[gdal-dev] read a catalog is slower then directly the raster ?

Dmitry Baryshnikov polimax at mail.ru
Tue Jan 31 10:10:10 EST 2012

31.01.2012 18:32, Even Rouault пишет:
> Selon Dmitry Baryshnikov<polimax at mail.ru>:
>> 31.01.2012 16:54, Andrea Peri ?????:
>>> Hi,
>>> using qgis I notice that it is more slow to open some raster from a
>>> catalog.vrt rather than opening the same raster directly.
>>> The slow is so big that when the catalog has many raster qgis take
>>> many time to open the catalog.
>>> Because qgis use gdal to read raster and also to read a catalog.
>>> I guess this is not an issue for qgis. But is a question of gdal.
>>> Before to start to do some test I like to know if there is some know
>>> issue on this.
>> I think this bug are mentioned in such tickets
>> http://trac.osgeo.org/gdal/ticket/2076 and
>> http://trac.osgeo.org/gdal/ticket/3482
>> QGIS more slow because it warp image (if it from VRT or have some
>> rotation in meta and etc.) and have no overviews. You can see if image
>> is warped in layer properties metadata tab ->  driver.
> Yes that's one possibility. A more pragmatic way would be to run qgis under
> GDB/DDD, and when it is stuck, Ctrl+C to see in which GDAL function it is
> stuck... That should give some hints.
>> Best regards,
>>       Dmitry
Hi, Even

I faced this problem before. I checked under debugger and saw that vrt 
sources were not creating overviews. So, gdal was making long RasterIO 
executions to the source raster, but not overviews. I provided a patch 
which took the overviews from origin raster and wrapped them too. May be 
this is not so elegant solution, but it works and speed up work with 
wrapped rasters considerably.

I don't thik that qgis under GDB/DDD will give any additional 
information (one will just see long RasterIO executions).

But I can still try to use GDB if you think this might help.

Best regards,

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