Sébastien Le Roux Sébastien Le Roux - 21 days ago 5x
C Question

How to get as many information as possible about an OpenGL context

Hello world and thanks for taking some time to read this !

I am writing a program in GTK2/3 + OpenGL, I got two versions of the program running:

  • (a) GTK+2 + GtkGlext Extention -> works great !

  • (b) GTK+3 + LibX11 -> works just fine !

Everything looks fine, except that the rendering in (a) is significantly faster that the rendering in (b) ... and I got no clue why.
Here are some example of the code parts used to create the OpenGL context:

  • (a)

    // To create the context, and the associated GtkWidget

    GdkGLConfig * glconfig = gdk_gl_config_new_by_mode (GDK_GL_MODE_RGBA | GDK_GL_MODE_DEPTH | GDK_GL_MODE_DOUBLE);
    GtkWidget * drawing_area = gtk_drawing_area_new ();
    gtk_widget_set_gl_capability (drawing_area, glconfig, NULL, TRUE, GDK_GL_RGBA_TYPE);
    g_signal_connect (G_OBJECT (drawing_area), "expose-event", G_CALLBACK (on_expose), data);

    // And later on to draw using the OpenGL context:

    gboolean on_expose (GtkWidget * widg, GdkEvent * event, gpointer data)
    GdkGLContext * glcontext = gtk_widget_get_gl_context (widg);
    GdkGLDrawable * gldrawable = gtk_widget_get_gl_drawable (widg);
    if (gdk_gl_drawable_gl_begin (gldrawable, glcontext))
    // OpenGL instructions to draw here !
    gdk_gl_drawable_swap_buffers (view -> gldrawable);
    gdk_gl_drawable_gl_end (view -> gldrawable);
    return TRUE;

  • (b)

    // To create the GtkWidget

    GtkWidget * drawing_area = gtk_drawing_area_new ();
    // Next line is required to avoid background flickering
    gtk_widget_set_double_buffered (drawing_area, FALSE);
    g_signal_connect (G_OBJECT (drawing_area), "realize", G_CALLBACK(on_realize), data);
    g_signal_connect (G_OBJECT (drawing_area), "draw", G_CALLBACK(on_expose), data);

    // To create the OpenGL context

    GLXContext glcontext;

    G_MODULE_EXPORT void on_realize (GtkWidget * widg, gpointer data)
    GdkWindow * xwin = gtk_widget_get_window (widg);
    GLint attr_list[] = {GLX_DOUBLEBUFFER,
    GLX_RED_SIZE, 8,
    XVisualInfo * visualinfo = glXChooseVisual (GDK_WINDOW_XDISPLAY (xwin), gdk_screen_get_number (gdk_window_get_screen (xwin)), attr_list);
    glcontext = glXCreateContext (GDK_WINDOW_XDISPLAY (xwin), visualinfo, NULL, TRUE);
    xfree (visualinfo);

    // To Draw using the OpenGL context

    G_MODULE_EXPORT gboolean on_expose (GtkWidget * widg, cairo_t * cr, gpointer data)
    GdkWindow * win = gtk_widget_get_window (widg);
    if (glXMakeCurrent (GDK_WINDOW_XDISPLAY (xwin), GDK_WINDOW_XID (xwin), glcontext))
    // OpenGL instructions to draw here !
    glXSwapBuffers (GDK_WINDOW_XDISPLAY (win), GDK_WINDOW_XID (win));
    return TRUE;

Trying to understand why (a) was faster than (b) I downloaded the sources of the GtkGLext library, read them, and find out that the commands were exactly the same with a call to X11.
Now my thoughts are either the following line in (b)

gtk_widget_set_double_buffered (drawing_area, FALSE);

Is messing with the rendering, and then there is nothing I can do ...
or there is/are difference(s) between the OpenGL contexts that might explain the behavior I noticed, If I follow up in this direction I need to compare both contexts with as many detail as possible ... so far I picked what seems to be the most usual way to get some information:

OpenGL Version : 3.0 Mesa 12.0.3
OpenGL Vendor : nouveau
OpenGL Renderer : Gallium 0.4 on NVCF
OpenGL Shading Version : 1.30

Color Bits (R,G,B,A) : 8, 8, 8, 0
Depth Bits : 24
Stencil Bits : 0
Max. Lights Allowed : 8
Max. Texture Size : 16384
Max. Clipping Planes : 8
Max. Modelview Matrix Stacks : 32
Max. Projection Matrix Stacks : 32
Max. Attribute Stacks : 16
Max. Texture Stacks : 10

Total number of OpenGL Extensions : 227
Extensions list:
N°1 : GL_AMD_conservative_depth
N°2 : GL_AMD_draw_buffers_blend

But both contexts give back exactly the same information ...

Thanks for getting there already ... now my question is:

Is there a way to output as many information as possible about an OpenGL context, and how ?

I welcome any other suggestion(s) on what I am doing !


PS: I am working on using the GtkGLArea Widget for GTK3, but as stated here I am not there yet.

[EDIT] Some of the OpenGL instructions:

// OpenGL instructions to draw here !

glLoadIdentity ();
glPushMatrix ();
// d is the depth ... calculated somewhere else
glTranslated (0.0, 0.0, -d);
// Skipping the rotation part for clarity, I am using a quaternion
rotate_camera ();
// r, g, b and a are GLFloat values
glClearColor (r,g,b,a);
glDisable (GL_LIGHTING);
int i;
// nbds is the number of chemical bonds
GLfloat * lineVertices;
// This is "roughly" what I do to draw chemical bonds, to give you an idea
for (i=0; i<nbds;i++)
// get_bonds (i) gives backs a 6 float array
lineVertices = get_bonds(i);
glLineWidth (1.0);
glEnableClientState (GL_VERTEX_ARRAY);
glVertexPointer (3, GL_FLOAT, 0, lineVertices);
glDrawArrays (GL_LINES, 0, 2);
glDisableClientState (GL_VERTEX_ARRAY);
glEnable (GL_LIGHTING);



thanks for your suggestions, the "ApiTrace" idea was amazing, not only did I discovered a great tool, but it helped me to get some clues about my problem. Using ApiTrace:

  1. I checked that both versions (a) and (b) of my program were using exactly the same OpenGL contexts ... with great details and ease I must add ... and therefore that the error was not coming from the context initialization.
  2. I found out that in version (b) the rendering was done 5 times for more often than in version (a) ... meaning 5 times for the same frame !

The only logical conclusion I am aiming at is the difference in GTK+ signals between version 2 and 3, in version (a) of my program I use an expose-event while in version (b) I use a draw event (new signal for the GtkDrawingArea) ... obviously there are some differences in the behavior of the GTK+ library between version 2 and 3 at this point ... I am working on finding a way around it ... I will edit this answer to provide further information.