[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?

? Offline

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.

? Offline

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