I'm pretty new to QT's graphic view frame, and I couldn't find anything about this in the docs or on Google.
I have a GUI application that draws a representation for some data. The application itself does some work with matrices / vectors (a neural net thing) and has to represent it on a QGraphicsScene. So far so good, but I've noticed that the app segfaults & crashes sooner or later (and usually sooner) if I try to update the QGraphicsScene from another thread. The QT Docs say nothing about thread-safety & Google gives nothing. What I want (and pretty much need) to do is run the calculations & update the GUI representation accordingly, but the GUI controls etc themself have to remain responsive. As I said, my first thought was to do the whole thing in another thread, but it crashes randomly if I try to.
Is there any "accepted practice" to do this kind of thing in QT or is there some gotcha that I don't know of in the graphics view framework itself?
The Qt docs actually say quite a lot about thread safety. If the docs for QGraphicsScene don't say anything it's because they are not thread-safe, consistent with the behaviour you are seeing.
What you need to do is run your calculations in another thread and synchronise that thread with the main GUI thread as appropriate. A simple way to do this would be to set a flag in the main thread when the calculations are ready for display. That way you can call the appropriate QGraphicsScene methods in the main thread at the right time by simply checking the flag.
Related
I am trying to use an external to Qt OpenGL context using a window handle that comes from Qt. The setup is:
A QMainWindow - contains various widgets including a QWebEngineView to make some web content available (in my case it's Leaflet for render and interact with OpenStreetMaps tiles)
Panda3D engine - rendered on top of my Qt application using a window handle of the central QWidget.
The setup works...when Panda3D is set to DirectX9 (aka pandadx9). When I switch the pipeline to OpenGL (aka pandagl) I get a black screen (Panda3D's window) and a very glitch OpenGL content (Qt). The reason is simple yet beyond my ability to fix it - QWebEngineView uses OpenGL. Somehow there is a conflict on a OpenGL context level between the engine and Qt. I am looking for a way to resolve this without removing the direct interaction with Panda3D's window (in my case using ShowBase) since the engine already offers a lot in terms of features for handling mouse events that I would otherwise be forced to reimplement in Qt and pass down to the engine. In addition I am not sure if I can make Panda3D render its scene as an FBO and how to load it in a - let's say - QOpenGLWidget. Activating shared OpenGL context before initializing QApplication allows multiple OpenGL widgets to render OpenGL content.
So far I have experimented to integrate Panda3D with Qt in two ways:
run two event loops in parallel - start the Panda3D engine in a child process and then using a pipe communicate with it
run a single event loop - use the event loop of Panda3D engine to also handle the main event loop of Qt by adding a task to the engine's task manager to run QApplication::processEvents() on every cycle
In both cases I am handing over a window Id (QWidget::winId()) as the parent window of Panda3D.
CASE 1 - parallel processes, separated event loops
This resolution comes with a lot of overhead. All the communication between the Qt content (running in the parent process) needs to be sent (via pipe hence involving IPC) to the engine (running in the child process). This adds a lot of code complexity and in case of my logging (using Python's logging module with a custom logging handler that writes records into an SQLite3 database) introduces a whole lot of issues. Concurrent write access to a file between processes is a tricky thing in general and I'm definitely not an expert. This case however does not exhibit the behaviour I'm describing below. In this case however the issue with OpenGL is not present!
CASE 2 - single process, single event loop
In my opinion this is a more elegant and is what I would like to go with (if possible). An example can be found here. I use the engine's main loop to process Qt's main loop. This is due to the fact that a 3D game engine usually would need to deal with far more events for a shorter period of time (processing rendering, audio, video, filesystem access, physics and so on) then a standard Qt GUI. This is also the recommended way as described in the official documentation of the engine. The other way around (Qt's main loop handling Panda3D's) is also possible. Imho neither have anything to do with my issue namely the moment I add anything Qt-ish that uses OpenGL, the problem I've described above occurs. On Windows this is not a huge deal breaker since I can also use DirectX for the engine, while Qt does it's OpenGL thing. On Linux it is not possible (without something like wine). In addition I want to use exclusively OpenGL including GLSL.
Here is a visual representation of case 1 and case 2 (but with mixed DX9 and OpenGL):
And below there are two visual representations of case 2 with only OpenGL:
While Panda3D offers CPU rendering (aka p3tinydisplay), QWebEngineView does not. Falling back to CPU rendering on the engine side is not an option considering the huge amount of polygons I have to render not to mention the fact that I can do something more useful with the CPU (e.g. processing the physics).
