QGLWidget: overpainting with QPainter - c++

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.

Answer 1

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