[Interest] QtQuick over Qt3D (Qt 5.15)

Paul Lemire paul.lemire at kdab.com
Thu Apr 1 07:40:47 CEST 2021


It sounds to me like Scene2D is what you want as it renders a QtQuick 
scene into an offscreen texture which can then be applied to a Qt 3D 
mesh. Unfortunately it can only be used from QML.


That being said, nothing is stopping you from having 90% of the Qt3D 
code in C++ and just having a small part in QML that essentially only 
instantiates your 3D Scene and the Scene2D and provides the 3D scene 
with the Scene2D texture. I think that would be the easiest.

If you want to do it purely in C++, another approach (but more complex 
to set up and that would have to be tested) is to use a QSharedGLTexture 
to tell Qt3D about an existing OpenGL texture by textureId. This means 
using the Qt shared OpenGL context, and manually rolling your own 
QQuickRenderControl to render QtQuick into a QOpenGLTexture, and then 
providing the QOpenGLTexture's id to the QSharedGLTexture. You'll also 
need to ensure proper synching using fences (QWaitFence/QSetFence) to 
ensure that Qt3D is not trying to render while the the shared texture is 
being updated. This would be more or less similar to one of the manual 
test we have in Qt3D 

I hope that helps,


On 3/29/21 12:28 PM, Konstantin Shegunov wrote:
> Hello,
> I want to overlay a QtQuick UI over a Qt3D scene I'm having, however 
> (here comes the "but") I want to do it from C++. I don't want to get 
> into instantiating Scene3D at all, I want to drive the Quick scene 
> from Qt3D, not vice versa (i.e. I don't want to render the 3D content 
> to an FBO, and I don't intend to depend on the QML engine to setup the 
> Qt3D parts). Sound(ed) like a simple-enough task at first, but I'm 
> having the worst time figuring it out.
> To start, I've set the Qt3D things up and I can show my mesh. For the 
> second part, I read a lot of the sources around Scene3D, Scene2D and 
> Qt3D examples but it doesn't appear this "obvious" thing to do is 
> covered anywhere, notwithstanding half the Qt3D examples being broken 
> (either segfaulting or not building at all).
> I was initially intending to get some ideas from Scene3D, but it 
> appears it works directly with the private classes for one, and for 
> two it forces synchronous rendering that's driven from the quick scene 
> ... and as a typical QtQuick item it doesn't expose a public C++ class 
> ...
> If I missed something there, please feel welcome to correct me.
> So after the next batch of sifting through Qt3D's own internals, the 
> renderer plugin and such, I am relatively convinced the only 
> reasonable way to do this is to render the UI offscreen to a texture 
> and to just slap that texture on the screen on each Qt3D frame. Is 
> this correct? If that's so, let's say I can render the quick scene to 
> a texture, how do I go about putting it on the screen after the 
> framegraph's been executed? I couldn't see a framegraph node that'd do 
> that for me. Am I to write my own? Is it the case that I need to 
> create a "fake" quad (i.e. a billboard) that I must reorient together 
> with the camera? I saw in one of Florian Blume's repositories that he 
> uses a second camera and another branch in the framegraph to put a 
> background image in. Is this the way to do it, or is there some better 
> way?
> Additional question(s):
> Do I spin my thread (which I sync manually) for rendering the quick scene?
> Thanks for any and all pointers.
> Kind regards,
> Konstantin.
> _______________________________________________
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> Interest at qt-project.org
> https://lists.qt-project.org/listinfo/interest

Paul Lemire | paul.lemire at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel: France +33 (0)4 90 84 08 53, http://www.kdab.fr
KDAB - The Qt, C++ and OpenGL Experts

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