## Tuesday, January 31, 2006

### Debugging OpenGL

There are only two debugging techniques in the universe:

1. printf.
2. /* */

(I would say a nice IDE that gives you stepping, logging, variable examination, etc. is just a really nice variant of these two techniques that avoids recompiles.)

OpenGL poses some particular challenges to debugging: often OpenGL’s failure mode will be to draw nothing, or fill the entire screen with solid black or white. A lot of OpenGL’s internal state is not easily printed by the application, and since OpenGL is state-based, removing code can have cascading effects.

You can still utilize these debugging techniques with OpenGL. Here are some things I’ve done when working on X-Plane to try to retain my sanity while debugging OpenGL code:

• Premature swapping: in a double-buffered context, you can swap the context and then wait for a mouse click. If you do this at intermediate points in your drawing, you can see your drawing in steps as it takes place. This is sort of like printf in that it lets you see internal program state that would otherwise be hidden.
• Removing drawing state: when something goes wrong you can start to remove the OpenGL tricks you do until you arrive at a simpler problem. Turn off texturing, turn off texture projection, turn off blending, etc. Lighting is a good one to turn off when things get strange, since bad normals can cause abnormal results.
• Use a different primitive. If your triangle fan looks funny, try drawing it as a line loop. Or set the polygon mode from fill to line - the wire frame may reveal something not obvious in a solid model.
• If you use display lists, set up your code to allow for immediate-mode execution as an option; this will let you catch display-list-related problems (I have seen drivers do things differently in display lists, and there are lots of ways to make coding mistakes with them) and also let you apply other techniques inside the list, like swapping to see work in progress.
• #define common modes: for all of the above tricks, if you’re going to be doing them a lot, design the code for debugging. I try to leave a series of #define switches at the top of my translation unit that apply debug-mode effects.

As a side thought, I have found Vertex Buffer Objects (VBOs) to be easier to debug than display lists for a few reasons: it’s pretty easy to revert VBOs to client vertex arrays (CVAs) at which point you can inspect the data in the debugger. Display lists can contain state commands as well as geometry, so it may not be obvious what the full effects of calling a display list is; since VBOs are just geometry you know what you are getting.

An example: in X-Plane we had some code that essentially did this:

static int has_dl = 0;if (has_dl == 0){// Build the car display list the firs time we need it.has_dl = glGenLists(1);glNewList(has_dl, GL_COMPILE);draw_a_car(); glEndList();}glCallList(has_dl);

This would be okay except that everything draw_a_car does gets compiled into the list. Furthermore, any non-OpenGL side-effects of draw_a_car get run only once. Neither of these problems are made clear by the code.