[QGIS-Developer] How to speed up rendering

[Martin Dobias]

Hi

On Fri, May 26, 2023 at 11:30 PM afernandez via QGIS-Developer <
qgis-developer at lists.osgeo.org> wrote:

> Hello,
> One of my project would greatly benefit from a much faster rendering.
> After some testing, the bulk (>99%) of the time is spent at rendering the
> layer with QgsRasterLayer(...,...). What I have also noticed is that
> rendering time does not change with the number of cores, which suggests
> that the renderer might be using a single thread. I checked the API
> documentation but couldn't find anything about multithreading or some other
> procedure to accelerate rendering. I was wondering if anyone has any
> suggestion or has faced a similar difficulty.
>

Multi-threaded rendering is done at the granularity of whole map layers, so
a single layer is rendered by a single CPU core. Therefore if you have just
one layer that is slow to render, the multi-threading is not going to help.
It is best to do some testing how you can improve the layer's rendering -
for example, it could be that you are using some sub-optimal raster format,
or there are overviews missing, or something else... one would need more
details.

Regards
Martin
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