Debugging OpenGL

Hardware accelerated graphics code is hard to debug. This is a fact. I know that from experience. Some of the most frustrating bugs I have ever encountered have been 3D related.

Over the years I have learned that catching the bugs early saves a lot of time and work in the end. It may also prevent you from going insane. I value my sanity, so I have decided to make my debug build help me as much as possible.

Apple recommends that you check the OpenGL error flag often and I concur. However, calling glGetError() too often results in a performance penalty. For this reason, I only check for GL errors in my debug build, where I actually check for errors after every single GL function call. That's how dedicated I am to squashing those pesky little bugs! (I like the movie Starship Troopers because it's all about bug killing.)

To be more specific, I have wrapped glGetError() in a function called checkGL() which programmatically triggers a breakpoint whenever an error has been detected.

Here's some code that you may adopt as your own, as long as you don't blame me if something goes wrong. Yes, this is a disclaimer. If you use any of my code, you are doing so at your own risk, I take no responsibility whatsoever. That said, I don't think there are any problems with this code...

Here's the header file (GLError.h):

#ifdef __cplusplus
extern "C" {
#endif
 
#if TARGET_IPHONE_SIMULATOR
#define BREAKPOINT __asm__ volatile ("int3");
#else
#define BREAKPOINT __asm__ volatile ("bkpt 1")
#endif
 
#ifdef DEBUG_OPENGL
#define CHECK_GL checkGL()
void checkGL(void);
#else
#define CHECK_GL
#endif
 
#ifdef __cplusplus
}
#endif

And here's the implementation file (GLError.m):

#import "GLError.h"
 
#import <OpenGLES/ES2/gl.h>
#import <OpenGLES/ES2/glext.h>
 
void checkGL(void)
{   
    GLenum err;
    if (err = glGetError())
    {
        switch (err)
        {
            case GL_INVALID_ENUM:
                NSLog(@"OPENGL ERROR: GL_INVALID_ENUM");
                BREAKPOINT;
                break;
 
            case GL_INVALID_VALUE:
                NSLog(@"OPENGL ERROR: GL_INVALID_VALUE");
                BREAKPOINT;
                break;
 
            case GL_INVALID_OPERATION:
                NSLog(@"OPENGL ERROR: GL_INVALID_OPERATION");
                BREAKPOINT;
                break;
 
            case GL_OUT_OF_MEMORY:
                NSLog(@"OPENGL ERROR: GL_OUT_OF_MEMORY");
                BREAKPOINT;
                break;              
 
            default:
                NSLog(@"OPENGL ERROR: 0x%x", err);
                BREAKPOINT;
                break;
        }
 
    }
}

That's it for today. Over and cout. (pathetic C++ joke)

Comments: 0 Tags: iPhone, OpenGL