[gdal-dev] Default GeoTransform for MEM driver

[Even Rouault]

Le mardi 10 décembre 2013 16:48:31, Ivan Lucena a écrit :
> Hi Even,
> 
> > From: even.rouault at mines-paris.org
> > To: gdal-dev at lists.osgeo.org
> > Subject: Re: [gdal-dev] Default GeoTransform for MEM driver
> > Date: Mon, 9 Dec 2013 20:54:35 +0100
> > CC: lucena_ivan at hotmail.com; tkeitt at utexas.edu; jukka.rahkonen at mmmtike.fi
> > 
> > Le lundi 09 décembre 2013 18:08:19, Ivan Lucena a écrit :
> > > The other problem with that documentation is the return code. CE_Failue
> > > means that the driver does not support it and/or that the dataset
> > > doesn't have it.
> > 
> > The current behaviour of the MEM driver doesn't particularly shock me. It
> > seems OK to return CE_Failure if SetGeoTransform() hasn't been
> > explicitely called to define it. And as far as the default geotransform,
> > it is a matter of taste. [ 0, 1, 0, 0, 0, 1] would be indeed a bit more
> > classical for a ungeoreferenced image, but one can consider that if
> > GetGeoTransform() returns CE_Failure, the returned geotransform is to be
> > ignored.
> 
> That is good to know. So it really doesn't matter if the driver support
> geotransformation or not.
> 
> But my concern is when it returns CE_None. What does it really means
> because I was checking some driver's implementation of GetGeoTransform and
> I noticed that some of then always return CE_None no matter what. 
> So would
> that be possible that a dataset is *ungeoreferenced* and a driver returns
> CE_None with the default values [0, 1, 0, 0, 0, 1].
> Should we trust that
> those values [0, 1, 0, 0, 0, 1] are correct or are they just a default
> filler?
> 
> For example, to get [0, 1, 0, 0, 0, 1] we should expect a world-file that
> should be something like:
> 
> 1.0000000000
> 0.0000000000
> 0.0000000000
> 1.0000000000
> 0.5000000000
> 0.5000000000
> 
> Not:
> 
> 1.0000000000
> 0.0000000000
> 0.0000000000
> 1.0000000000
> 0.0000000000
> 0.0000000000
> 
> Anyway, I will trust that the CE_None means that the geotransformation
> values are correct but should I will also check if the values are equal to
> [0, 1, 0, 0, 0, 1] just in case? And what to do if they are?

I'm not sure to know the ultimate answer to your questions. My understanding 
of http://www.gdal.org/classGDALDataset.html#af9593cc241e7d140f5f3c4798a43a668 
is that when there's no projection space associated to a raster, it should 
return (0,1,0,0,0,1) and CE_Failure. But they are certainly drivers that 
return (0,1,0,0,0,1) and CE_None. They should likely be fixed to return 
CE_Failure instead. But you can also test if the geotransform is (0,1,0,0,0,1) 
since it isn't a likely real geotransform.

Even

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