[Gdal-dev] Problem for build overviews

[Mateusz Loskot]

mouhamad mbaye wrote:
> When i replace GA_ReadOnly with GA_Update
> i have this error
>  
> Une exception non gérée du type 'System.ApplicationException' s'est
> produite dans gdal_gdal_csharp.dll
> Informations supplémentaires : The JPEG driver does not support update
> access to existing datasets.

I missed the detail explained in JPEG driver docs.
GDAL doesn't support embedded overviews for JPEG format.

For JPEG format you can only build overviews in external .ovr file:

http://www.gdal.org/frmt_jpeg.html

This is automatically handled by GDALDataset::BuildOverviews() for
JPEG driver. You can see apps/gdaladdo.cpp sources to see how it's
build. Also, using gdaladdo utility should be good for testing.

Note, .ovr files creation is a default method for drivers which do not
support embedded overviews natively.

If you're playing with JPEG2000, check note at the bottom of this page:

http://www.gdal.org/frmt_jp2ecw.html


p.s. Thanks to Andrey for teaching me about overviews support :-)

Cheers
-- 
Mateusz Loskot
http://mateusz.loskot.net