[gdal-dev] Feasibility of expanding VRT schema to allow users to specify X/Y dimension for HDF data?

[Even Rouault]

On Thursday 04 August 2016 16:31:25 H. Joe Lee wrote:
> Hi,
> 
>   My name is Joe Lee and I'm very interested in improving GDAL's
> capability to access NASA HDF4/HDF5 data so that users can work with
> HDF easily through GDAL. For example, my goal is to allow users to
> translate any HDF data into GeoTIFF via gdal_translate.
> 
>   I've worked with diverse NASA HDF products and provided solution for
> visualizing data correctly through Python/MATLAB/IDL/NCL [1] and I
> know that many products, other than HDF-EOS, may not work well with
> GDAL because HDF is flexible and NASA data producers do not
> necessarily follow the conventions that GDAL uses.
> 
>   By default, GDAL HDF4/HDF5 driver uses the following convention for
> unknown products.
> 
>     For HDF4 (frmts/hdf4/hdf4imagedataset.cpp):
> 
>     // Search for the starting "X" and "Y" in the names or take
>     // the last two dimensions as X and Y sizes
>     iXDim = nDimCount - 1;
>     iYDim = nDimCount - 2;
> 
>   For HDF5 (frmts/hdf5/hdf5imagedataset.cpp):
> 
>     int     GetYIndex() const { return IsComplexCSKL1A() ? 0 : ndims - 2; }
>     int     GetXIndex() const { return IsComplexCSKL1A() ? 1 : ndims - 1; }
> 
>  The above code works well as long as Unknown HDF product follows the
> above convention. However, in reality, HDF data can have an arbitrary
> order in terms of Band, X and Y dimension like this:
> 
>   dset4D[XDim=360][YDim=180][Band1=2][Band2=3]
>   dimindex:    0                      1            2             3
> 
>   Since ndims=4, ndims-2 becomes 2 and ndims-1=3. In such case, GDAL
> generates 360x180 bands of 2x3 images, instead of the desired 2x3
> bands of 360x180 images.
> 
>   Thus, I'm wondering if GDAL can expand VRT schema so that VRT allows
> users to specify the correct dimension index because specifying
> dimension order for each different NASA product in [1]  is
> impractical. For example, I'd like suggest a new tag like
> "SourceDimension" like below:
> 
>   <VRTRasterBand dataType="UInt16" band="1">
>   <SimpleSource>
>     <SourceFilename
> relativeToVRT="0">HDF4_SDS:UNKNOWN:"DATA_WITH_4D_DATASET.hdf":7</SourceFilen
> ame> <SourceDimension RasterXDim="0" RasterYDim="1" />
>     <SourceBand>1</SourceBand>
>     <SourceProperties RasterXSize="360" RasterYSize="180"
> DataType="UInt16" BlockXSize="360" BlockYSize="180" />
>    ...
>   </SimpleSource>
> </VRTRasterBand>
> 
>   Once user specifies correct dimensions by editing VRT, it can be
> used by GDAL HDF4/HDF5 drivers and the HDF drivers read the data
> correctly for GDAL's image buffer.
> 
>   Do you think it's right and feasible approach to solve wrong X/Y
> dimension order problem? Or do you have any other existing solution
> that can mitigate this problem in GDAL? The GEE project team has
> experimented the idea by creating another separate XML file [2] but I
> think it's time to sync with GDAL development team and come up with
> the most elegant solution. In my opinion, VRT looks best and I wish
> GDAL development team can give me some feedback on this idea.

Joe,

I would rather suggest to add open options to the drivers and pass them with 
the exiting VRT OpenOptions element, rather than adding a new element in the 
VRT that would be specific of a few drivers

 <SimpleSource>
    <SourceFilename> 
relativeToVRT="0">HDF4_SDS:UNKNOWN:"DATA_WITH_4D_DATASET.hdf":7</SourceFilename>
   <OpenOptions>
      <OOI key="RASTERXDIM">0</OOI>
      <OOI key="RASTERYDIM">1</OOI>
   </OpenOptions>
    <SourceBand>1</SourceBand>
    <SourceProperties RasterXSize="360" RasterYSize="180" DataType="UInt16" 
BlockXSize="360" BlockYSize="180" />
   ...
  </SimpleSource>

Which is equivalent to:

gdalinfo HDF4_SDS:UNKNOWN:"DATA_WITH_4D_DATASET.hdf":7 -oo RASTERXDIM=0 -oo 
RASTERYDIM=0


Even

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