[GtkGLExt] GdkGLPixmap troubles

[Lindley M French <lfrench1 gmu edu>]

I'm trying to allow an offscreen render to a pixmap to occur now, instead of trying to force an expose event while a window is offscreen like I was before.

I've looked over the example code in pixmap.c that comes with GtkGLExt, but my adaptation of it isn't working.

My configure-event handler:

gboolean reshapeGL(GtkWidget *widget, GdkEventConfigure *evnt, gpointer data)
{
    GUIWindow* me = (GUIWindow*)data;
    GdkGLContext *glcontext = gtk_widget_get_gl_context (widget);
    GdkGLDrawable *gldrawable = gtk_widget_get_gl_drawable (widget);

    assert(pthread_equal(me->mainthread, pthread_self()));
    fprintf(stderr,"reshaping window %d, %d %d %d %d\n",me->window_id,evnt->x,evnt->y,evnt->width,evnt->height);

    if (!gdk_gl_drawable_gl_begin (gldrawable, glcontext))
        return FALSE;

    me->lock();
    me->xviewport = evnt->x;
    me->widthvp = evnt->width;
    me->yviewport = evnt->y;
    me->heightvp = evnt->height;
    glViewport(me->xviewport,me->yviewport,me->widthvp,me->heightvp);
    me->unlock();

    gdk_gl_drawable_gl_end (gldrawable);

    if (me->osPixmap)
        g_object_unref(me->osPixmap);
    if (me->osContext)
        gdk_gl_context_destroy(me->osContext);
    me->osPixmap = gdk_pixmap_new (widget->window,
                                   widget->allocation.width,
                                   widget->allocation.height,
                                   -1);
    GdkGLConfig *glconfig = gdk_gl_config_new_by_mode (
        GdkGLConfigMode(GDK_GL_MODE_RGB    |
                        GDK_GL_MODE_SINGLE));
    GdkGLDrawable *osDrawable = GDK_GL_DRAWABLE (
        gdk_pixmap_set_gl_capability (me->osPixmap,
                                      glconfig,
                                      NULL));
    me->osContext = gdk_gl_context_new (osDrawable,
                                        glcontext,
                                        FALSE,
                                        GDK_GL_RGBA_TYPE);
    
    return FALSE;
}

That all works fine so far as I can tell. There may be some issues with when exactly I'm supposed to unref something; the whole reference counting scheme is less than well-documented. However, I'm not concerned about that for now.

I also have this function, which is not linked to any particular event:

gboolean doTakeScreenshot(gpointer data)
{
    GUIWindow *me = (GUIWindow *)data;

    assert(pthread_equal(me->mainthread, pthread_self()));

    me->lock();

    GdkGLDrawable *osDrawable = GDK_GL_DRAWABLE(me->osPixmap);
    if (!osDrawable || !me->osContext)
    {
        fprintf(stderr, "Warning, no offscreen pixbuf configured in aeGraphWindow::doTakeScreenshot.\n");
        me->signal(&me->screenshot_complete);
        me->unlock();
        return FALSE;
    }

    if (!gdk_gl_drawable_gl_begin (osDrawable, me->osContext))
    {
        me->signal(&me->screenshot_complete);
        me->unlock();
        return FALSE;
    }
    
    // OPENGL DRAWING HERE
    
    gdk_gl_drawable_wait_gl(osDrawable);
    glReadBuffer(GL_FRONT);
    glReadPixels(me->xviewport,me->yviewport,me->widthvp,me->heightvp,
                 GL_RGBA,GL_UNSIGNED_BYTE,me->screenshot_dest->data[0]);

    me->signal(&me->screenshot_complete);

    me->unlock();

    gdk_gl_drawable_gl_end (osDrawable);

    return FALSE;
}

It appears that gdk_gl_drawable_gl_begin() is failing here. What could cause that? Both the context and drawable being passed are valid pointers. However, I did notice that gdk_gl_context_get_gl_drawable(me->osContext) returned NULL, which I thought was odd.

I'm not positive that my glReadPixels() is valid either, but I want to get past this hurdle before worrying about that one.

Any ideas?