xdr_double failed for index %d."), n);

BTW, it doesn't apply to cell_values_int, which just does pointer
arithmetic to locate the source value.

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

-------------- next part --------------
--- src/libes/gis/get_row.c~	Sun Sep  5 13:04:59 2004
+++ src/libes/gis/get_row.c	Mon Sep  6 07:04:40 2004
@@ -367,6 +367,12 @@
 	    continue;
 	}
 
+	if (cmap[i] < cmapold)
+	{
+	    xdr_setpos(xdrs, 0);
+	    cmapold = 0;
+	}
+
 	while (cmapold++ != cmap[i]) /* skip */
 	    if (!xdr_float(xdrs, &c[i]))
 	    {
@@ -406,6 +412,12 @@
 	{
 	    c[i] = c[i-1];
 	    continue;
+	}
+
+	if (cmap[i] < cmapold)
+	{
+	    xdr_setpos(xdrs, 0);
+	    cmapold = 0;
 	}
 
 	while (cmapold++ != cmap[i]) /* skip */