[GRASS-dev] Using GRASS to calculate an index

Glynn Clements glynn at gclements.plus.com
Mon Nov 23 18:06:08 EST 2009

Luis Lisboa wrote:

> I'm planning on developing an index to use over RASter images. Is quite an
> easy expression with a few sums and square roots. All those operations are
> included in r.mapcalc but instead of doing each operation using image
> calculation in GUI I want to do an automatic procedure in which I select
> image A, image B and Image C and he does the rest.
> I can use a script file but I want the user to select which is image A, B
> and C and only then the script process the images.
> Question: How can I call a wxpython form (just like the others) from a
> script file?

The GUI parameter forms are auto-generated from the parameter
descriptions (the same information that is used for the --help output
and the HTML documentation).

If you look at any of the provided scripts (in the "scripts"
directory), you will see that they start with a block of comments like
(this example was taken from r.blend):

	#%  description: Blends color components of two raster maps by a given ratio.
	#%  keywords: raster
	#% key: first
	#% type: string
	#% gisprompt: old,cell,raster
	#% description: Name of first raster map for blending
	#% key_desc : name
	#% required : yes

Shortly after the comments will be something like:

	if [ "$1" != "@ARGS_PARSED@" ] ; then
	    exec g.parser "$0" "$@"

The g.parser module reads the script, and uses the comments to
generate a parameter dialog. Once the user fills in the parameters and
presses "OK", the script is executed with the supplied parameters
passed through environment variables (and with the string
"@ARGS_PARSED@" as the only argument, so that it doesn't just call
g.parser again).

See the g.parser manual page for more details.

This mechanism can be used in any language which treats the "#"
character as starting a comment (although for Python, there is a
standardised framework for scripts which includes an interface to

Glynn Clements <glynn at gclements.plus.com>

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