QGLWidget for graphics and QT for GUI

5
votes

I am trying to implement a custom OpenGL scene (wrapped in a QGLWidget), but still use QT controls (such as slide bars and buttons). I tried to make my main widget a QGLWidget, which has QWidgets as children. The child widgets seem to work (when I click a pulldown menu, its options appear), but the widget fails to draw (I see a white square). I changed the base class of my children widgets from QWidget to QGLWidget, and now QPainter calls work, but QT GUI stuff is still not displaying...

Some claim that a QGLWidget can not have QWidgets as children... I am not sure about that and I'm not willing to give up. By the way, I am using Visualization Library to draw the OpenGL scene, but it is just an OpenGL wrapper...

Any clues where the problem might be? Also, key events stop being processed for some reason when I add subwidgets to the GQLWidget.

Update: I tried various combinations of widgets and layouts. It seems that the QGLWidget just gets drawn on top of anything. I even tried with raise() to arrange Z-depth of the widgets, to no avail. Is overlayGL() the only way to draw on top of an OpenGL widget?

Update 2 After months of trying, I figured out it is something related to QT. Whenever a QGLWidget is drawn on top of another QGLWidget, the first's background() function is not called. So a button is there and can be clicked, but it's not drawn. My workaround was to draw everything myself using QPainter - that works. What I found curious is, one has to use QPainter::startNativePainting() in order to draw with it. One would expect that they would overload start() in QPainter to call startNativePainting() whenever a QGLWidget is the painting device...

Another (maybe useful) fact is that QPainter calls CHANGE the Opengl context. So if you have another tool in cooperation with QPainter (in my case Visualization Library), chances are that one of them will crash (in my case VL which checks if the context state has been cleared before each frame).

4
qtopengl
Can you not create a non-GL widget as the main window, and a GL widget as it's child? Therefore, the other controls you want in your window will be siblings and cousins of the GL window rather than children. – Nicol Bolas
I tried that first - it fails complaining about some GL_SRC_ALPHA issues. The QGLWidget is constructed by VLlink and is very complex and therefore hard to debug... – Violin Yanev
Are you trying to have QWidget derived objects floating over the top of a QGLWidget? – cmannett85
That's exactly what I'm trying to do! – Violin Yanev

4 Answers

2
votes

There is a labs example of rendering Qt widgets using opengl it's a little old and I haven't tried it with the improved qglwidget in 4.8

There is also an issue with drawing over the top of a fullscreen QGLwidget - it was supposedly a bug in NVidia's openGL driver. It doesn't seem to have been resolved https://bugreports.qt-project.org/browse/QTBUG-7556

1
votes

If I were you I would use the QGLWidget as a viewport for a QGraphicsScene, then you can draw your widgets using a QGraphicsProxyWidget. That way you get a proper OpenGL viewport with the ability to put widgets in and manipulate them easily.

0
votes

Did u try to call update() explicitly? I find that sometimes QGLWidget update the display stuff kind of slow.

0
votes

This is an old question, but I recently had quite a bit of trouble with this, and this is what helped me -

Use QML/QT Quick. QML is a javascript-like language that allows you to layout your widget elements on a window and do basic processing on events like mouse clicks. There are a few other cool features, but the one that is most relevant to this question is that you can import qt widgets from C++ code.

So you can make a custom QGLWidget, import it into the QML window, and set the size/position/stick widgets on top of it in whatever you want, very easily. QT's Scenegraph example does a great job explaining this. Scenegraph's code is also included as an example in at least Qt Creator 3.2.1 Open Source (that's the version I have anyway)