在OS X的Qt 4.8.6中更改QGLWidgets的OpenGL上下文版本

Question

I want to use Qt 4.8.6 to render OpenGL content with a QGLWidget. The machine i'm working on is a macbook pro with OS X 10.9.4.

The QGLWidget is created by passing a QGLFormat object with a requested format version of the 3.2 core profile. The problem i am encountering is that the OpenGL version reported by the QGLContext remains 1.0, no matter what GLFormat I specify.

After researching the topic i found the Qt OpenGL Core Profile Tutorial . However the example source code reports the same OpenGL version 1.0 from before. Curiously the call

qDebug() << "Widget OpenGl: " << format().majorVersion() << "." << format().minorVersion();
qDebug() << "Context valid: " << context()->isValid();
qDebug() << "Really used OpenGl: " << context()->format().majorVersion() << "." << context()->format().minorVersion();
qDebug() << "OpenGl information: VENDOR:       " << (const char*)glGetString(GL_VENDOR);
qDebug() << "                    RENDERDER:    " << (const char*)glGetString(GL_RENDERER);
qDebug() << "                    VERSION:      " << (const char*)glGetString(GL_VERSION);
qDebug() << "                    GLSL VERSION: " << (const char*)glGetString(GL_SHADING_LANGUAGE_VERSION);

reported a version string of 2.1

Widget OpenGl:  1 . 0 
Context valid:  true 
Really used OpenGl:  1 . 0 
OpenGl information: VENDOR:        NVIDIA Corporation 
                    RENDERDER:     NVIDIA GeForce GT 750M OpenGL Engine 
                    VERSION:       2.1 NVIDIA-8.26.26 310.40.45f01 
                    GLSL VERSION:  1.20 

Using the Cocoa code suggested in this OS X opengl context discussion from 2011 the output of the version numbers changed to

Widget OpenGl:  1 . 0 
Context valid:  true 
Really used OpenGl:  1 . 0
OpenGl information: VENDOR:        NVIDIA Corporation 
                    RENDERDER:     NVIDIA GeForce GT 750M OpenGL Engine 
                    VERSION:       4.1 NVIDIA-8.26.26 310.40.45f01 
                    GLSL VERSION:  4.10 

While the driver is now reporting expected OpenGL version number, i am still only able to get a 1.0 QGLWidget context. The QGLFormat object that is passed to the QGLWidget constructor is set up using

QGLFormat fmt;
fmt.setProfile(QGLFormat::CoreProfile);
fmt.setVersion(3, 2);
fmt.setSampleBuffers(true);

I am somewhat at a loss as to why i am still only getting a version 1.0 context. Even without the Cocoa framework generated OpenGL Context it should be possible to increase the context version to 2.1, but it remains fixed at 1.0 regardless of the QGLFormat passed to the constructor.

Any pointers as to why the QGLWidget Context remains at version 1.0 are very much appreciated.

Update 1

Further experimentation showed that the code returns the requested OpenGL version on a Ubuntu 13.04 Linux. The issue seems to be specific to OS X.

Update 2

I build a minimal non-/working example

#include <QtOpenGL/QGLFormat>
#include <QtOpenGL/QGLWidget>
#include <QtGui/QApplication>
#include <QtCore/QDebug>

int main(int argc, char **argv) {
    QApplication app(argc, argv);

    QGLFormat fmt = QGLFormat::defaultFormat();
    fmt.setVersion(3,2);
    fmt.setProfile(QGLFormat::CoreProfile);
    fmt.setSampleBuffers(true);

    QGLWidget c(fmt);
    c.show();
    qDebug() << c.context()->requestedFormat();
    qDebug() << c.context()->format();

    return app.exec();
}

which can be build in Ubuntu using

g++ main.cpp -I/usr/include/qt4 -lQtGui -lQtCore -lQtOpenGL -lGL -o test

or under OS X

g++ main.cpp -framework OpenGL -framework QtGui -framework QtCore -framework QtOpenGL -o test

It prints two lines of QGLFormat debug output. The first is the requested format and the second line is the actual context format. Both are supposed to show a major.minor version number of 3.2. It seems to be working under Ubuntu Linux, but fails when using OS X.

Update 3

Fun times. It might be a bug in Qt4.8.6, since the issue does not occur when compiling the example agains Qt5.3.1. A bug report has been filed.

Can someone else verify this behaviour?

Answer 1

Yes. That's platform specific. Please find solution here .

Override QGLContex::chooseMacVisual to specify platform specific initialization.

CustomGLContext.hpp:

#ifdef Q_WS_MAC
void* select_3_2_mac_visual(GDHandle handle);
#endif // Q_WS_MAC

class CustomGLContext : public QGlContext {
  ...
#ifdef Q_WS_MAC
  void* chooseMacVisual(GDHandle handle) override {
    return select_3_2_mac_visual(handle); // call cocoa code
  }
#endif // Q_WS_MAC
};

gl_mac_specific.mm:

void* select_3_2_mac_visual(GDHandle handle)
{
static const int Max = 40;
NSOpenGLPixelFormatAttribute attribs[Max];
int cnt = 0;

attribs[cnt++] = NSOpenGLPFAOpenGLProfile;
attribs[cnt++] = NSOpenGLProfileVersion3_2Core;

attribs[cnt++] = NSOpenGLPFADoubleBuffer;

attribs[cnt++] = NSOpenGLPFADepthSize;
attribs[cnt++] = (NSOpenGLPixelFormatAttribute)16;

attribs[cnt] = 0;
Q_ASSERT(cnt < Max);


return [[NSOpenGLPixelFormat alloc] initWithAttributes:attribs];
}