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

问题

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_RGBA,
                       GLX_DEPTH_SIZE, 16,
                       GLX_RED_SIZE,   8,
                       GLX_GREEN_SIZE, 8,
                       GLX_BLUE_SIZE,  8,
                       None};
   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); 
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT); 
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);
   glPushMatrix(); 
   glLineWidth (1.0); 
   glEnableClientState (GL_VERTEX_ARRAY); 
   glVertexPointer (3, GL_FLOAT, 0, lineVertices); 
   glDrawArrays (GL_LINES, 0, 2); 
   glDisableClientState (GL_VERTEX_ARRAY); 
   glPopMatrix();
}
glEnable (GL_LIGHTING);

[/EDIT]

回答1:

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:

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.
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.

[EDIT]Hello world, answering my own question, hopefully to help someone to avoid the same mistake I did. To re-draw my OpenGL window I was using:

void update (GtkWidget * plot)
{
  gtk_widget_hide (plot);
  gtw_widget_show (plot);
}

Instead I should have been using:

gtk_widget_queue_draw (plot);

All problems solved ![/EDIT]

来源：https://stackoverflow.com/questions/39789414/how-to-get-as-many-information-as-possible-about-an-opengl-context

标签

opengl

gtk

gtk3

gtk2