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

[Javier Jimenez Shaw]

If I remember correctly, the arithmetic is done with floating 64b numbers,
right? Doing bitwise operations there sound a bit (pun intended) strange.
I know a bit portion of images used are RGB (byte images), there bitwise
operatios are fine.

On Wed, 3 Dec 2025 at 21:14, Even Rouault via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

> 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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>> gdal-dev at lists.osgeo.org
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>>
>
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