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

[Andrea Peri]

Hi,

Unfortunately I was unable to test with GDB.
My skill was not enough :(

However I try other ways.

I will use a utility to test the disk activity and after put the qgis
canvas on a little portion of land covered I add the catalog.vrt.
Using this utility I can see that qggis try to open all the raster instead
of open only the single raster necessary to visualize the portion of land
in the canvas.

After this I see the xml in the catalog:

<SimpleSource>
  <SourceFilename relativeToVRT="0">F:\....\216110_2010.ecw</SourceFilename>
      <SourceBand>1</SourceBand>
      <SourceProperties RasterXSize="13900" RasterYSize="11760"
DataType="Byte" BlockXSize="13900" BlockYSize="1" />
      <SrcRect xOff="0" yOff="0" xSize="13900" ySize="11760" />
      <DstRect xOff="26320" yOff="140" xSize="13900" ySize="11760" />
    </SimpleSource>

And I notice also that the catalog don't store on every tag of a raster the
single bbox of that raster.
So qgis necessary must open all the raster to understand what raster is on
the visible portion on the canvas.

I guess this is a good reason to explain because it is so slow when in the
catalog there and many hundred of rasters.

Unfortunately I don't find any option in gdalbuildvrt to add the extension
of every raster in the catalog.

Andrea.

2012/1/31 Even Rouault <even.rouault at mines-paris.org>

> 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
> >
>
>
>


-- 
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Andrea Peri
. . . . . . . . .
qwerty àèìòù
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