[Qgis-developer] Unloading a layer in Sextante script crashes QGIS

Tue Feb 19 04:10:22 PST 2013

Thanks Victor... I tried a few other ways to load the layer, but none of
them seem to work.

Another approach would be to somehow render the layer without loading it to
the canvas first. Is it possible to add a layer to a QgsMapRenderer() layer
set without it being loaded in the project's QgsMapLayerRegistry? I guess
not since it requires a layer id, which I guess is provided by the
registry. Is it possible to have more than one QgsMapLayerRegistry? If it
is, I could create an output registry for each map... if not, looks like
I'll need to make a full plugin.

Thanks,
Rudi

On Tue, Feb 19, 2013 at 1:13 PM, Victor Olaya <volayaf at gmail.com> wrote:

> Interacting with the canvas or any other elements in QGIS is not a
> good idea from SEXTANTE. It might cause that freezing you mention,
> since you are calling GUI methods from a different thread and that
> might result in strange behaviour.
>
> Ideally, a SEXTANTE algorithm should just take data and produce new
> data, and then, if it is declared correctly as algorithm output, it
> will be loaded automatically by SEXTANTE. I am afraid that your
> algorithm might go beyond what SEXTANTE expects....
>
> Anyway, the problem might be that you are trying to remove the layer
> and maybe it is not there since the load method in SEXTANTE does not
> add the layer to the canvas, but just open it so SEXTANTE algorithms
> can use it if it is not loaded.
>
> Hope this helps
>
> 2013/2/19 Rudi von Staden <rudivs at gmail.com>:
> > I've put together a Sextante script to automate the production of
> grid-based
> > species distribution maps (png image output) by modifying [1] and [2]. It
> > takes a vector layer of point localities (with species name as a field)
> and
> > a grid layer as input, and produces output maps by selecting grids based
> on
> > point localities.
> >
> > For each species it creates a distribution shapefile, which consists of
> the
> > grid polygons where the species occurs. As far as I could tell, this
> layer
> > needs to be loaded to the canvas before it can be included in the
> layerset
> > to be rendered for the output images (is there a better way?). Sextante
> > includes a function for loading layers, but not removing them. From what
> I
> > could find,
> > "QgsMapLayerRegistry.instance().removeMapLayers(taxon_layer.id())"
> should
> > remove the layer, but it crashes QGIS when I try (current git version).
> If I
> > comment out the line, my canvas gets clogged. Any idea where the problem
> > lies? Script below.
> >
> > Thanks,
> > Rudi
> >
> > [1] http://www.qgis.org/pyqgis-cookbook/composer.html#simple-rendering
> > [2]
> http://qgissextante.blogspot.com/2013/01/using-selection-algorithms.html
> >
> >
> > #Definition of inputs and outputs
> > #==================================
> > ##[Scratch]=group
> > ##all_localities=vector
> > ##taxon_field=field all_localities
> > ##africa_map=vector
> > ##sa_map=vector
> > ##grid_layer=vector
> > ##distribution_style_file=file
> > ##output=output file
> >
> > #Algorithm body
> > #==================================
> > from qgis.core import *
> > from PyQt4.QtCore import *
> > from PyQt4.QtGui import *
> > from sextante.core.QGisLayers import QGisLayers
> > from sextante.core.SextanteVectorWriter import SextanteVectorWriter
> > import tempfile
> > import os
> >
> > def print_map(taxon,taxon_shp):
> >     #load taxon layer (necessary?)
> >     QGisLayers.load(taxon_shp,name = "taxon",style =
> > distribution_style_file)
> >
> >     # create image (dimensions 325x299)
> >     img = QImage(QSize(325,299), QImage.Format_ARGB32_Premultiplied)
> >
> >     # set image's background color
> >     color = QColor(192,192,255)   # blue sea
> >     img.fill(color.rgb())
> >
> >     # create painter
> >     p = QPainter()
> >     p.begin(img)
> >     p.setRenderHint(QPainter.Antialiasing)
> >
> >     render = QgsMapRenderer()
> >
> >     # create layer set
> >     africa_layer = QGisLayers.getObjectFromUri(africa_map)
> >     sa_layer = QGisLayers.getObjectFromUri(sa_map)
> >     taxon_layer = QGisLayers.getObjectFromUri(taxon_shp)
> >
> >     lst = []
> >     lst.append(taxon_layer.id())
> >     lst.append(sa_layer.id())
> >     lst.append(africa_layer.id())
> >
> >     render.setLayerSet(lst)
> >
> >     # set extent (xmin,ymin,xmax,ymax)
> >     rect = QgsRectangle(14.75,-36.00,34.00,-21.00)
> >     render.setExtent(rect)
> >
> >     # set output size
> >     render.setOutputSize(img.size(), img.logicalDpiX())
> >
> >     # do the rendering
> >     render.render(p)
> >     p.end()
> >
> >     # save image
> >     outdir = os.path.dirname(os.path.abspath(output))
> >     img.save(os.path.join(outdir,taxon+".png"),"png")
> >
> >     # remove taxon layer from project
> >     QgsMapLayerRegistry.instance().removeMapLayers(taxon_layer.id())
> > #crashes QGIS
> >
> > tempdir = tempfile.gettempdir()
> > taxa = sextante.runalg('qgis:listuniquevalues', all_localities,
> taxon_field,
> > None)['UNIQUE_VALUES'].split(";")
> > counter = 0   # limit for testing
> > for taxon in taxa:
> >     if counter < 10:
> >         sextante.runalg('qgis:selectbyattribute', all_localities,
> > taxon_field, 0, taxon)
> >         sextante.runalg('qgis:selectbylocation', grid_layer,
> all_localities,
> > 0)
> >         filename = os.path.join(tempdir,"taxon.shp")    #memory file
> better?
> >         sextante.runalg('qgis:saveselectedfeatures', grid_layer,
> filename)
> >         print_map(taxon,filename)
> >         counter+=1
> >
> > _______________________________________________
> > Qgis-developer mailing list
> > Qgis-developer at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/qgis-developer
> >
>
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