Fully offscreen QML rendering

skinkie · 11 Dec 2023, 06:41

I am using a slightly modified version of qml_movie_renderer which gives the ability to render QML-output into separate images. I have noticed that this, in one way or the other, requires to have a platform such as eglfs available. Which indirectly requires a monitor to be connected. I am interested in the following things.

Is it possible (like with a raspberry pi) to specify in the Linux cmdline directly that a certain HDMI-output must be always available, even when no screen is connected, to prevent the "No screens available" errors. Something like a non-hardware based dummy output would do too.
The movierenderer code fails on a segmentation fault after QOpenGLFramebufferObject not being available. This is true for platform vnc as well. Is there a way to have a fully software backend QOpenGLFramebufferObject not dependent on hardware?

sierdzio · 17 Dec 2023, 06:49

@skinkie You can try running your app with -platform offscreen command line argument. Not sure if it will fix the issue, but it is the simplest to try :-)

I don't know about command line, but you can search online and buy a "dummy monitor" device (also called monitor emulator), they are available for HDMI and not very expensive. You just plug it in and your Raspberry will think a monitor is connected.
I think it should be possible with llvm-pipe, yes. MESA does have software-based OpenGL renderer. I don't know how to do it, though. But if you are using Raspberry Pi, there is no need for it, as it does have a GPU and good OpenGL drivers.

skinkie · wrote on 17 Dec 2023, 06:49

@sierdzio the "-platform offscreen" option gives the same crash on the specific PC. The issue does not happen on a RaspberryPi since it allows for the "hotplug" option, and likely the proper drivers, I am trying to achieve that on a regular PC.

