[gdal-dev] Default GeoTransform for MEM driver

[Tim Keitt]

On Tue, Dec 10, 2013 at 12:44 PM, Even Rouault <even.rouault at mines-paris.org
> wrote:

> 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.
>

It would make things easier if all drivers returned the same default: (0,
1, 0, 0, 0, -1) for any dataset where the transform has not been assigned.
(I reworked my test to use the assumption that spatial y and line index y
are opposite sign and it works correctly.) I think it should be considered
a bug in the docs and any driver the does not return that default.
Otherwise users have to add a lot of ugly conditional code.

THK


>
> Even
>
> --
> Geospatial professional services
> http://even.rouault.free.fr/services.html
>



-- 
http://www.keittlab.org/
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