How to call processing toolbox in Qgis' python console

Question

I have been trying to use processing toolbox via Python console in QGIS 2.0.1. I am able to write the following commands:

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I am particularly interested to run 'r.statistics' from GRASS module.

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My question is how to arrange the input parameters sequentially and what does each parameter mean? For example, in the above screenshot, when I type the help, I can see the parameter inputs,

processing.runalg('grass:r.statistics', base, cover, method, -c, GRASS region parameter, Grass region cell size, output)

I can understand that 'base = input raster', 'method = avg, mean, std' and 'output=outputfile to eb declared'.

But I am not sure what are 'cover=', '-c','GRASS region means'? How can I get further help on that? If I were to run the function 'r.statistics' via processing toolbox, how should the exact/correct command look like?

processing.runalg('grass:r.statistics', 'C:\mydata\dem.tif', cover, '1', -c, GRASS region parameter, Grass region cell size, 'C:\mydata\outputdem.tif') 

Can anyone help me what to put in place of rest of the parameters?

Thanks in advance.

Answer 1

To learn the processing module in Python, the easiest solution for me is:

1. execute the Processing command (from the Toolbox), for example v.to.rast.val
2. examine the "processing_qgis.log" file in the ".qgis2/processing/" folder

where you can examine all the running algorithms:

 ALGORITHM|Mon Oct 28 2013 
 12:30:34|processing.runalg("grass:v.to.rast.value","/Users/Shared/polygon.shp",0,1,"202563.92575,204206.182887,89106.9864984,89783.2100251",0,-1,0.0001,"/Users/Shared/polyraster.tif")

The actual commands are in:

• "grass_batch_job.sh" or "grass_batch_job.bat" files (in the ".qgis2/processing/" folder) for GRASS GIS;
• "saga_batch_job.sh" or "saga_batch_job.bat" files for SAGA GIS;
• "processing_script.r" file for R.

So, in the Python console:

import processing
processing.runalg("grass:v.to.rast.value","/Users/Shared/polygon.shp",0,1,"202563.92575,204206.182887,89106.9864984,89783.2100251",0,-1,0.0001,"/Users/Shared/polyraster.tif")
processing.runalg("grass:r.statistics","/Users/Shared/polyraster.tif","/Users/Shared/mydem.asc",1,True,"202086.577,205625.414407,88411.048,90534.3504441",0,"/Users/Shared/test.tif")

You can also use one of the displayed layers:

vectorlayer = qgis.utils.iface.mapCanvas().currentLayer().source()
processing.runalg("grass:v.to.rast.value",vectorlayer,0,1,"202563.92575,204206.182887,89106.9864984,89783.2100251",0,-1,0.0001,"/Users/Shared/polyraster.tif")

but before, you need to understand all the parameters of the command r.statistics and the problems as r.statistics limitation to CELL

• underdark: You can actually access the log from within QGIS. It's not necessary to dig out the file from some folder.
• gene: Perhaps, but these files are interesting if you really want to understand how processing (or sextante) works.
• Zaman: Hi All,Thanks for the reply. As suggested by gene, how would I execute the Processing command of 'v.to.rast.value'? Do I have to run the v.to.rast.value from GRASS gui via Qgis first to see log file? Then shall I check qgis/processing ?
• Zaman: gene, how did you generate the following log file? ALGORITHM|Mon Oct 28 2013 12:30:34|processing.runalg("grass:v.to.rast.value","/Users/Shared/polygon.shp",0,1,"202563.92575,204206.182887 Did you run v.to.rast.value from GRASS gui initially to see the log file in qgis2/processing folder?
• gene: I use the Toolbox command GRASS.../vector (v*)/v.to.rast.value (not from the GRASS plugin)