Am 20.06.22 um 10:08 schrieb Mike Krus:

On 20 Jun 2022, at 08:37, Yves Maurischat <> wrote:

Hi all,

I've got a C++/QWidgets application with 2 Qt3DWindow embedded. The 3D windows 
can be hidden (i.e. the user switches to another view), while work is done in 
other parts of the application. Whenever the user switches back to the view 
with the embedded Qt3DWindow there are a lot of changes to be made to the 

Rendering is set to OnDemand, but whenever the view is switched back to the 3D 
windows there is a big lag, up to several seconds. Profiling the application 
has shown that a lot of work is done for rendering at that point because the 
scenegraph is changing (most time is spent in the material classes).

My working theory is, that each change to the scenegraph (i.e. removing 
entities, adding a new ones) causes a separate render update, which leads to 
the lag due to the amount of separate changes.
that is not correct, updates are batched on a per-frame basis. If you’re not 
scheduling frames, no updates will be made.

If you are using Qt version 5.15.0 or later this should be pretty fast. Prior 
version had a slower update mechanism.

I'm on Qt 5.14.2 for the application, but this is a reason to switch to Qt 5.15.x

One thing to keep in mind though is that changing materials will lead shaders 
needing to be recompiled, potentially textures to be updates, etc. Changing 
geometry means bounding volumes need updating, buffers need to be uploaded, etc.
So the time might just be due to the GL state needing to be updated. You should 
take care to minimise such changes.

I see. Currently large parts of the scenegraph are removed for an update but I guess I could optimize changes so only the absolute necessary parts are modified. Thanks!

Another thing to note, if you have 2 Qt3DWindows, then you have 2 underlying 
Qt3DEngine instances. These do not share resources so everything might be done 

Of course it’s impossible to provide accurate feedback without profiling the 
actual code.


So my question is: Is there a way to disable all render updates until the scenegraph has 
been updated/modified completely? What would be the "correct" way to make such 
changes? Is there  something similar to the beginResetModel() and endResetModel() methods 
in QAbstractItemModel?



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