Hi,
I am not a raster math genius but I will give this a shot.
1)". In this case all cells outside the extent of rasterB get assigned "nodata"
which is the default -3.4028234663852886e+38 in my case. Is that correct? 
-Yes that is a standard null data for float 16 or float 32 files.  It can
be change with gdal-warp or gdal-translate. I suspect that is you save the file
in ASCII grid format, you will have no more null values... And less hard drive
space!
-If you multiply a value by a null value, you will get a null value in the
answer.  That explains what is going on in point 1 and 2.
As for point 4, you can probably do it with a mask. Something like
rasterA/rasterA should give you a mask with 1 where there is data and zero when
not (I think. If not reset with gdal-translate). Then you can use the AND
function to combine areas and find where raster A and B intersect and work your
way from there. You could also find the extent of the rasters (in
vectors) and then use the vectors to raster operation to create a mask.
Stuff I found:
http://gis.stackexchange.com/questions/22786/how-to-exclude-missing-values-from-raster-layer
http://gis.stackexchange.com/questions/12418/how-to-redefine-the-nodata-value-into-zero-in-qgis
http://gis-lab.info/qa/rastercalc-eng.html#How_does_it_work
http://gis.stackexchange.com/questions/33764/how-to-change-null-values-raster-values-to-0-in-qgis
http://docs.qgis.org/2.6/en/docs/training_manual/rasters/terrain_analysis.html#moderate-fa-using-the-raster-calculator
http://docs.qgis.org/2.2/en/docs/training_manual/processing/no_data.html
Good luck
Nicolas Cadieux M.Sc.
Les Entreprises Archéotec inc. 
8548, rue Saint-Denis Montréal H2P 2H2
Téléphone: 514.381.5112  Fax: 514.381.4995
www.archeotec.ca
Le 2015-02-24 12:26, "Michael.Scholz [via OSGeo.org] "
<[email protected]> a écrit :
Hey!
In QGIS 2.6.1 I spent the whole day on the raster calculator for some
super-simple overlay calculations and I'm ending up in rage and fury. I
don't want to complain about the unintuitive syntax. Rather I have some
questions:
1) I have two raster with different extents. rasterA entirely contains rasterB.
When I add these rasters (rasterA + rasterB) and set the output extent to
the same as of rasterA, my result is only defined at the overlapping area of
both rasters. In this case all cells outside the extent of rasterB get assigned
"nodata" which is the default -3.4028234663852886e+38 in my case.
Is that correct?
2) I extended rasterB to have the same extent as rasterA by filling the missing
cells with "nodata". rasterA + rasterB still gives the same result
as in 1). Why is "rasterA + nodata = nodata"? I just want
values from rasterB to be added to values of rasterA AND keeping rasterA
entirely where rasterB has no values.
3) How can I query the "nodata" value of a band?? Without being able to
query it, I cannot set it to 0 which would solve all my problems.
4) I would like to express something like "(rasterB = nodata AND
rasterA != nodata) * rasterA + (rasterB != nodata) * (rasterA +
rasterB)". Is that possible?
Still in love with QGIS
Michi Scholz
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