[gdal-dev] gdal & WMS

Ming Chin mingchin.st at gmail.com
Thu Jun 17 11:02:29 PDT 2021

Hello Even,
thank you for your input.
I will review the raster API as you have suggested.  However, as I
understand the requirements of my task, I need to support the vector API as
well, because we do need to support vector layers.
Can you, or anyone else here, point to a recommended guide/tutorial to
implementing gdal wms vector layers (in c++)?

Thank you,

On Thu, 17 Jun 2021 at 08:47, Even Rouault <even.rouault at spatialys.com>

> Ming,
> A OGRLayer is a *vector* layer, not a raster one. So you should rather
> look at the raster API: https://gdal.org/tutorials/raster_api_tut.html
> Depending on the kind of URL you provide to GDAL, you may get directly a
> workable dataset, or just a placeholder that links to real datasets with
> the subdataset (which can be seen as a king of raster layer) mechanism,
> using GDALDataset::GetMetadata("SUBDATASETS") to get the list of
> subdatasets, and then pass to GDALOpen() the value of one of the
> Even
> Le 17/06/2021 à 17:41, Ming Chin a écrit :
> Hello GDAL devs,
> I am a GDAL rookie, and I am trying to add WMS support to an existing GDAL
> (2.2.4) implementation.
> I saw in this example:
> https://gis.stackexchange.com/questions/67416/using-gdal-to-connect-to-a-wfs
> that its possible to provide OGRSFDriverRegistrar::Open(sURL,  FALSE );
> with a url and use it to open an OGRDataSource.
> And in other examples I see that OGRSFDriverRegistrar::Open() has been
> deprecated and that GDALOpen() should be used instead.
> So I did that, adding cURL to our GDAL build, and then in my
> implementation, I provided GDALOpen() with a URL like this:
> GDALDataset *pDataset = (GDALDataset*)::GDALOpen(sURL, GA_ReadOnly);
> if( pDataset  )
> {
>     int nLayers =  pDataset->GetLayerCount();
>     OGRLayer *pLayer =  pDataset->GetLayerByName(sLayerName);
> }
> This seems to partially work, in the sense that the  GDALOpen function
> returns a non-NULL pointer to a GDALDataset, but when I try to call the
> member functions of the resulting GDALDataset, everything seems to return
> zeros or nulls;
> The above call returns nLayers = 0, and  pLayer=NULL
> but if I feed the same sURL into my web browser (Chrome), the browser
> receives a properly formatted xml file that clearly has <Layer>(s) defined
> in it.
> What am I missing?  Why am I unable to retrieve useful data from the
> dataset?
> Ming
> _______________________________________________
> gdal-dev mailing listgdal-dev at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/gdal-dev
> -- http://www.spatialys.com
> My software is free, but my time generally not.
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