[QGIS-Developer] Caching of raster images

[Patrick Dunford]

I probably don't have the technical means to use that.


On 06/10/17 01:12, Régis Haubourg wrote:
> 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 
> <mailto: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 <mailto: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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