Hi what kind of error are you getting?  Is Qgis  just crashing? Do you get an 
error message?  Does the script work with smaller files?  

Nicolas

Envoyé de mon iPad

> Le 14 sept. 2016 à 02:05, R HG [via OSGeo.org] 
> <ml-node+s1560n5285719...@n6.nabble.com> a écrit :
> 
> 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.
> 
>> 
>> 
>> Hi,
>> 
>> 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 
>> processing.
>> 
>> Nicolas
>> 
>> 
>> Envoyé de mon iPad
>> 
>> > Le 13 sept. 2016 à 05:42, ROMAN HG [via OSGeo.org] <<a href="javascript:;" 
>> > onclick="_e(event, &#39;cvml&#39;, 
>> > &#39;ml-node+s1560n5285491...@n6.nabble.com&#39;)">ml-node+s1560n5285491h82@...>
>> >  a écrit :
>> >
>> > 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(), 
>> > entries).processCalculation()
>> >
>> > 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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