[GRASS-dev] New module: r.grow.distance

[Dylan Beaudette]

On Wednesday 23 July 2008, Glynn Clements wrote:
> Hamish wrote:
> > > I have added a new module, r.grow.distance, which generates a map
> > > where the value of each cell is the distance to the nearest non-null
> > > cell in the input map (similar to r.cost with a constant cost, except
> > > that you get real circles rather than approximations).
> >
> > ie better than r.cost's knight's move / octagon. (?)
>
> AFAICT, r.cost gives octagons without -k and hexadecagons (16 sides)
> with -k.
>
> Even if that's an adequate approximation, r.grow.distance is
> noticeably faster, especially if the map won't fit into memory.
>
> In the process of quantifying that, I found a bug in the way that
> r.cost computes the number of segments:
>
>     if (maxmem > 0)
> 	segments_in_memory =
> 	    2 + maxmem * (nrows / SEGCOLSIZE) * (ncols / SEGCOLSIZE) / 100;
>     else
> 	segments_in_memory = 4 * (nrows / SEGCOLSIZE + ncols / SEGCOLSIZE + 2);
>
> Because integer division rounds down, if the map is
> one-point-something segments in each direction, it allocates 2+1*1 = 3
> segments for a map which really needs 4.
>
> [With "spearfish".]
>
> $ g.region res=45
> $ time r.cost -k --o start_rast=test input=one output=test.grow
> 318 rows, 423 cols
> real	1m18.267s
> user	0m2.876s
> sys	1m15.358s
>
> 78 seconds, of which 75 is spent in the kernel (i.e. swapping segments
> to file). At res=30 (spearfish's default), I Ctrl-C'd it after a
> couple of minutes.
>
> OTOH, with percent_memory=0 (4*(1+1+2) = 16 segments):
>
> $ g.region res=45
> $ time r.cost -k --o start_rast=test input=one output=test.grow
> percent_memory=0 318 rows, 423 cols
> real	0m1.518s
> user	0m1.477s
> sys	0m0.040s
>
> 1.5 seconds. A significant improvement over 78 seconds.
>
> Fixed in SVN trunk (r32239).

Nice work. Is there any schedule for fixes like these to make it back to 6.4?

Thanks,

Dylan

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