Last but not least I have seen a third integration attempt, which I quickly discarded - rendering the scene as an image to RAM, reading it in Qt, generating a QPixmap from it and painting on top of a QLabel. Needless to say this is a no go for my scenario due to the heavy hit on performance among others.
Any ideas how to tackle this?
I don't think event loops have to do with anything here; the problem is that by default child windows get the same device-context (DC) as the parent. In your case that's a problem because two different components (Qt framework, and Panda3D engine) try to ChoosePixelFormat and initialize OpenGL context twice on the same DC, which is not supported.
The proper solution is to create the Panda3D engine QWidget from a QWindow with the Qt::MSWindowsOwnDC style, which corresponds to the CS_OWNDC window-class style. Normally QWidget doesn't create any window at all -- but is rather implemented entirely within the Qt framework by drawing itself on the parent window.
There is a lot of discussion (e.g. here) going on about spinning or moving busy indicators in a GUI but I could not find a single one that clearly states that it is impossible to re-paint/update any content in a GUI application while the main thread is blocked.
This question is actually a general one and is not necessarily directly related to Qt GUI applications.
When the main thread of a GUI is performing a blocking operation no events are processed and no content can be re-painted. There are two "recommended" approaches:
Using worker threads
Splitting the work in chunks and updating the UI "just not that often"
The problem is that there are operations that simply cannot be moved to worker threads or any other asynchronous mechanism. The best example is the serialization and de-serialization of the UI itself. In both scenarios the user must not click happily in the UI and change settings while the settings, properties, etc. are taken from the widgets (during the saving) or applied to the widgets (during the loading). This is true for the threading approach as well as the splitting into chunks.
Edit: Another example is the application of stylesheets to a complete GUI. Imagine that a user wants so select a "dark" scheme. All of the widgets need to be updated and the currently visible ones needs to be re-painted. During this step the event loop cannot run. A similar discussion can be found here.
In my case I'm working on an embedded device so there is another approach:
Directly overwriting a specific region of the framebuffer
This approach feels very ugly and comes with a lot of problematic scenarios and surly involves lots of debugging.
The last but sad approach:
Do not use any moving/updating content at all and just show something static such as "...in progress..."
Well, this is just sad...
Do you agree on these observations? Does anyone know of a different approach or concept in general un-related to Qt?
The problem is that there are operations that simply cannot be moved to worker threads or any other asynchronous mechanism.
I can't agree. These operations should be split into blocking and non-blocking operations. Everything that blocks should be handled asynchronously, or, if no non-blocking APIs are available, handed off to a worker thread.
The best example is the serialization and de-serialization of the UI itself.
I find it a particularly poor example mainly because I've yet to run into a need for blocking the GUI, and serialization doesn't call for it.
In both scenarios the user must not click happily in the UI and change settings while the settings, properties, etc. are saved or loaded.
Construction and destruction of the widgets should be very quick, if that's what you mean by "deserializing" the UI. Recall that the blocking I/O and long parsing has been done in another thread. Almost all Qt widgets certainly are quick to set up, and those that are not are a necessary evil that you have no choice but to live with. If you have your own widgets that do blocking operations like disk or registry access in their constructors or event handlers (plenty of such "code" exists), fix them.
If you're merely talking about setting widget values, this again is super-quick and can be done all in one batch. You will probably need a viewmodel to asynchronously interface between the view (widgets, QML view, or a QAbstractItemView) and the data source.
Disk I/O and parsing/output of the on-disk representation belongs in a separate worker. Once you create an internal representation, it can be passed to the gui thread to build the widget tree or populate the interface.
You should implement thread-safe models using QAbstractItemModel or a similar interface, and populate them in a worker thread, and let the GUI thread react to the updates in real time.
If you wish to indicate to the user that a part of the interface is not usable yet, e.g. while a model gets populated, you could do so via an overlay, making sure that you temporarily disable the focus on the widgets that are under the overlay. This is not hard and I'd find it amusing if your entire question could be reduced to "how do I make a part of the UI inaccessible while it's being modified".
The key thing missing is that the UI should handle asynchronously reacting to a model changing its state. For all I care, it could take an hour to load the data needed to fully populate the model. It doesn't mean that your application should be unresponsive. Simply make the parts of the UI that can't be interacted with inaccessible for interaction. Ensure that other parts of the application that need the configuration data are similarly either inaccessible or in a partially usable state that will asynchronously revert to full state once the configuration becomes available.
