[gdal-dev] Using bitwise & with muparser VRT expressions

[Even Rouault]

I see there's a mu::ParserInt class in 
https://github.com/beltoforion/muparser/blob/master/include/muParserInt.h 
, that we don't use in GDAL. A bit strange it wasn't mentionned in [1]. 
Perhaps because of this comment: "This version of the parser handles 
only integer numbers. It disables the built in operators thus it 
is slower than muParser. Integer values are stored in the double 
value_type and converted if needed."

Le 03/12/2025 à 21:07, Daniel Baston via gdal-dev a écrit :
> Hugh,
>
> My read of this discussion [1] is that bitwise operators are not 
> supported and we would need to add them as a GDAL extension. This is 
> pretty easy to do: see [2]. Alternatively, bitwise operators are 
> probably supported by the ExprTk expression parser [3] (supported by 
> GDAL) or this pull request [4].
>
> Dan
>
> [1] 
> https://github.com/beltoforion/muparser/discussions/154#discussioncomment-13104418
> [2] https://github.com/OSGeo/gdal/pull/13187/files
> [3] https://www.partow.net/programming/exprtk/
> [4] https://github.com/OSGeo/gdal/pull/13470
>
> On Wed, Dec 3, 2025 at 2:58 PM Hugh Graham via gdal-dev 
> <gdal-dev at lists.osgeo.org> wrote:
>
>     Hi,
>
>     I am using VRT muparser expressions for bitwise operations when
>     creating masks and wanted to understand why I can't get the
>     /bitwise & /operator working? My current solution is to use fmod
>     which is fine (and I think equivalent) but using "&" would be
>     convenient and a bit easier to read...
>
>     To illustrate, the following file works:
>     https://gist.github.com/h-a-graham/6db16e70e064ae8173f1052537c59761#file-fmask-fmodmask-vrt
>
>     the expression used here is:"(fmod(Fmask, 16) >= 8) ||
>     (fmod(Fmask, 8) >= 4) ? 0 : 1"
>
>     ```
>     gdal convert
>     "/vsicurl/https://gist.githubusercontent.com/h-a-graham/6db16e70e064ae8173f1052537c59761/raw/c48b15852bcc7c5252cae4b906c14b9a3205a33d/fmask-fmodmask.vrt"
>     "test.tif"
>     ```
>
>     However, the following file fails:
>     https://gist.github.com/h-a-graham/6db16e70e064ae8173f1052537c59761#file-fmask-bitmask-vrt
>     the expression parsed as: "((Fmask & 8) > 0 || (Fmask &
>     4) > 0) ? 0 : 1" in the XML but provided as (to my xml parser):
>
>     "((Fmask & 8) > 0 || (Fmask & 4) > 0) ? 0 : 1"
>
>     ```
>     gdal convert
>     "/vsicurl/https://gist.githubusercontent.com/h-a-graham/6db16e70e064ae8173f1052537c59761/raw/c48b15852bcc7c5252cae4b906c14b9a3205a33d/fmask-bitmask.vrt"
>     "test.tif"
>     0...10...20..ERROR 1: Unexpected token "& 8) > 0 || (Fmask & 4) >
>     0) ? 0 : 1 " found at position 8.
>     ```
>
>     So, is using "&" possible, or should I just stick with fmod and
>     not worry about it?
>
>     Any help would be much appreciated.
>
>     Hugh
>
>
>
>
>
>
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