[gdal-dev] GTiff optimisation

Yves Jacolin yjacolin at free.fr
Wed Aug 31 09:58:26 EDT 2011

Hello Even,

Le mercredi 31 août 2011 14:38:53, Even Rouault a écrit :
> Selon Yves Jacolin <yjacolin at free.fr>:
> > List,
> > 
> > I successfully assigned my projection information but still have some
> > difference in the file size. This is not so important but I wanted to
> > understand
> > the reason. Idea welcome :)
> Yves,
> You have a difference between the input file and the output file produced
> with gdal_translate (not gdalwarp) with identical compression method and
> NBITS value, right ? [..]

hum my apologies, I mixed my files from gdalwarp and gdal_translate or there is 
an improvement for gdal_transalte from GDAL 1.6.0 (my desktop) to GDAL 1.8.0 
(on my server).

> > Another question: I have 2 000 ECW files (40 Mo each), 49 ECW files (1 Go
> > each)
> > and 2 ECW files (2 Go each). I worked first on 2 Go ECW file, it takes
> > around 1
> > day to assign the projection information in the files.
> > 
> > Do you think this is normal? How can I improve this?
> It sounds particularly slow indeed, even if it isn't totally unexexpected,
> gdal_translate being a format agnostic tool, and when doing ECW -> ECW
> transformation, it will decompress and recompress the data, which is not
> ideal from a speed point of view and, in the case of lossy compression
> methods, from the data integrity point of view. But a day for 2 ECW files
> is admitedly slow. You might hit that ticket about ECW -> ECW slowness :
> http://trac.osgeo.org/gdal/ticket/3454 . It contains a possible workaround
> (altough you will find it quite ironic if you keep in memory what has been
> said previously in this thread about gdalwarp being slower than
> gdal_translate ;-)). Another solution if you only want to correct the
> projection information is to use the "ECW Header Editor". I have never
> used that tool myself, but a search in your favorite engine should bring
> some results. However it might not be appropriate for batch processing.

Interesting, I will test with gdalwarp soon. I don't know if it could be a 
problem but I am using VRT format. I saw in one of your comment in GeoRezo 
forum that it could be faster thant a direct ECW to ECW conversion. Am I 

I also find this script http://jrepetto.free.fr/ecwhed/ which seems usefull, 
but I have not test it yet.

"ECW Header Editor" doesn't work on Linux or in batch mode :/


Yves Jacolin


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