how to pass in a GDALResampleAlg to GDAL's RasterIO?

Question

The the RasterIO method has a GDALResampleAlg option using the GDALRasterIOExtraArg struct:

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The GDAL tutorial has an example like this for reading data from a raster band:

  poBand->RasterIO( GF_Read, Xoffset, Yoffset, nXSize, nYSize,
                    pafScanline, outXSize, outYSize, GDT_Float32,
                    0, 0);

and the unused 12th argument from that can take the resampling option within the GDALRasterIOExtraArg.

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How does one write the C++ to actually pass in the option? I can instantiate the option from the constants:

 // how to pass this option in to RasterIO?
  GDALResampleAlg eResampleAlg = GRA_NearestNeighbour;

But I don't know C++ well enough to see how to actually use it, and I can't find any examples that I can follow enough to make it work.

Answer 1

Here is a way, there is a macro INIT_RASTERIO_EXTRA_ARG that sets up the extra-arg struct, that is listed here:

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Then set the eResampleAlg property to the appropriate constant. Some of the other extra-arg parameters are needed for certain cases, but that is outside of scope of this question.

float *pafScanline;

GDALRasterIOExtraArg psExtraArg;
INIT_RASTERIO_EXTRA_ARG(psExtraArg);
// TODO expose the resampling options to user
psExtraArg.eResampleAlg = GRIORA_NearestNeighbour;
pafScanline = (float *) CPLMalloc(sizeof(float)*outXSize*outYSize);
CPLErr err = poBand->RasterIO( GF_Read, Xoffset, Yoffset, nXSize, nYSize,
                pafScanline, outXSize, outYSize, GDT_Float32,
                0, 0, &psExtraArg);

See here for more Link