Conflict between shaders and QPainter in paintGL() for rendering 2D text

Question

I have the following class inheriting from QOpenGLWidget and QOpenGLFunctions:

class OpenGLWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
    Q_OBJECT
public:
    OpenGLWidget();
    virtual ~OpenGLWidget();

    void initializeGL();
    void paintGL()
    {
       QPainter painter(this);

       painter.beginNativePainting();
       glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
       // Calls OpenGL draw functions with VBOs
       m_viewport.render(m_shader, m_entities);
       painter.endNativePainting();

       painter.drawText(0, 0, width(), height(), Qt::AlignCenter, "Hello World!");

    }
    void resizeGL(int width, int height);

    [...]
}

"Hello World" is drawn as intended, but the 3D scene is broken. I should have 3D axis in the center and in the top-right of the screen:

To me, it seems that the vertex and fragment shaders I'm using are the source of the problem. Otherwise, given the simplicity of the code and the examples I've found, it should work.

A good output would be:

with the "Hello World" at the center. This is what I get when I comment the QPainter calls.

Answer 1

It seems that your shader program is released when you use QPainter. Bind the shader program before OpenGL calls and release it afterwards. It should fix it.

painter.beginNativePainting();
// Bind shader program
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// Calls OpenGL draw functions with VBOs
m_viewport.render(m_shader, m_entities);
// Release shader program
painter.endNativePainting();