[GRASS-user] Fwd: How to obtain selected adjacent pixel values?

[Nikos Alexandris]

Yasser Said Lopez de Olmos Reyes wrote:
> How could I obtain the differences between central pixel values and
> cardinal directions pixel values related with a central pixel?
> 
> Or in a different manner, how to do this in grass gis?
> 
> RASTER_OUT= round(abs((RASTER_IN - RASTER_IN_PUSHED_ONE_ROW_UP) +
> (RASTER_IN - RASTER_IN_PUSHED_ONE_ROW_DOWN) + (RASTER_IN -
> RASTER_IN_PUSHED_ONE_COLUMN_LEFT) + (RASTER_IN -
> RASTER_IN_PUSHED_ONE_COLUMN_RIGHT))

This boils down to calculating

(RASTER_IN - RASTER_IN_PUSHED_ONE_ROW_UP) +
(RASTER_IN - RASTER_IN_PUSHED_ONE_ROW_DOWN) +
(RASTER_IN - RASTER_IN_PUSHED_ONE_COLUMN_LEFT) +
(RASTER_IN - RASTER_IN_PUSHED_ONE_COLUMN_RIGHT)

Please check the manual of r.mapcalc.  There is

--%<---
THE NEIGHBORHOOD MODIFIER

Maps and images are data base files stored in raster format, i.e., two-
dimensional matrices of integer values. In r.mapcalc, maps may be followed by 
a neighborhood modifier that specifies a relative offset from the current cell 
being evaluated. The format is map[r,c], where r is the row offset and c is 
the column offset. For example, map[1,2] refers to the cell one row below and 
two columns to the right of the current cell, map[-2,-1] refers to the cell 
two rows above and one column to the left of the current cell, and map[0,1] 
refers to the cell one column to the right of the current cell. This syntax 
permits the development of neighborhood-type filters within a single map or 
across multiple maps.
--->%--

which I think can be used to obtain what is asked.

So, for each of the off-current pixel, the modifiers would be like

RASTER_IN_PUSHED_ONE_ROW_UP:  map[-1,0]
RASTER_IN_PUSHED_ONE_ROW_DOWN:  map[1,0]
RASTER_IN_PUSHED_ONE_COLUMN_LEFT:  map[0,-1]
RASTER_IN_PUSHED_ONE_COLUMN_RIGHT:  map[0,1]

I am not exactly sure about the implementation though.

Nikos