[GRASS-dev] Fwd: QGIS Processing & GRASS

[Markus Neteler]

Forwarded message from Giovanni Manghi:

Hi all,

as you may know GRASS modules are available in QGIS through the
QGIS/GRASS plugin but also through Processing (ex Sextante), that makes it
particularly easy to use them because users don't
need to know about locations/mapsets.

A few GRASS modules are not available in the Processing toolbox,
but with a little help this would be probably possible. We are looking
for help (a not too advanced python knowledge should be enough) in adding
them.

As it stands now Processing always expects a GRASS module to create
one (or more outputs) from one (or more) inputs. There are obvious
exceptions to this, examples:

* r.ros (maybe others): the output in this GRASS module is just a
prefix for 3/4 output maps with an hardcoded name. Processing does not
actually support such case.

*r.null/v.distance: this modules modify the input layer instead of
creating a new output

* r.mapcalc: in Processing there is already r.mapcalculator, but it is
limited to 6 inputs. R.mapcalc is missing because it uses real map
name instead of parameters like "A", because it is not limited to 6
inputs and because Processing uses random names for temporary input
GRASS layers

* a few modules would benefit from a coordinate picker from the QGIS
map canvas. There is already something similar that allows to choose
(from canvas) the region size, so it should be relatively simple

* support GRASS layers as input: at the moment Processing imports
vector/raster data (non GRASS) and place them on the fly in a
temporary mapset, then runs the GRASS module against them. As QGIS can
use GRASS data it would be nice to support them also in Processing, it
would obviously necessary to skip the import (and eventually export)
phase.

for who may be interested this is good starting point

https://github.com/qgis/QGIS/blob/master/python/plugins/processing/algs/grass/GrassAlgorithm.py
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