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

[Barry DeZonia]

This might be ill behaved for floating point. Rounding errors and
correct behavior for Infs and NaNs etc can spring surprises on you.
Java has a floating point modulus operator. I am betting the Double or
Float class java api javadoc might discuss implementation notes.

On Tue, Oct 7, 2025 at 4:24 PM Scott via gdal-dev
<gdal-dev at lists.osgeo.org> wrote:
>
> 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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> >
>
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