[Interest] Few questions regarding Qt Quick Scene Graph

[Artem Fedoskin]

1. I have experimented a little with the Scene Graph and got following
results:

   1. Rectangles with the same color were batched
   2. Differently colored rectangles weren't batched
   3. Textures were batched if the QQuickWindow::TextureCanUseAtlas flag
   was set but only if QSG_ATLAS_WIDTH and QSG_ATLAS_HEIGHT were big enough.

2. I also found one of your E-mails here http://lists.qt-project.org/
pipermail/interest/2014-January/010674.html where you suggested to use
GL_MAX_TEXTURE_SIZE
for setting atlas width and height, however, it didn't work when I set it
as a value for environmental variable QSG_ATLAS_WIDTH and QSG_ATLAS_HEIGHT.

3. If I'm going to create say 100 000 objects and change X and Y
coordinates with setX() setY() funcitions and at the same time set some
objects to invisible state with setVisible(), can signals emitted from
calling this setters cause significant drop in performance if I'm not
connecting any slots on my side?

4. How setX() and setY() changes the position of the QQuickItem? I didn't
noticed any calls to updatePaintNode() and is it better to set coordinates
this way then directly changing coordinates of graphic nodes in
updatePaintNode() and constantly call update()?

Sorry for so much questions, but my project has to display and process a
lot of items on the screen and it has to work good on mobile platforms.

Thank you very much for all your advices and suggestions.

Regards,

Artem Fedoskin

2016-04-26 9:19 GMT+02:00 Gunnar Sletta <gunnar at sletta.org>:

