[QGIS-Developer] Caching of raster images

[Mathieu Pellerin]

You can also build your VRT using QGIS' processing toolbox.

On Oct 5, 2017 7:26 PM, "Régis Haubourg" <regis.haubourg at gmail.com> wrote:

> GDAL is probably installed on your system, you need it for QGIS :)
>
> something like this should do the trick in a linux bash or from the
> osgeo4w embedded shells (either windows or msys )
> gdalbuildvrt all_ecw.vrt *.tiff -a_srs "EPSG:yourSridHere"
> See http://geoserver.geo-solutions.it/edu/en/raster_
> data/advanced_gdal/example1.html for instance
>
>
>
>
> 2017-10-05 14:19 GMT+02:00 Patrick Dunford <blackwhitekez at gmail.com>:
>
>> 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>:
>>
>>> 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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