GLIntercept

Here’s a cool debugging tool for OpenGL on Windows: GLIntercept. You can use it to track every OpenGL function call along with its parameters. (For those of you familiar with UNIX, this is like truss(1) or strace(1), only for OpenGL calls instead of system calls.)

Here’s a 30-second walkthrough:

  1. Download and run the Windows installer. You’ll end up with a new directory, C:\Program Files\GLIntercept0_5.
  2. Copy OpenGL32.dll and gliConfig_FullDebug.ini to the same directory as your program. Rename gliConfig_FullDebug.ini to gliConfig.ini
  3. Run your program. A new file will be created in the same directory called gliInterceptLog.txt, containing every OpenGL call your program makes. (You may also find that your program is a little slower).

Here’s some of the output from running assignment5.c:

glClearColor(0.000000,0.000000,0.000000,0.000000)
wglGetCurrentContext()=0x10000
wglGetCurrentDC()=0xb70114f8
wglGetCurrentContext()=0x10000
wglGetCurrentDC()=0xb70114f8
glViewport(0,0,600,300)
glMatrixMode(GL_PROJECTION)
glLoadIdentity()
glOrtho(0.000000,2.000000,0.000000,1.000000,-1.000000,1.000000)
glMatrixMode(GL_MODELVIEW)
glLoadIdentity()
glClear(GL_COLOR_BUFFER_BIT)
glColor3f(1.000000,1.000000,0.000000)
glBegin(GL_LINES)
glVertex3f(1.000000,0.000000,0.000000)
glVertex3f(1.000000,1.000000,0.000000)
glEnd()
glViewport(300,0,300,300)
glMatrixMode(GL_PROJECTION)
glLoadIdentity()
glFrustum(-1.500000,1.500000,-1.500000,1.500000,2.000000,8.000000)
glMatrixMode(GL_MODELVIEW)
glLoadIdentity()
glMultMatrixf([1.000000,0.000000,0.000000,0.000000,
             0.000000,1.000000,0.000000,0.000000,
             0.000000,0.000000,1.000000,0.000000,
             0.000000,0.000000,0.000000,1.000000])
glTranslated(0.000000,0.000000,-6.500000)
glPushMatrix()
glPushMatrix()
glColor3f(1.000000,0.000000,0.000000)
glBegin(GL_LINE_LOOP)
glNormal3fv([0.000000,0.000000,-1.000000])
glVertex3fv([0.500000,0.500000,-0.500000])
glVertex3fv([0.500000,-0.500000,-0.500000])
glVertex3fv([-0.500000,-0.500000,-0.500000])
glVertex3fv([-0.500000,0.500000,-0.500000])
glEnd()

Running with gliConfig_FullDebug.ini also makes sure that every call to an OpenGL function is checked with glGetError(), which will catch things like passing bad values to functions or trying to pop the matrix stack more times that it’s been pushed.

To make the best use of a tool like this, you’ll probably want a utility to tail the log as your program is running. (I’m told there are also GUI versions).

Note that you don’t need access to the source code — if you have a game at home that uses OpenGL, you can watch what it’s doing, too. (Just don’t steal their code, and don’t tell them I’m the one who told you how.)

GLIntercept has a few more tricks up its sleeve, but I’ll leave it up to you to read about them. For Linux people, there’s a similar program called BuGLe, but I haven’t tried it. And, of course, we smug OS X weenies have the OpenGL Profiler.

For those of you using Igloo, I’ve put together a new debug package that includes GLIntercept and Windows port of GNU tail. I’m not entirely sure about the legality of all this — GLIntercept and GNU tail are available under the GNU General Public license, but EiC is still MIA.

2 Responses to “GLIntercept”

  1. Alexandra Lee says:

    Anyone know where to get GLIntercept - the download website is down.

    If you have the installer, please email it to me: alxthor@yahoo.com.

    Thanks!