[QGIS-Developer] Preview job and slow datasources

[Matthias Kuhn]

Hi Patrick,

Would be interesting to do this test with previews enabled and disabled.
Then we'll see if it's actually the previews or some other mechanism
that is causing this.

IIRC only a composite image is saved in the preview jobs and not each
layer separately, but that's just what I remember and not evidence based.

You can enable them in the python console with

    iface.mapCanvas().setPreviewJobsEnabled(True)

and disable with

    iface.mapCanvas().setPreviewJobsEnabled(False)

Thanks for a feedback

Matthias


On 11/16/2017 08:42 AM, Patrick Dunford wrote:
>
> In my current project using Qgis 2.99, turning off the rasters uses 16
> GB, turning them on uses 52 GB
>
> I do not believe it is independent from the number of layers that are
> being displayed.
>
> Previous versions of the software did not cache every single raster
> (the number of rasters actually being displayed on the canvas at any
> one time is a small fraction of the total number in the project)
>
> On 16/11/17 20:11, Matthias Kuhn wrote:
>>
>> Hi Patrick,
>>
>> This uses some memory (~ canvas width pixels * canvas height pixels *
>> 8 preview images * 32 bit RGBA), so let's assume 50 MB to 100 MB.
>>
>> This consumption is independent from the number or type of layers.
>>
>> Matthias
>>
>>
>> On 11/16/2017 08:02 AM, Patrick Dunford wrote:
>>>
>>> So to put it another way this is the reason why Qgis wants to use a
>>> huge amount of memory (40 GB) when I have a lot of raster images
>>> loaded in the background.
>>>
>>>
>>> On 16/11/17 18:56, Tim Sutton wrote:
>>>> Hi
>>>>
>>>>> On 16 Nov 2017, at 04:35, Patrick Dunford <enzedrailmaps at gmail.com
>>>>> <mailto:enzedrailmaps at gmail.com>> wrote:
>>>>>
>>>>> What is this "preview job" function?
>>>>>
>>>>>
>>>>
>>>> Its application logic to prefetch / pre-render offscreen content in
>>>> anticipation of user panning the map.
>>>>
>>>> Regards
>>>>
>>>> Tim
>>>>>
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>>>> —
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> *Tim Sutton*
>>>>
>>>> *Co-founder:* Kartoza
>>>> *Project chair:* QGIS.org <http://QGIS.org>
>>>>
>>>> Visit http://kartoza.com <http://kartoza.com/> to find out about
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>>>>
>>>
>>>
>>>
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