[gdal-dev] gdal raster calc - a modulo operator (%)?

[Scott]

Thanks for the idea Dan. Building on that, I added it upstream to the 
MuParser source. Now I have it in libmuparser as well:

./muparser-2.3.5/include/muParserTemplateMagic.h
    static T Mod(T pixel, T div) { return lround(pixel) - 
((lround(pixel) / lround(div)) * lround(div)); }

./muparser-2.3.5/src/muParser.cpp
    DefineFun(_T("mod"), MathImpl<value_type>::Mod);

After rebuild, leads to:

gdal raster calc \
    --input "p=input.tif" \
    --calc="mod(p[1], 50) == 0 ? p[1] : 255" \
    --ot=Byte \
    --output="output.tif" --co COMPRESS=DEFLATE --overwrite --progress

Ideally, though, having a mod function in GDAL itself would be awesome.

Scott


On 10/7/25 09:52, Daniel Baston wrote:
> It's fairly straightforward to define functions in C++ and register them 
> with muparser. You can see how we do this with "isnan" and "isnodata" here:
> 
> https://github.com/OSGeo/gdal/blob/ 
> ca7a2d38893a8d6cdc03653b47712f4442d99d65/frmts/vrt/ 
> vrtexpression_muparser.cpp#L27 <https://github.com/OSGeo/gdal/blob/ 
> ca7a2d38893a8d6cdc03653b47712f4442d99d65/frmts/vrt/ 
> vrtexpression_muparser.cpp#L27>
> 
> I don't think we want too many custom functions, but adding "mod" seems 
> like a good idea.
> 
> Dan
> 
> On Tue, Oct 7, 2025 at 12:30 PM Scott via gdal-dev <gdal- 
> dev at lists.osgeo.org <mailto:gdal-dev at lists.osgeo.org>> wrote:
> 
>     Greetings!
> 
>     I've been unable to find a modulo operator in CLI calc. It's not in the
>     muparser functions or appear to be in the forth-coming 3.12 C pixel
>     functions.
> 
>     This has been working with muparser, but it's messy every time you need
>     a simple modulo. pixel is real or int, div is int:
> 
>     rint(pixel) - (rint(rint(pixel) / div) * div)
> 
>     Am I missing something? If not, consider this a feature request! :)
> 
>     Thanks!
>     Scott
> 
>     -- 
>     www.postholer.com <http://www.postholer.com>
> 
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