Qt Graphics Framework and OpenGL

[hanaxi]

Hello everyone, I use c++17, ubuntu 20.04 and qt 5.12. I have a project where I use QGraphicsView, QGraphicsScene and QGraphicsItem. To increase performance, I switched to OpenGL render . So QGraphicsView 's viewport is plain QOpenGLWidget :

std::unique_ptr<GraphicsView> _view;
std::unique_ptr<QOpenGLWidget> _glViewport;
// ......
_view->setViewport(_glViewport.get());

std::unique_ptr<GraphicsView> _view;
std::unique_ptr<QOpenGLWidget> _glViewport;
// ......
_view->setViewport(_glViewport.get());

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And I also use OpenGL inside QGraphicsItem:aint() method:

void GLitem::paint(QPainter *painter,
                      const QStyleOptionGraphicsItem * option,
                      QWidget * widget)
{
 
    painter->beginNativePainting();
    initializeOpenGLFunctions();
 
std::call_once(_onceFlag, &GLitem::initStuff, this);
// draw stuff 
 
_vao.release();
_shaders.release();
 
    painter->endNativePainting();
 
}
 
void GLitem::UploadGeometry()
{
// here i create VAO, VBO and shaders
// this function is called only once in a valid context, 'cause std::call_once
//    QOpenGLVertexArrayObject _vao;
//    QOpenGLBuffer _vbo;
}

void GLitem::paint(QPainter *painter,
                      const QStyleOptionGraphicsItem * option,
                      QWidget * widget)
{

    painter->beginNativePainting();
    initializeOpenGLFunctions();

std::call_once(_onceFlag, &GLitem::initStuff, this);
// draw stuff 

_vao.release();
_shaders.release();

    painter->endNativePainting();

}

void GLitem::UploadGeometry()
{
// here i create VAO, VBO and shaders
// this function is called only once in a valid context, 'cause std::call_once
//    QOpenGLVertexArrayObject _vao;
//    QOpenGLBuffer _vbo;
}

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So the problem is, that when I close this program, it crashes. And the problem is in GLitem D-tor:

~GLitem()
{
        if (_vao.isCreated())
            _vao.destroy();
        if (_vbo.isCreated())
            _vbo.destroy();
        if (_shaderProgram->isLinked())
            _shaderProgram->removeAllShaders();
}

~GLitem()
{
        if (_vao.isCreated())
            _vao.destroy();
        if (_vbo.isCreated())
            _vbo.destroy();
        if (_shaderProgram->isLinked())
            _shaderProgram->removeAllShaders();
}

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I've read in the documentation, that you need

void QOpenGLVertexArrayObject::destroy()
Destroys the underlying OpenGL vertex array object. There must be a valid OpenGL context that supports vertex array objects current for this function to succeed.

QOpenGLVertexArrayObject QOpenGLBuffer
So I need QOpenGLContext, but I don't create it explicitly anywhere in my program. And I tried to create it in D-tor but it did not help. I also figured out, that since I use QGraphicsView, then my Context is viewport and I tried to work with that, but it also didn't help. So the main problem is that my program crashes when D-tor is getting called. I don't know how to fix this. I know, that it's not the best approach in Qt+OpenGL, but I can't switch to the better method (like defining my own QOpenGLWidgets)

One caveat is that I initialize vao, vbo and shaders inside member function using std::call_once inside paint() method, so with that I can be assured that all that stuff is going to be created during a valid context, I don't think that it is a good approach, but I don't know how to do it otherwise.

[d_stranz]

In my use of QOpenGLVertexArrayObject and QOpenGLBuffer in a 3D plotting library, I create these as non-pointer member variables in each of the classes I use to draw graphical objects in a widget derived from QOpenGLWidget. For example, the class that defines the 3D axes cube, I have these member variables:

// Vertex array object for the axis lines and box
	QOpenGLVertexArrayObject	mAxisVAO;
 
	// Holds the vertex coordinates
	QOpenGLBuffer	mVBO = QOpenGLBuffer( QOpenGLBuffer::VertexBuffer );
 
	// Holds the indices for the box
	QOpenGLBuffer	mBoxIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );
 
	// Holds the indices for the axes
	QOpenGLBuffer	mAxisIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );
 
	// Vertex array object for the ticks
	QOpenGLVertexArrayObject	mTickVAO;
 
	// Holds the tick vertex coordinates
	QOpenGLBuffer	mTickVBO = QOpenGLBuffer( QOpenGLBuffer::VertexBuffer );
 
	// Holds the indices for the tick vertices
	QOpenGLBuffer	mTickIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );

// Vertex array object for the axis lines and box
	QOpenGLVertexArrayObject	mAxisVAO;

	// Holds the vertex coordinates
	QOpenGLBuffer	mVBO = QOpenGLBuffer( QOpenGLBuffer::VertexBuffer );

	// Holds the indices for the box
	QOpenGLBuffer	mBoxIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );

	// Holds the indices for the axes
	QOpenGLBuffer	mAxisIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );

	// Vertex array object for the ticks
	QOpenGLVertexArrayObject	mTickVAO;

	// Holds the tick vertex coordinates
	QOpenGLBuffer	mTickVBO = QOpenGLBuffer( QOpenGLBuffer::VertexBuffer );

	// Holds the indices for the tick vertices
	QOpenGLBuffer	mTickIBO = QOpenGLBuffer( QOpenGLBuffer::IndexBuffer );

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They are created on the stack during construction and are automatically destroyed in the destructor. The two VAO instances are initialized in the constructor with "this" as the parent. The axis class is derived from QObject. I do not create any OpenGL context.

The QOpenGLShaderProgram is a pointer member variable in the axis class. It is created and compiled on demand before plotting starts. Because it is derived from QObject, it is also given "this" as a parent so it is automatically destroyed when the axis class instance is destroyed.

During rendering, first the shader program is bound, then the tick VAO is bound and the ticks rendered, and the tick VAO released. Next the axis VAO is bound and the axes are rendered, and the axis VAO released. Finally the shader program is released. Native QPainter calls are used to draw the labels into a QOpenGLPaintDevice the same size as the widget, which overlays them onto the OpenGL rendering..

Here's a screenshot of the axes cube.

Capture.jpg