>
> > On 25 Apr 2016, at 19:46, Artem Fedoskin <afedoskin3 at gmail.com> wrote:
> >
> > Thank you for your reply Gunnar,
> >
> > Can you please tell me - can I store a pointer to the object of external
> class (even not a QQuickItem derived one) in QSGNode derived class? Each
> object of the class Data is associated with corresponding Triangle in View.
> Whenever View is updated, it checks in loop Data objects on special
> condition and based on that sets the corresponding Triangle to visible or
> invisible state. If Triangle is visible, Triangle itself calls a bunch of
> functions from external classes and uses pointer on object Data to retrieve
> Data's info about coordinates then accesses function from another class to
> convert the received coordinates to x,y positions.
> >
> > I understand that it is not safe to access QSGNode not from
> updatePaintNode(), but is it actually safe to access other classes from
> QSGNode itself? How far can I go in QSGNode in accessing things in GUI
> thread?
>
> Normal threading considerations apply. You can reference anything to your
> liking from objects that live on the render thread, there is nothing
> special about it. We just strongly encourage to not cross reference between
> GUI and render threads because the two run independently and objects can
> come and go on both.
>
> >
> > Could you please also tell me - what is the way to hide QSGNode
> explicitly like for example setting QQuickItem::visible property to false.
> I found your E-Mail here
> http://lists.qt-project.org/pipermail/interest/2015-February/015285.html
> but I'm afraid that deallocating and allocating QSGNode will take
> unnecessary overhead as the visibility of nodes in my app will be often
> turned off/on. Is it a good idea to use QSGNode::removeChildNode() for this?
>
> You would have to have an opacity node in there then and set its opacity
> to 0. That way, they would still be in the scene graph (and still
> potentiallly consume processing and resources).
>
> Alternatively you could experiment with explicitly managing the node. If
> you during updatePaintNode stored the QSGNode that is passed in and return
> 0, you will effectively have taken ownership over that subtree from the SG.
> You could at a later time pass that stored node back to the SG. If there
> are GL resources associated with it, you will need to clean those up
> according to the section aboue "Graphics Resource Handling" in
> http://doc.qt.io/qt-5/qquickitem.html#custom-scene-graph-items.
>
> > How do you think - maybe instead of allocating directly QSGNodes I
> should just subclass instance of Data from QQuickItem and set its visual
> parent to View? Is allocating 1000 QSGNodes in View significantly faster
> than creating 1000 QQuickItems with the same painting logic and then
> setting its visual parent to View?
>
> Constructing a QSGNode in C++ is orders of magnitude faster than
> constructing a QQuickItem object through QML. Compared to a QQuickItem
> constructed in C++, I would still expect it to be faster, but probably not
> by a large margin.
>
> >
> > Thank you
> >
> > Regards,
> > Artem
> >
> > 2016-04-25 9:06 GMT+02:00 Gunnar Sletta <gunnar at sletta.org>:
> >
> > > On 24 Apr 2016, at 16:49, Artem Fedoskin <afedoskin3 at gmail.com> wrote:
> > >
> > > I'm sorry. My first E-Mail was sent accidentally. Here is the final
> version of my E-Mail.
> > > Hello everybody. I have a project where a lot of objects are painted
> on the screen whenever the user interacts with the app (pan, zoom, move) I
> would be really grateful to you if you could answer a few questions
> regarding the internals of Qt Quick Scene Graph:
> > >
> > >       • Imagine that I add 1000 nodes with the same color inside
> QQuickItem::updatePaintNode() function of some QQuickItem derived class.
> Will they be drawn in batch as described in
> http://doc.qt.io/qt-5/qtquick-visualcanvas-scenegraph-renderer.html#batching
> ?
> >
> > Depends on the cirucmstances, but if they share clipping their
> QSGMaterial::compare() indicates they are all identical, they will be
> batched, yes. Verify with QSG_VISUALIZE=batches, as indicated by this same
> document.
> >
> > >       • Can I call the function of some other class in
> QQuickItem::updatePaintNode()? I have some class named Data, class named
> View and a subclass of QSGGeometryNode Triangle. Imagine that after objects
> of these classes are created the Data asks View to create 10 Triangles. The
> View creates 10 Triangles during the updatePaintNode() call and passes to
> them pointers to the object of class Data, that required their
> instantiation. The Triangles refers to the Data and get some values from it
> and converts them to x,y coordinates. Is it something possible?
> > > Do I violate the rule stated here
> http://doc.qt.io/qt-5/qtquick-visualcanvas-scenegraph.html#scene-graph-and-rendering
> "Warning: It is crucial that OpenGL operations and interaction with the
> scene graph happens exclusively on the render thread, primarily during the
> updatePaintNode() call. The rule of thumb is to only use classes with the
> "QSG" prefix inside the QQuickItem::updatePaintNode() function." ?
> >
> > The point here is really that on many systems, there will be two threads
> that are running. The GUI thread where your QQuickItems live and the render
> thread where your QSGXxx objects live. updatePaintNode() is called on the
> render thread while the GUI thread is blocked, so you are safe to read
> state out of the QQuickItem at that time and use that to
> create/update/modify/delete your QSGXxx objects accordingly. At any other
> time, the two worlds live separate lives and should have no connection.
> >
> > There are other places where interaction between the threads are also
> ok, as outlined in the QQuickItem docs, but by sticking to the rule
> outlined in the updatePaintNode() function, 99% of all usecases will be
> covered.
> >
> > For your usecase you are free to construct View, Data, x number of
> Triangles during updatePaintNode() and then store that under some subclass
> of a QSGNode. On the next call,  you can update your structure if needed.
> Your QQuickItem does not store a pointer to this QSGNode, it only exists
> inside the scene graph and during the updatePaintNode() function.
> >
> > cheers,
> > Gunnar
> >
> > >
> > > At first I was thinking about deriving classes like Triangle from
> QQuickItem and then reparenting them to the View but the direct creation of
> QSGGeometryNodes seems to me to be less memory requiring approach.
> > >
> > > Regards,
> > > Artem Fedoskin
> > >
> > > 2016-04-24 16:41 GMT+02:00 Artem Fedoskin <afedoskin3 at gmail.com>:
> > > Hello everybody. I have a project where a lot of objects are painted
> on the screen whenever the user interacts with the app (pan, zoom, move) I
> would be really grateful to you if you could answer a few questions
> regarding the internals of Qt Quick Scene Graph:
> > >
> > >       • Imagine that I add 1000 nodes with the same color inside
> QQuickItem::updatePaintNode() function of some QQuickItem derived class.
> Will they be drawn in batch as described in
> http://doc.qt.io/qt-5/qtquick-visualcanvas-scenegraph-renderer.html#batching
> ?
> > >       • Can I call the function of some other class in
> QQuickItem::updatePaintNode()? I have some class named Data, class named
> View and a subclass of QSGGeometryNode Triangle. Imagine that after objects
> of these classes are created the Data asks View to create 10 Triangles. The
> View creates 10 Triangles during the updatePaintNode() call and passes
> pointers to the object of class Data, that required their instantiation.
> The Triangles refers to the Data and get coordinates from it. Is it
> something possible. Do I violate the rule stated here "Warning: It is
> crucial that OpenGL operations and interaction with the scene graph happens
> exclusively on the render thread, primarily during the updatePaintNode()
> call. The rule of thumb is to only use classes with the "QSG" prefix inside
> the QQuickItem::updatePaintNode() function."
> > >
> > >
> > >
> > >       • For example, I want to get new coordinates of the object and
> to do that I should refer to function of some other not QSGNode derived
> object and then use these coordinates for rendering one of the nodes.
> > >
> > > _______________________________________________
> > > Interest mailing list
> > > Interest at qt-project.org
> > > http://lists.qt-project.org/mailman/listinfo/interest
> >
> >
>
>
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