QOpenGlWidget: usage of native opengl code and QPainter class

[dunaj]

Hello everyone!
I am preparing a desktop application to draw some 2D plots. I am using C++: Qt 5 and visual studio 2013. I created a simple GUI and put myQOpenGLWidget in it. I am already drawing the plots, axes and ticks using my own openGl shaders. Everything works fine and now I want to add the description to my axes and ticks to make the graph possible to analysis. As in OpenGL itself there is no dedicated functions to render text, I came up with using QPainter object just to add the desired description to already created plot. And here are problems starting...

Here is my functions of my QopenGLWidget implementation:

void GLWidget::initializeGL() {
	initializeOpenGLFunctions();
	glEnable(GL_POINT_SPRITE);
	glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

	vertexShad = new QOpenGLShader(QOpenGLShader::Vertex);
	fragmentShad = new QOpenGLShader(QOpenGLShader::Fragment);

	bool flag = vertexShad->compileSourceFile(QString("vs.glsl"));
	if (flag) printf("compiled vertex Shader\n");
	flag = fragmentShad->compileSourceFile(QString("fs.glsl"));
	if (flag) printf("compiled fragment Shader\n");
	flag = program.addShader(vertexShad);
	if (flag) printf("linked vertex Shader\n");
	flag = false;
	flag =program.addShader(fragmentShad);
	if (flag) printf("linked fragment Shader\n");

	program.link();
	program.bind();
}

paintGL:

void GLWidget::paintGL() {
	glViewport(0, 0, this->width(), this->height());
	glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
	glClear(GL_COLOR_BUFFER_BIT);
	drawAxes();
	glViewport(MARGIN, MARGIN, this->width() - MARGIN * 2, this->height() - MARGIN * 2);
	glScissor(MARGIN, MARGIN, this->width() - MARGIN * 2, this->height() - MARGIN * 2);
	glEnable(GL_SCISSOR_TEST);
	// functions that uses openGL native code
       // ....
//
// labels description:
	QPainter painter(this);
	char * maxText = new char[4];
	sprintf(maxText, "%d", maxY);
	char * minText = new char[4];
	sprintf(minText, "%d", minY);
	painter.drawText(0, MARGIN, QString(maxText));
	painter.drawText(0, height() - MARGIN, QString(minText));
	painter.drawText(MARGIN, height() - MARGIN + 20, QString("0"));
	painter.drawText(width() - MARGIN, height() - MARGIN + 20, QString(100));
	update();
	painter.end();
}

Unfortunately, when I run the code, with uncommented QPainter part, the axis and plots are cleared, only the numbers I paint with QPainter are shown. Any suggestions how I can add the text with QPainter to my plot instead of clearing it?

Thank you for any comments!

[dunaj]

I finally managed to get things work.

First of all, I had to change and set the format (you have to change it before widget is shown), for example in main function , before my app was shown:
main:

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);

	SymmGaitModels w;
       // GLWidget is my implemantation of QOpenGLWidget
	GLWidget* gl = w.findChild<GLWidget*>("OpenGLWidget");
	QSurfaceFormat  format;
	//format.setSamples(4);
	//format.setDepthBufferSize(24);
	//format.setStencilBufferSize(8);
	format.setProfile(QSurfaceFormat::OpenGLContextProfile::CompatibilityProfile);
	QSurfaceFormat::setDefaultFormat(format);
	
	gl->setFormat(format);
	w.show();
	return a.exec();
}

Moreover, I had to move all (even enabling and disabling things like blend, point_sprite etc, which caused my problems) of my openGL functions between

QPainter-> beginNativePainting()
...
QPainter->endNativePainting();

And I had to implement paintEvent function. I just put paintGL() expression there.
Now my functions look like that:

void GLWidget::initializeGL() {
	printf("autoFILL: %d\n",this->autoFillBackground());
	initializeOpenGLFunctions(); // this function has to stay in initializeGL
	// commented functions below has to be removed and put after beginNativePainting
        //glEnable(GL_POINT_SPRITE);
	//glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
	//glEnable(GL_BLEND);
	//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
       
       // shaders may be compiled and added here, but program has to be linked and built after beginNativePainting()
	vertexShad = new QOpenGLShader(QOpenGLShader::Vertex);
	fragmentShad = new QOpenGLShader(QOpenGLShader::Fragment);

	bool flag = vertexShad->compileSourceFile(QString("vs.glsl"));
	//bool flag = vertexShad->compileSourceCode(vertexShadSrc);
	if (flag) printf("compiled vertex Shader\n");
	flag = fragmentShad->compileSourceFile(QString("fs.glsl"));
	//flag = fragmentShad->compileSourceCode(fragmentShadSrc);
	if (flag) printf("compiled fragment Shader\n");
	flag = program.addShader(vertexShad);
	if (flag) printf("linked vertex Shader\n");
	flag = false;
	flag =program.addShader(fragmentShad);
	if (flag) printf("linked fragment Shader\n");
}

void GLWidget::paintEvent(QPaintEvent *e) {
	paintGL(); // still this, widget has to be now refreshed by widget->update()
}

void GLWidget::paintGL() {
	painter = new QPainter(this);
	painter->beginNativePainting();
	//
	glEnable(GL_POINT_SPRITE);
	glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

	program.link();
	program.bind();
	//
	glViewport(0, 0, this->width(), this->height());
	glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
	glClear(GL_COLOR_BUFFER_BIT);
	drawAxes();
	glViewport(MARGIN, MARGIN, this->width() - MARGIN * 2, this->height() - MARGIN * 2);
	glScissor(MARGIN, MARGIN, this->width() - MARGIN * 2, this->height() - MARGIN * 2);
	glEnable(GL_SCISSOR_TEST);
	// functions using native openGL functions here
        //  .....
        //
	glDisable(GL_SCISSOR_TEST);
	glDisable(GL_POINT_SPRITE);
	glDisable(GL_VERTEX_PROGRAM_POINT_SIZE);
	glDisable(GL_BLEND);
	program.disconnect();
	painter->endNativePainting();
	drawTicksValues();
	painter->end();
}

After adding paintEvent implementation, the openGL widget has to be now refreshed using widget->update().

Hope it will be helpful for other people who are struggling with using both QPainter and native openGL functions, as Qt doc seems to be a little shallow on this topic.

Thanks!
Adam