[gdal-dev] Advice on `GDALCreateAndReprojectImage` and destination no-data value.

[Fitch, Simeon]

Hi,

I'm one of the maintainers at the `georust/gdal` bindings project. 👋

I'm currently working on exposing some of the GDAL Warp functionality in
Rust, starting with a simple wrapper around `GDALCreateAndReprojectImage`.
I'm having trouble getting the output file to include a no-data value in
the band's metadata, despite the Warp operation properly respecting no-data
values specified in `GDALWarpOptions`.

My C++ skills are very "rusty", so I'm hoping I can get away with
explaining my problem with some Rust code that is pretty close to being 1:1
with the C API. My suspicion is I'm missing a step in the `GDALWarpOptions`
setup.

This is the CLI operation I'm trying to replicate:

    gdalwarp -t_srs EPSG:4269 -dstnodata 255 -r near input.tif output.tif

Here's the bare-bones Rust bindgen version:

    <https://gist.github.com/metasim/d2a288d5b6126438aaa84b9e95707aff>

The single band in `input.tif` is described by `gdalinfo` like this:

    Band 1 Block=186x44 Type=Byte, ColorInterp=Gray
      NoData Value=255
      Metadata:
        CLASSES=3

After running the Rust code, the results are correct (including Warp
numerically respecting source and destination no-data values) _except_
`output.tif` does not have a no-data value in the band metadata. Here's
what `gdalinfo` on `output.tif` shows:

    Band 1 Block=210x39 Type=Byte, ColorInterp=Gray

The only way I've been able to get the behavior I need is to open the
written dataset from the filesystem and set the no-data value explicitly in
the band, which is inefficient and awkward. Given the CLI version does the
right thing, it's obviously possible. I've spent several hours visually
inspecting the C++ source code, but the CLI version doesn't call
`GDALCreateAndReprojectImage` directly; rather it constructs the Warp
pipeline itself.

Anyone have tips that might help me run this down? Am I missing a no-data
handling step required when working with Warp routines? Could
`GDALCreateAndReprojectImage` have a bug?

Thanks,

Simeon

-- 
Simeon Fitch
Co-founder & CTO
Astraea, Inc.

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