Selon Frank Warmerdam <warmer...@pobox.com>: Craig,
Sorry for giving you wrong directions. Frank is obviously right. I somehow got confused by the fact that the VRT driver doesn't expose overviews, but its implementation of RasterIO() can indeed use overviews of the underlying rasters. I could check it with OpenEV by setting to 0 an entire overview level of an underlying raster. As far as VRTSourcedRasterBand is concerned, VRTSourcedRasterBand is a subclass (in C++ terms) of VRTRasterBand. VRTRasterBand has 4 subclasses : VRTSourcedRasterBand, VRTDerivedRasterBand, VRTRawRasterBand, VRTWarpedRasterBand. In the XML file, this is specified by the subClass attribute of the VRTRasterBand element. For example : <VRTRasterBand dataType="Float32" band="1" subClass="VRTDerivedRasterBand">. If the subclass attribute is not specified, its default value is VRTSourcedRasterBand. > Craig / Even, > > I think this is slightly inaccurate. The VRT driver will not expose > overviews > to the application (via GetOverviewCount()), but a downsampling RasterIO() > request to the VRT driver is in turned passed on to the underlying driver as > a downsampling RasterIO() request, and should take advantage of the overviews > available in the underlying driver at that point. > > I am not sure why the QGIS performance on the VRT is poor. It might be that > QGIS makes requests in an odd way (ie. via IReadBlock instead of IRasterIO) > or > that you are starting with a view with several mrsid files in view. But > I don't think the problem is just that the VRT driver hides the overviews. > > Best regards, > -- > ---------------------------------------+-------------------------------------- > I set the clouds in motion - turn up | Frank Warmerdam, warmer...@pobox.com > light and sound - activate the windows | http://pobox.com/~warmerdam > and watch the world go round - Rush | Geospatial Programmer for Rent > > _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
