[SOLVED] Overpainting on QOpenGLWidget erases opengl scene

[BHeaney]

I'm trying to render 2D graphics & text using QPainter over my OpenGL scene, using a QOpenGLWidget in Qt 5.4.1. The painter 2D graphics works, but the opengl scene is erased. I've tried to modify the 'hellogl2' sample program by adding a painter->drawText() call and get the same results.

Here's my class definition:
class GLWidget : public QOpenGLWidget, protected QOpenGLFunctions

Here's my paintGL() function:

GLWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

drawMyOpenGlScene();   //  draws 3D 'Q' with a shader, from the sample program

QPainter painter(this);
painter.drawText(50, 50, "Testing");
painter.end();

}

I see the text, but not the 3D scene. If I comment out the 3 painter lines, I see the 3D 'Q'.

Note that the GLWindow has
setAutoFillBackground(false);

How do I get QPainter graphics to overpaint my 3D scene?

[A Former User]

Hi!

Have a look at this: http://www.informit.com/articles/article.aspx?p=1405557&seqNum=2 . The article explains everything in great detail.

Hope this helps!

[BHeaney]

Excellent article on using QPainter & OpenGL, particularly with the now-deprecated QGLWidget.

However, I'm trying to use QPainter with the new Qt5.4 QOpenGLWidget class. I tried using all the tips from this article with QOpenGLWidget, but it did not work. I also tried following all the overpainting instructions in the QOpenGLWidget documentation.

Does anyone have a sample of using QPainter with QOpenGLWidget ?

I'll need go back to using the old QGLWidget until QOpenGLWidget supports overpainting.

[A Former User]

Hi again @BHeaney!

Just made a small example. Works for me on Ubuntu 14.10 and Qt 5.4.0.

#ifndef WIDGET_H
#define WIDGET_H

#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QMatrix4x4>

class Widget : public QOpenGLWidget, protected QOpenGLFunctions
{
public:
    Widget(QWidget *parent = 0);
    void initializeGL();
    void resizeGL(int w, int h);
    void paintGL();

private:
    QMatrix4x4 m_projection;
};

#endif // WIDGET_H

#include "widget.h"

#include <QPainter>

Widget::Widget(QWidget *parent) : QOpenGLWidget(parent)
{
    QSurfaceFormat format;
    format.setDepthBufferSize(24);
    setFormat(format);
}

void Widget::initializeGL()
{
    initializeOpenGLFunctions();
}

void Widget::resizeGL(int w, int h)
{
    m_projection.setToIdentity();
    m_projection.perspective(60.0f, w / float(h), 0.01f, 1000.0f);
}

void Widget::paintGL()
{
    QPainter painter(this);

    painter.beginNativePainting();
    glClearColor(0,0.5,7,0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glBegin(GL_LINES);
     glColor3f(1.0,1.0,1.0);
      glVertex2f(0,0);
      glVertex2f(200,200);
     glEnd();
    painter.endNativePainting();

    // ----
    painter.drawText(50, 50, "Testing");
    painter.end();
}

[BHeaney]

Your example worked perfectly and is very easy to understand. Thanks!

My shader-rendered object still disappears when I draw anything with QPainter. I think the shader may not be releasing all it's buffers/objects before I draw with QPainter. I'm using the shader code from the hellog2 sample program.

I'll close this issue since you've shown me how to overpaint on top of an OpenGL scene.

[Wild Bill]

@BHeaney

BHeaney,

Did you ever get this to work with and did you have to use a compatibility profile? Would you mind posting your complete code?

WB