here it is: http://www.gdal.org/gdal_vrttut.html
you can assemble raster of vector files, apply filters, reprojections, build pyramids to avoid fetching the lowest resolution data. This is pretty efficient. Cheers Régis 2017-10-05 14:10 GMT+02:00 Patrick Dunford <blackwhite...@gmail.com>: > I don't know what this is. > > On 06/10/17 01:08, Régis Haubourg wrote: > > Hi Patrick , > did you consider using GDAL VRT to avoid opening so much files? > In my experience, this works well. > Régis > > > 2017-10-05 13:48 GMT+02:00 Patrick Dunford <blackwhite...@gmail.com>: > >> Some time ago in a discussion of a particular bug a contributor expressed >> concern that the refresh of background rasters (aerial photography) in >> Windows was too slow. >> >> Maybe this is the reason that recent versions of master appear to be >> loading all of the background imagery into memory (I use a master from >> January this year to work around issues with later ones, and that master >> does not have this feature). >> >> Unfortunately if there are a lot of rasters then the memory demand is >> excessive and unsurprisingly slows down the computer negating any purported >> benefit of caching. >> >> As an example a project I am currently working on has about 900 aerial >> photo images (GeoJpeg). When the layer is turned on for display, Qgis >> requires about 46 GB of virtual memory. Since my computer only has 24 GB of >> physical memory, it is required to dip into the swap space considerably. >> Even with 60 GB of swap space on an SSD, the swapping needed to refresh the >> canvas is substantial and dramatically reduces performance resulting in >> substantial delays. Compare with the January master referred to above which >> only requires about 7 GB of virtual memory total with the aerial photo >> layer displayed. The time needed to refresh the canvas is less than 1 >> second, most of the time. >> >> I know that the canvas refresh in Windows with aerial photos can be >> substantially slower than in Linux. This does not affect me, because I >> don't use Windows now that I have a stable platform for running an older >> Linux master alongside the most recent one. What I do know is that the >> memory demands are making it difficult to evaluate the recent masters. I >> need some kind of setting to turn this caching off. With the aerial photo >> layer turned off, the memory usage of the current master is about the same >> as the old one, and it's much quicker to update. >> >> >> _______________________________________________ >> QGIS-Developer mailing list >> QGIS-Developer@lists.osgeo.org >> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer >> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer > > > >
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