[GRASS-dev] [bug #3729] (grass) g.region zoom= off by one error on eastern side

[Glynn Clements]

Maciek Sieczka via RT wrote:

> This bug is still present and it's bad. In spearfish60:
> 
> $ g.region -a n=4916070 s=4915800 w=602940 e=603240 res=30
> 
> $ r.mapcalc 'map1=1'
> 
> $ g.region -a n=4916250 s=4915620 w=602730 e=603450 res=30
> 
> $ g.region zoom=map1
> 
> $ g.region -g
> n=4916070
> s=4915800
> w=602940
> e=603270
> nsres=30
> ewres=30
> rows=9
> cols=11
> 
> WRONG: 'e' should be '603240' -> 'cols' should be '10'.

I've committed this change:

--- general/g.region/cmd/zoom.c	20 Aug 2006 14:40:05 -0000	2.3
+++ general/g.region/cmd/zoom.c	25 Sep 2006 14:45:10 -0000
@@ -47,7 +47,7 @@
 	if (row < top) top = row;
 	if (row > bottom) bottom = row;
 	if (col < left) left = col;
-	for (mark = col++; col < ncols; col++)
+	for (mark = col; col < ncols; col++)
 	{
 	    if (!G_is_null_value(rast_ptr, map_type))
 		mark = col;


col gets incremented (the fact that we got here means that we already
know that raster[col] is non-null, so we can skip testing it again),
but rast_ptr doesn't, so the two are out of sync. Either rast_ptr
needs to be explicitly incremented along with col, or col should not
be incremented (the latter is simpler, and probably clearer; the
inefficiency is trivial).

Also, the issue is much worse than your example suggests. Because col
is one too high, the loop never examines the rightmost column. 
Consider:

	$ g.region -a n=4916070 s=4915800 w=602940 e=603240 res=30
	$ r.mapcalc 'map1 = if(col() == 1 || col() == 10,1,null())'
	$ g.region -a n=4916250 s=4915620 w=602730 e=603450 res=30
	$ g.region rast=map1
	$ g.region zoom=map1
	$ g.region -p
	projection: 1 (UTM)
	zone:       13
	datum:      nad27
	ellipsoid:  clark66
	north:      4916070
	south:      4915800
	west:       602940
	east:       603000
	nsres:      30
	ewres:      30
	rows:       9
***>	cols:       2
	cells:      18

Thus, "g.region zoom=..." can actually make the region too small,
discarding valid data.

-- 
Glynn Clements <glynn at gclements.plus.com>