[Qgis-user] Two different raster calculators in QGIS and negative value handling - any explanation?

[Alessandro Pasotti]

On Wed, Sep 7, 2022 at 11:43 AM Michael Dufty via Qgis-user
<qgis-user at lists.osgeo.org> wrote:
>
> It appears that there are two different native raster calculator tools in QGIS
>
> One is found in at the top of the raster menu
>
> The other is in the processing toolbox in the raster analysis group
>

Yeah, we know it's a mess.

Both are based on the same QGIS core implementations, they just use a
different GUI but they must give the same results.

>
>
> (Not to mention the GDAL raster calculator and SAGA raster calculator)
>

This one is based on GDAL instead and it may give different results.

>
>
> Can anyone explain the reason for there being two, and what the differences are?
>
> I’ve been very confused by it, mainly because the one in the raster menu does not appear to handle negative values correctly.  I’d been wondering why NDVI type expressions with negatives randomly worked or not, and discovered it just depended on which raster calculator I used.
>
> Is it expected that the one in the raster menu doesn’t handle negative values correctly?  It looks like it might do calculations based on unsigned integers regardless of the input and output raster formats.
>

You could file a bug report attaching a small dataset to reproduce the
issue and if you are using OpenCL acceleration (see global QGIS
settings).


Kind regards.


-- 
Alessandro Pasotti
QCooperative:  www.qcooperative.net
ItOpen:   www.itopen.it