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

[Victor Olaya]

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
>