Even Rouault <even.rouault <at> mines-paris.org> writes: > > Jukka, > > I think you could achieve what you want by adding a <Overview> element (see > http://www.gdal.org/gdal_vrttut.html ) inside the <VRTRasterBand> element. > The > dataset pointed by the <SourceFilename> in <Overview> can be a .vrt. > > So, schematically, you would have something like 100_percent.vrt : > > <VRTDataset> > <VRTRasterBand> > <Overview> > <SourceFilename relativeToVRT="1">50_percent.vrt</SourceFilename> > <SourceBand>1</SourceBand> > </Overview> > </VRTRasterBand> > </VRTDataset> > > and in 50_percent.vrt : > > <VRTDataset> > <VRTRasterBand> > <Overview> > <SourceFilename relativeToVRT="1">25_percent.vrt</SourceFilename> > <SourceBand>1</SourceBand> > </Overview> > </VRTRasterBand> > </VRTDataset>
I finally tried how it goes with VRT overviews. The result is rather well usable with QGis. The VRT file containing an overview chain opens a bit slow but once it is open it works totally transparently for the user. I had luck in my trial because all my images do not have same extents which relealed one trouble. Finland gets squeezed when zoomed far out because two images with smallest scales are wider than the others. For preventing wrong scaling I guess I should clip all overview images to suit the extents of the primary VRT. Anyway, I consider that this kind of system could be useful sometimes. Test set is downloadable from http://latuviitta.org/documents/GDAL_Scale_dependent_VRT_test.zip Archive contains 5 original tiffs, each with a different pixel size and rendering styles, and corresponding .VRT files. Unzip and open the "t0320.vrt" file with QGis and that's all. QGis and user believes it is just one map even all five originals are used one by one depending on the scale. The native SRID is EPSG:3067. -Jukka Rahkonen- _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
