[Qgis-psc] 2019 grant report: Fix problems in larger 3D scenes

[Martin Dobias]

Hi all

I am pleased to announce that my 2019 grant proposal "Fix problems in
larger 3D scenes" has been finally implemented and merged in time for
3.12 release.

To summarize the work, the focus was on two issues within 3D map
views. The first one was that map tiles were only being prepared using
a single CPU core - this is now fixed and we may use multiple CPUs to
load tiles of 3D scenes faster. The other (and greater) problem was
that data from vector layers (when they have 3D renderer assigned)
were all being prepared at once for the whole layer in the main
thread. That resulted in possibly long freeze of the whole user
interface while data were being loaded. This is now resolved as well
and data from vector layers are being loaded in smaller tiles in
background threads (and using multiple CPU cores). As a result, the
overall user experience is now much  smoother.

Many thanks for supporting this proposal and also big thanks to
everyone who helped with reviews, testing and provided feedback. I
believe this is one of the things that take us from the point where 3D
map view is just a toy that works with small scenes to the point where
the 3D view may be used for serious work with larger amounts of data.

Relevant pull requests:
- https://github.com/qgis/QGIS/pull/32737
- https://github.com/qgis/QGIS/pull/33480

Regards
Martin