[QGIS-Developer] Caching of raster images

Régis Haubourg regis.haubourg at gmail.com
Thu Oct 5 05:12:59 PDT 2017


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 <blackwhitekez at 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 <blackwhitekez at 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.
>>
>>
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>
>
>
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