How to create a QOffscreenSurface correctly?

MartinChan 0

When I try to do some offscreen rendering jobs , I create a QOffscreenSurface in my program but it encounters error.The source code about this part says:

qoffscreensurface.cpp 129(Qt5.3):

    //if your applications aborts here, then chances are your creating a QOffscreenSurface before
    //the screen list is populated.
    Q_ASSERT(d->screen);

So, what should I do when I want to use a QOffscreenSurface correctly ?

jsulm

@MartinChan-0 said in How to create a QOffscreenSurface correctly?:

So, what should I do when I want to use a QOffscreenSurface correctly ?

Can you show what you're doing now?

MartinChan 0

@jsulm

Here is my code,I want to use a QGLFramebufferObject to bind the offscreen surface to do some opengl painting ,then use QGLFramebufferObject to output it .

qtestoffscreen.h

#ifndef QTESTOFFSCREEN_H
#define QTESTOFFSCREEN_H

#include <QOffscreenSurface>
#include <QtGui/QOpenGLFunctions_3_3_Core>
#include <QImage>
#include <QGLFramebufferObject>
#include <QOpenGLPaintDevice>

class QTestOffScreen : public QOffscreenSurface,
                       protected QOpenGLFunctions_3_3_Core
{
public:
    explicit QTestOffScreen(QWindow *parent = 0);
    ~QTestOffScreen();

    virtual void render();
    virtual void initialize();
    void renderNow();
    void setAnimating(bool animating);

private:
    QGLFramebufferObject *fbo;
    bool m_animating;

    QOpenGLContext *m_context;
    QOpenGLPaintDevice *m_device;

};

#endif // QTESTOFFSCREEN_H

qtestoffscreen.cpp

#include "qtestoffscreen.h"
#include <QTime>

QTestOffScreen::QTestOffScreen(QWindow *parent):
    fbo(Q_NULLPTR),
    m_context(Q_NULLPTR),
    m_device(Q_NULLPTR)
{
    Q_UNUSED(parent);
}

QTestOffScreen::~QTestOffScreen()
{
    delete m_context;
    delete m_device;
}

void QTestOffScreen::render()
{
    glClear(GL_COLOR_BUFFER_BIT
          | GL_DEPTH_BUFFER_BIT
          | GL_STENCIL_BUFFER_BIT);

    glViewport(0,0,100,100);
    glOrtho(0,100,0,100,0,1);

    glColor3f(1.0,0.0,1.0);

    glBegin(GL_LINE_STRIP);
        glVertex2f(10.0f,10.0f);
        glVertex2f(90.0f,10.0f);
        glVertex2f(90.0f,90.0f);
        glVertex2f(90.0f,10.0f);
    glEnd();

}

void QTestOffScreen::initialize()
{

    if (!fbo)
    {
        fbo = new QGLFramebufferObject(1920,1080,0);
    }

    fbo->bind();
}

void QTestOffScreen::renderNow()
{
    bool needsInitialize = false;
    if (!m_context)
    {
        m_context = new QOpenGLContext(this);
        m_context->setFormat(requestedFormat());
        m_context->create();
    }

    m_context->makeCurrent(this);

    if (needsInitialize)
    {
        initializeOpenGLFunctions();
        initialize();
    }

    render();

    fbo->toImage().save(QString("buffer") + QString(".png"));
}

void QTestOffScreen::setAnimating(bool animating)
{
    m_animating = animating;
}

main.cpp

#include <QCoreApplication>
#include "qtestoffscreen.h"

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

    QTestOffScreen screen;   ///It is wrong here.
    screen.renderNow();
    
    return a.exec();
}

MartinChan 0

When I use the most simplified situation -- creating a QOffscreenSurface in an original main() function,it still fails.

jsulm

@MartinChan-0 I'm not an expert on this topic, but my guess is that you need to initialize OpenGL before you create offscreenscreen surface.

MartinChan 0

@jsulm Thank u anyway and if I have any solutions I will update this topic.

MartinChan 0

I have solved it (althought it still has some problems) like the link in this topic