QGLWidget: overpainting with QPainter

Question

I would like to draw some overlay on my image viewer : a dashed rectangle around the image, indicating the bounding box. Here 's what I do in the paintEvent function:

void ViewerGL::paintEvent(QPaintEvent* event){
    makeCurrent();
    QPainter p(this);
    glClearColor(0.0,0.0,0.0,0.0);
    glClear(GL_COLOR_BUFFER_BIT); // << If I don't do this it clears out my viewer white
// and I want it black

    p.setBackgroundMode(Qt::TransparentMode); // < was a test, doesn't seem to do anything
    p.setBackground(QColor(0,0,0,0));

     //loading the appropriate shader before texture mapping
    if(rgbMode() && _shaderLoaded){
        shaderRGB->bind();
    }else if(!rgbMode() && _shaderLoaded){
        shaderLC->bind();
    }
    paintGL(); // render function (texture mapping)
    //drawing a rect around the texture on the viewer
    QPen pen(Qt::DashLine); 
    pen.setColor(QColor(233,233,233));
    p.setPen(pen);
    QPoint btmRight=mousePosFromOpenGL(dataWindow().w(),dataWindow().h() +transY*2 + ( zoomY-dataWindow().h()/2)*2);
    QPoint btmLeft=mousePosFromOpenGL(0,dataWindow().h() +transY*2 + ( zoomY-dataWindow().h()/2)*2);
    QPoint topLeft=mousePosFromOpenGL(0,0 +transY*2 + ( zoomY-dataWindow().h()/2)*2);
    QPoint topRight=mousePosFromOpenGL(dataWindow().w(), 0+ +transY*2 + ( zoomY-dataWindow().h()/2)*2);
    p.drawLine(topLeft,topRight);
    p.drawLine(topRight,btmRight);
    p.drawLine(btmRight,btmLeft);
    p.drawLine(btmLeft,topLeft);
    QPoint pos = mousePosFromOpenGL( (dataWindow().w()) + 10  ,
                                    (dataWindow().h()) +transY*2 + ( zoomY-dataWindow().h()/2)*2  +10); // bottom right of the texture +10
    p.drawText(pos, _resolutionOverlay);

    p.end();

}

And here is what I do in the paintGL function:

    glPushAttrib(GL_ALL_ATTRIB_BITS);
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();

... my drawing code

    glMatrixMode(GL_MODELVIEW);
    glPopMatrix();

    glMatrixMode(GL_PROJECTION);
    glPopMatrix();

    glPopAttrib();

And the initializeGL function :

    initAndCheckGlExtensions();
    glClearColor(0.0,0.0,0.0,0.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glGenTextures (1, texId);
    glGenTextures (1, texBlack);

    glGenBuffersARB(1, &texBuffer);

    shaderBlack=new QGLShaderProgram(context());
    shaderBlack=new QGLShaderProgram(context());
    if(!shaderBlack->addShaderFromSourceCode(QGLShader::Vertex,vertRGB))
        cout << shaderBlack->log().toStdString().c_str() << endl;
    if(!shaderBlack->addShaderFromSourceCode(QGLShader::Fragment,blackFrag))
        cout << shaderBlack->log().toStdString().c_str() << endl;
    if(!shaderBlack->link()){
        cout << shaderBlack->log().toStdString().c_str() << endl;
    }

So far it works, I have what I want, but my program is flooding stderr on exit with : 'QGLContext::makeCurrent: Cannot make invalid context current'. I know this is coming from the QPainter and not from something else in my program.

I tried to move the code in the paintGL function to another function that is not virtual but that did not change anything.

well the QGLContext object is created automatically with default settings, but here's the initialiazeGL function (in the main post) — Lex

Sir Digby Chicken Caesar Sir Digby Chicken Caesar · Accepted Answer · 2013-02-26T11:06:24

You should first perform OpenGL calls, clear your OpenGL state, then wrap ALL QPainter draw calls between QPainter::begin(QGLWidget *) and QPainter::end(QGLWidget *).

Your program is most likely causing a problem because you interleave QPainter and OpenGL drawing calls. When you instantiate the QPainter object with QPainter(QPaintDevice *) you tell the QPainter to use the native drawing facilities of the QGLWidget to perform QPainter operations. So... QPainter uses OpenGL fixed-function calls to perform 2D drawing and it may interfere with your OpenGL rendering calls when your state has not been cleared.

I suggest following this guide:

https://doc.qt.io/archives/qt-4.8/qt-opengl-overpainting-example.html

QGLWidget: overpainting with QPainter

1 Answers