/**************************************************************************\
    |* Save the offscreen image if it doesn't already exist
    \**************************************************************************/
 #ifdef DEBUG_BUFFERS
    QFileInfo check_file("/tmp/tst.png");
    if (!check_file.exists())
        _gl->dumpBuffer("/tmp/tst.png", 0);

    QFileInfo check_file2("/tmp/tst2.png");
    if (!check_file2.exists())
        {
        _gl->dumpBuffer("/tmp/tst2.png", 1);
        fprintf(stderr, "width: %d, height:%d\n",
            _gl->screenSize().width(),
            _gl->screenSize().height());
        }
#endif

    /**************************************************************************\
    |* And focus back onto the main backbuffer
    \**************************************************************************/
    _gl->focusOnscreen();

    /**************************************************************************\
    |* Blit the framebuffer to the screen
    \**************************************************************************/
    int w                           = _gl->screenSize().width();
    int h                           = _gl->screenSize().height();
    QRect trect                     = QRect(0,0,w,h);
    QRect srect                     = QRect(0,0,w,h);
    QOpenGLFramebufferObject *fbo   = _gl->fbo();
    GLbitfield what                 = GL_COLOR_BUFFER_BIT;
    GLenum filter                   = GL_NEAREST;
    int which                       = GL_COLOR_ATTACHMENT0;

    QOpenGLFramebufferObject::blitFramebuffer(0,        // Target = default
                                              trect,    // Target rect
                                              fbo,      // source
                                              srect,    // source rect
                                              what,     // what to copy
                                              filter,   // how to copy
                                              which,    // which buffer to copy
                                              0         // No restore
                                              );

which prints out the width and height of the image in the offscreen buffer (1920x1080) and the program also prints out the dimensions of the window (1920x1080), so they ought to match...

OpenGL version :  4.1 ATI-3.8.24 ( CoreProfile )  - renderToMultiple: yes
Texture 'textures/map.ppm.gz' has id 3 [file type:5, tex type:8058
Texture 'textures/fog.pgm.gz' has id 4 [file type:5, tex type:8058
buffer width: 1920, height:1080
window : 1920, 1080

The problem is that when the rendering is done, all I see is a quarter of the screen...

As opposed to the frame buffer that the above code rendered:

Interestingly the size of the quarter-rendered image if I screenshot it, is 3838 x 2200. Am I running afoul of hi-dpi or something ?