Thanks Nicolas for your reply.
It does seem to be a memory problem, although it doesnt show the typical
"Memory Error". Without the script I could do that operation with the same
rasters. It involves four files and it is basically (a-b)*(c-d).
GDAL also failed to load all files at once, so I finally splited the
operation in 3. (a-b) then (c-d) and finaly the multiplication of the
outputs. Its much more time consuming but it works.
I still dont understand why using the tool directly from the UI it wrks and
it doesnt from the script. Will write back if I find the correct way.
> I have never use the Qgis raster calculator via Python so I can't help
> there. What is the problem? Does Qgis crash? If so, the rasters may be to
> big for the memory. You mentioned the rasters were very big and that may
> be the problem. I recommend you test the calculation with smaller
> rasters. I think they are working for a solution for the big raster math.
> Saga and gdal calculator may help. Saga will probably try to load
> everything in memory an may crash. Gdal is made to calculate very big
> rasters but layers must be in the same CRS and have the same pixel posting
> and extent. You can use it direction in the Osgeo command prompt or via
> Envoyé de mon iPad
> > Le 13 sept. 2016 à 05:42, ROMAN HG [via OSGeo.org] <
> > Hi,
> > I'm trying to do raster calculations with big and multiple rasters. I am
> new to python, but I managed to use the QgsRasterCalculator algorithm to do
> the average of three rasters.
> > The problem comes when I introduce a forth layer and try to do a more
> complex equation:
> > #calculating c4s ( [c1-c2]*[c0-c3] )
> > #F1_30
> > A=processing.getObjectFromUri(path + "/F1_30c1.tif")
> > B=processing.getObjectFromUri(path + "/F1_c2.tif")
> > C=processing.getObjectFromUri(path + "/F1_30c0.tif")
> > D=processing.getObjectFromUri(path + "/F1_c3.tif")
> > entries=
> > rasA=QgsRasterCalculatorEntry()
> > rasA.ref='A@1'
> > rasA.raster=A
> > rasA.bandNumber=1
> > entries.append(rasA)
> > rasB=QgsRasterCalculatorEntry()
> > rasB.ref='B@1'
> > rasB.raster=B
> > rasB.bandNumber=1
> > entries.append(rasB)
> > rasC=QgsRasterCalculatorEntry()
> > rasC.ref='C@1'
> > rasC.raster=C
> > rasC.bandNumber=1
> > entries.append(rasC)
> > rasD=QgsRasterCalculatorEntry()
> > rasD.ref='D@1'
> > rasD.raster=D
> > rasD.bandNumber=1
> > entries.append(rasD)
> > F1_30c4=QgsRasterCalculator('(( A@1 - B@1 ) * ( C@1 - D@1 ))', path +
> '/F1_30c4.tif', 'GTiff', base.extent(), base.width(), base.height(),
> > I think that the problem is recognising the formula. I have tried it on
> the console doing several changes and it doesnt work.
> > Is it possible to do this operation with QgsRasterCalculator?? Or how
> can I use the gdal_calc algorithm or the saga raster calculator??
> > Thanks all
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