I'm porting over a OpenGL application from a win32 project to a Qt project.
In the win32 project I simply had a mainloop that kept executing the next frame as soon as the previous was finished.
In the tutorials I find about Qt and OpenGL, they're using timers to control the update of the scene. However, I'd like to render as many frames as frequent as possible - how to do that?
However, I'd like to render as many frames as frequent as possible - how to do that?
Not with Qt. Qt takes full control of the event loop thus not giving you any means of directly performing idle operations. The standard way to perform idle operations in Qt is to use a QTime with a timeout of 0.
Another possibility was using a second thread just for OpenGL operations. However to work reliably you should create a dedicated OpenGL context for that one thread.
Qt simply is not the right toolkit for those kinds of program where the primary task of the main loop is rendering pictures and input and event management are only secondary.
Try calling update() at the end of your paint handler.
That will queue up another repaint event.
The intrinsic principles of Qt and OpenGL are not about rendering as soon as possible, but it's rendering whenever we want (huge difference).
You have to ask for the system max framerate (search for samples in OpenGL i'm sorry I don't rememember where I saw that). Once you've done that, just create a timer based on this max framerate, so you'll get what you want.
However, I'd like to render as many frames as frequent as possible - how to do that?
Use QTimer with zero interval.
I'm currently writing a programme which has a function to hash a number of files in the background. I've read the Qt4 documentation a number of times over and I still can't really figure out which threading option is best for this.
http://doc.qt.io/qt-5/thread-basics.html
There's really no need to update the GUI when it's done with each file, I just don't wish to block the GUI and I really only need a single signal/slot connection upon completion. I'm thinking of extending QThread for a hashing thread. Does this sound reasonable/right?
I have this article bookmarked as it nicely illustrates the use of QThread and highlights some common misconceptions about it. Sample code available, which runs without blocking the GUI. Sample is hosted on RapidShare, but they seem to have implemented some sort of timed waiting period since I last used it.
This sounds like a good place to use the QtConcurrent::map() function. The map function can apply the same operation to a container of objects, in your case, files. Once you start the map function, you can create a QFutureWatcher and connect to its finished signal to be notified when all of the work is done.
i wan't to use Native OpenGL in the paint function of my widgets(QPainter), to improve performance.
i saw that there is function QPainter::begin/endNativePainting(), that can help me.
but i can't find examples for that...
i wanted to know if those functions are low cost, or evry use of them reduce performance?
2.can i define beginNativePainting() and endNativePainting(), in general for all the widgets i use, instead of using that in every paint function i have.
tnx for any help....
There is some basic example code right in the documentation: http://doc.qt.io/qt-4.8/qpainter.html#beginNativePainting
The functions themselves should be fairly low-cost, but calling them might still cause a noticeably overhead, because Qt has to flush its internal painting queue on the beginNativePainting() call and probably has to assume that everything is changed as soon as endNativePainting() is called.
For the second part I am not sure if I understand what you are aiming at. Basically if you have a QPainter object, you can call beginNativePainting() once. But you have to match it with an endNativePainting() call. So the usual place would be the paint() method.
Qt is using a range of OpenGL functionalities to implement its 2D painting, including custom shaders and various frame buffers. It puts OpenGL into a pretty messy state.
beginNativePainting / endNativePainting are there to allow Qt's drawing engine to save this context and retrieve it once the user is done drawing.
It would have been nice to have the xxxNativePainting methods do the contrary (i.e. automatically save and restore user configuration of OpenGL), but since Qt allows to call OpenGL primitives directly, saving the global state is nigh impossible without tons of code and potential serious performance hit.
Instead, these methods simply save Qt's internal OpenGL state and, rather than having user code start in a configuration that would be meaningless anyway (and likely to change with each new Qt release), reset OpenGL to a "neutral" state.
It means that, inside a begin/end section, you will start with a clean slate: no shader linked, no vertex array, most of global parameters reset, etc.
Contrary to a simple QGLWidget / PaintGL scenario where you can afford to setup the global OpenGL state once and for all and simply call the rendering primitives each frame, you will have to restore pretty much everything just after the call to beginNativePainting (link/bind your shaders, set global parameters, select and enable various buffers, etc).
It also means that you should use native painting sparringly. Having each single widget do custom painting might soon bring your rendering to its knees.