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<p>I probably don't have the technical means to use that.<br>
</p>
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<div class="moz-cite-prefix">On 06/10/17 01:12, Régis Haubourg
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABgOYCf2mMMFBG0u9RmLjYVEvboDR7suE4nGRH=dHE+=95TJSg@mail.gmail.com">
<div dir="ltr">here it is: <a
href="http://www.gdal.org/gdal_vrttut.html"
moz-do-not-send="true">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>
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<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"
moz-do-not-send="true">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>
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<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" moz-do-not-send="true">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>
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