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There are other modules which have this problem?

Replying to [comment:1 rugginoso]:

There are other modules which have this problem?

Yes there should be very many. Sorry for posting a non-standard module as an example.

I seem to recall that Radim may have done something about this some weeks ago when he was working on the QGIS/GRASS parser code???

Sorry I don't have a newish version of QGIS in front of me (due to lack of official Debian package...), I can give some solid examples later.

But a quick grep of the man pages gives some probables:

$  grep 'name</em>]' [rv]\\.*html | cut -f1 -d: | uniq | sed -e 's/.html//'
r.carve
r.category
r.cats
r.colors
r.cost
r.external
r.grow.distance
r.gwflow
r.in.ascii
r.in.gdal
r.kappa
r.lake
r.le.setup
r.los
r.out.gdal
r.out.ppm
r.out.xyz
r.proj
r.random
r.reclass
r.recode
r.region
r.sim.sediment
r.sim.water
r.statistics
r.stats
v.buffer
v.build
v.category
v.clean
v.convert
v.db.connect
v.distance
v.drape
v.edit
v.external
v.extract
v.extrude
v.in.ascii
v.in.db
v.in.dxf
v.in.ogr
v.kernel
v.label
v.label.sa
v.lrs.create
v.lrs.segment
v.net
v.net.iso
v.net.path
v.out.ascii
v.outlier
v.patch
v.proj
v.random
v.reclass
v.segment
v.surf.bspline
v.surf.rst
v.to.3d
v.to.db
v.to.rast
v.transform

may I suggest the r.cost start_rast= or start_points= option.

thanks,
Hamish

I can't understand what do you mean about "can't unselect an optional map".
In the example you provided me (r.cost) you must select a layer into each combobox, so the choice to leave empty one of them is incorrect.
Waiting your explainations :D

Replying to [comment:3 rugginoso]:

I can't understand what do you mean about "can't unselect an optional map".
In the example you provided me (r.cost) you must select a layer into each combobox, so the choice to leave empty one of them is incorrect.

yeah sorry, that's a bad example. in GRASS it is optional but in the QGIS wrapper it is not.

In GRASS you can use specific coordinates, vector map points, or raster map pixels for the starting points. Apparently the QGIS wrapper for it only offers the chance to use a raster map for that, so it becomes a required option in QGIS.

I'll have to hunt around for one that really shows the behaviour, although the effort to minimize module options in the QGIS wrappers may make it hard.

the general idea is this: some modules take optional input maps with a default of none. (eg if input data comes from a vector map instead) If you use the pulldown menu to select a map for that option there is no way to get back to the "none" state.

Hamish

Uhm undestood.
The solution used in the past seems to be to create a new module.
For example, r.cost. We could create two modules, r.cost.raster and r.cost.points.
What about this solution?