[QGIS-Developer] Caching of raster images

[Régis Haubourg]

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.
>>>
>>>
>>> _______________________________________________
>>> QGIS-Developer mailing list
>>> QGIS-Developer at lists.osgeo.org
>>> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer
>>> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer
>>
>>
>>
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-developer/attachments/20171005/42e85d4e/attachment.html>