Thread 1 "renderzmq" received signal SIGSEGV, Segmentation fault.
0x00007fffeed8900a in QOpenGLContext::format() const () from /usr/lib/libQt5Gui.so.5
(gdb) bt
#0  0x00007fffeed8900a in QOpenGLContext::format() const () at /usr/lib/libQt5Gui.so.5
#1  0x00007fffeed89089 in QOpenGLContext::isOpenGLES() const () at /usr/lib/libQt5Gui.so.5
#2  0x00007fffef08110f in QOpenGLFramebufferObject::QOpenGLFramebufferObject(QSize const&, QOpenGLFramebufferObject::Attachment, unsigned int, unsigned int) () at /usr/lib/libQt5Gui.so.5
#3  0x0000555555566099 in MovieRenderer::createFbo() (this=0x5555555e31d0) at /home/skinkie/render-zmq/renderzmq/movierenderer.cpp:196
#4  0x0000555555565eac in MovieRenderer::start() (this=0x5555555e31d0) at /home/skinkie/render-zmq/renderzmq/movierenderer.cpp:166
#5  0x000055555555bd9d in MovieRenderer::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=0x5555555e31d0, _c=QMetaObject::InvokeMetaMethod, _id=4, _a=0x7fffffffbe50)
    at /home/skinkie/render-zmq/build-renderzmq-Desktop-Debug/renderzmq_autogen/EWIEGA46WW/moc_movierenderer.cpp:129
#6  0x00007fffee2d1253 in  () at /usr/lib/libQt5Core.so.5
#7  0x00007fffee837305 in QQmlVMEMetaObject::metaCall(QObject*, QMetaObject::Call, int, void**) () at /usr/lib/libQt5Qml.so.5
#8  0x00007fffee87ce04 in  () at /usr/lib/libQt5Qml.so.5
#9  0x00007fffee77e006 in  () at /usr/lib/libQt5Qml.so.5
#10 0x00007fffee780036 in QV4::QObjectMethod::callInternal(QV4::Value const*, QV4::Value const*, int) const () at /usr/lib/libQt5Qml.so.5
#11 0x00007fffee7903b7 in  () at /usr/lib/libQt5Qml.so.5
#12 0x00007fffee793d74 in  () at /usr/lib/libQt5Qml.so.5
#13 0x00007fffee737100 in  () at /usr/lib/libQt5Qml.so.5
#14 0x00007fffee7903b7 in  () at /usr/lib/libQt5Qml.so.5
#15 0x00007fffee793d74 in  () at /usr/lib/libQt5Qml.so.5
#16 0x00007fffee7372fa in QV4::Function::call(QV4::Value const*, QV4::Value const*, int, QV4::ExecutionContext const*) () at /usr/lib/libQt5Qml.so.5
#17 0x00007fffee89837d in QQmlJavaScriptExpression::evaluate(QV4::CallData*, bool*) () at /usr/lib/libQt5Qml.so.5
#18 0x00007fffee84f541 in QQmlBoundSignalExpression::evaluate(void**) () at /usr/lib/libQt5Qml.so.5
#19 0x00007fffee84fc2c in  () at /usr/lib/libQt5Qml.so.5
#20 0x00007fffee87cd25 in QQmlNotifier::emitNotify(QQmlNotifierEndpoint*, void**) () at /usr/lib/libQt5Qml.so.5
#21 0x00007fffee2d0980 in  () at /usr/lib/libQt5Core.so.5
#22 0x00007fffee8e1e79 in QQmlTimer::event(QEvent*) () at /usr/lib/libQt5Qml.so.5
#23 0x00005555555619ed in RenderApp::notify(QObject*, QEvent*) (this=0x7fffffffe900, obj=0x555555625970, event=0x55555578b9c0)
    at /home/skinkie/render-zmq/build-renderzmq-Desktop-Debug/renderzmq_autogen/EWIEGA46WW/../../../renderzmq/renderapp.h:25
#24 0x00007fffee29c168 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#25 0x00007fffee2a10cb in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /usr/lib/libQt5Core.so.5
#26 0x00007fffee2e7138 in  () at /usr/lib/libQt5Core.so.5
#27 0x00007fffe7d34f69 in  () at /usr/lib/libglib-2.0.so.0
#28 0x00007fffe7d93367 in  () at /usr/lib/libglib-2.0.so.0
#29 0x00007fffe7d33162 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#30 0x00007fffee2eaf7c in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#31 0x00007fffee29ae74 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#32 0x00007fffee29c313 in QCoreApplication::exec() () at /usr/lib/libQt5Core.so.5
#33 0x00005555555652d8 in main(int, char**) (argc=1, argv=0x7fffffffea48) at /home/skinkie/render-zmq/renderzmq/main.cpp:59
(gdb) up
#1  0x00007fffeed89089 in QOpenGLContext::isOpenGLES() const () from /usr/lib/libQt5Gui.so.5
(gdb) up
#2  0x00007fffef08110f in QOpenGLFramebufferObject::QOpenGLFramebufferObject(QSize const&, QOpenGLFramebufferObject::Attachment, unsigned int, unsigned int) () from /usr/lib/libQt5Gui.so.5
(gdb) up
#3  0x0000555555566099 in MovieRenderer::createFbo (this=0x5555555e31d0) at /home/skinkie/render-zmq/renderzmq/movierenderer.cpp:196
196	    m_fbo = new QOpenGLFramebufferObject(m_size * m_dpr, QOpenGLFramebufferObject::CombinedDepthStencil);
(gdb) list
191	    destroyFbo();
192	}
193	
194	void MovieRenderer::createFbo()
195	{
196	    m_fbo = new QOpenGLFramebufferObject(m_size * m_dpr, QOpenGLFramebufferObject::CombinedDepthStencil);
197	    m_quickWindow->setRenderTarget(m_fbo);
198	}
199	
200	void MovieRenderer::destroyFbo()

I have also tried setting export LIBGL_ALWAYS_SOFTWARE=1. And QCoreApplication::setAttribute(Qt::AA_UseSoftwareOpenGL); in the main function of the application.

Now the obvious question is EGL available? According to eglinfo there is. But LIBGL_DEBUG=verbose does not give output either.

skinkie · wrote on 26 Dec 2023, 18:59

@skinkie said in Fully offscreen QML rendering:

Is it possible (like with a raspberry pi) to specify in the Linux cmdline directly that a certain HDMI-output must be always available, even when no screen is connected, to prevent the "No screens available" errors. Something like a non-hardware based dummy output would do too.

I can answer the first question. You can specify video=HDMI-A-1:1280x720-24@50D to force the digital output on (notice the D in the end, e will work too). My current system makes that interlaced output, so it has some particularities.

If you want interlacing in Qt, on EGLFS that would still need this bug+patch to be applied. Sadly...

For question two. Still stumbling.