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Thread: qglwidget overpaint

  1. #1
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    Default qglwidget overpaint

    Hi there,

    I would like to achieve what is being done in the overpaint example of QGLViewer: render a 3D scene with OpenGL and paint something *on top* of that with a QPainter. I followed the instructions on that website and did this:

    Qt Code:
    1. class OpenGLWidget: public QGLViewer
    2. {
    3. Q_OBJECT
    4.  
    5. public:
    6. OpenGLWidget(QWidget* parent=0);
    7. ~OpenGLWidget();
    8.  
    9. protected:
    10. void paintEvent(QPaintEvent *event); // override paintEvent
    11.  
    12. };
    13.  
    14. OpenGLWidget::OpenGLWidget(QWidget *parent) : QGLViewer(parent)
    15. {
    16. setAttribute(Qt::WA_NoSystemBackground);
    17. }
    18.  
    19. void OpenGLWidget::paintEvent(QPaintEvent *event)
    20. {
    21. QGLViewer::paintEvent(event); // Do what the overridden method would do.
    22.  
    23. // Paint something with QPainter.
    24. QPainter painter;
    25. painter.setBackgroundMode(Qt::TransparentMode);
    26. painter.begin(this); // This is where it happens.
    27. painter.setPen(colorUtil.penBlueThick);
    28. painter.drawLine(0, 0, 100, 100);
    29. }
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    Unfortunately this does not have the desired effect. When I begin() the painter, the background is cleared and the 3D rendering disappears. Setting the background mode to transparent, and setting the widget attribute to NoSystemBackground have no effect. Can anyone help me along please?

    Thanks
    Cruz

  2. #2
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    Default Re: qglwidget overpaint

    Good, by now I managed to make it work like this:

    Qt Code:
    1. void OpenGLWidget::paintEvent(QPaintEvent *event)
    2. {
    3. QPainter painter;
    4. painter.begin(this);
    5. painter.setRenderHint(QPainter::Antialiasing);
    6.  
    7. // Save current OpenGL state
    8. glPushAttrib(GL_ALL_ATTRIB_BITS);
    9. glMatrixMode(GL_PROJECTION);
    10. glPushMatrix();
    11. glMatrixMode(GL_MODELVIEW);
    12. glPushMatrix();
    13.  
    14. // Setup OpenGL context.
    15. setBackgroundColor(QColor(255,255,255));
    16. setForegroundColor(QColor(0,0,0));
    17. setFont(QFont("Helvetica", 18));
    18. glEnable(GL_DEPTH_TEST);
    19. glDisable(GL_LIGHTING);
    20. glEnable(GL_BLEND);
    21. glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    22.  
    23. drawOpenGL(); // draw a bunch of opengl things
    24.  
    25. // Restore OpenGL state
    26. glMatrixMode(GL_MODELVIEW);
    27. glPopMatrix();
    28. glMatrixMode(GL_PROJECTION);
    29. glPopMatrix();
    30. glPopAttrib();
    31.  
    32. drawOverPaint(&painter); // 2D QPainter drawing
    33.  
    34. painter.end();
    35. }
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    What I don't understand is, on one computer this works perfectly and the QPainter code is drawn over the OpenGL, on another computer the OpenGL is on top of the QPainter drawing and covers it fully.
    How do I have control over what gets painted over what?

  3. #3
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    Default Re: qglwidget overpaint

    QGLViewer is derived from QOpenGLWidget. See the docs for that class on how to combine QPainter and OpenGL in a single paint event:

    It is also possible to draw 2D graphics onto a QOpenGLWidget subclass using QPainter:

    - In paintGL(), instead of issuing OpenGL commands, construct a QPainter object for use on the widget.
    - Draw primitives using QPainter's member functions.
    - Direct OpenGL commands can still be issued. However, you must make sure these are enclosed by a call to the painter's beginNativePainting() and endNativePainting().
    The order in which you do this would likely affect which graphics are "on top". You may not have to do all of the state saving / restoring.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  4. The following user says thank you to d_stranz for this useful post:

    Cruz (16th May 2019)

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