<div dir="ltr">here it is: <a href="http://www.gdal.org/gdal_vrttut.html">http://www.gdal.org/gdal_vrttut.html</a><div><br></div><div>you can assemble raster of vector files, apply filters, reprojections, build pyramids to avoid fetching the lowest resolution data. This is pretty efficient. </div><div><br></div><div>Cheers</div><div>Régis</div></div><div class="gmail_extra"><br><div class="gmail_quote">2017-10-05 14:10 GMT+02:00 Patrick Dunford <span dir="ltr"><<a href="mailto:blackwhitekez@gmail.com" target="_blank">blackwhitekez@gmail.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>I don't know what this is.<br>
</p><div><div class="h5">
<br>
<div class="m_-8794353157527591599moz-cite-prefix">On 06/10/17 01:08, Régis Haubourg
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Hi Patrick ,
<div> did you consider using GDAL VRT to avoid opening so much
files? </div>
<div>In my experience, this works well. </div>
<div>Régis</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2017-10-05 13:48 GMT+02:00 Patrick
Dunford <span dir="ltr"><<a href="mailto:blackwhitekez@gmail.com" target="_blank">blackwhitekez@gmail.com</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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.<br>
<br>
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).<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
<br>
______________________________<wbr>_________________<br>
QGIS-Developer mailing list<br>
<a href="mailto:QGIS-Developer@lists.osgeo.org" target="_blank">QGIS-Developer@lists.osgeo.org</a><br>
List info: <a href="https://lists.osgeo.org/mailman/listinfo/qgis-developer" rel="noreferrer" target="_blank">https://lists.osgeo.org/mailma<wbr>n/listinfo/qgis-developer</a><br>
Unsubscribe: <a href="https://lists.osgeo.org/mailman/listinfo/qgis-developer" rel="noreferrer" target="_blank">https://lists.osgeo.org/mailma<wbr>n/listinfo/qgis-developer</a></blockquote>
</div>
<br>
</div>
</blockquote>
<br>
</div></div></div>
</blockquote></div><br></div>