Why does calling glMatrixMode(GL_PROJECTION) give me EXC_BAD_ACCESS in an iPhone app?

Question

I have an iphone app where I call these three functions in appDidFinishLaunching:

glMatrixMode(GL_PROJECTION);
glOrthof(0, rect.size.width, 0, rect.size.height, -1, 1);
glMatrixMode(GL_MODELVIEW);

When stepping through with the debugger I get EXC BAD ACCESS when I execute the first line. Any ideas why this is happening?

Btw I have another application where I do the same thing and it works fine. So I've tried to duplicate everything in that app (#imports, adding OpenGLES framework, etc) but now I'm just stuck.

Answer 1

I've run into this with OpenGL calls if two threads are attempting to draw to the OpenGL scene at once. However, that doesn't sound like what you're doing.

Have you properly initialized your display context and framebuffer before this call? For example, in my UIView subclass that does OpenGL drawing, I call the following in its initWithCoder: method:

context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];

if (!context || ![EAGLContext setCurrentContext:context] || ![self createFramebuffer]) 
{
    [self release];
    return nil;
}

The createFramebuffer method looks like the following:

- (BOOL)createFramebuffer 
{   
    glGenFramebuffersOES(1, &viewFramebuffer);
    glGenRenderbuffersOES(1, &viewRenderbuffer);

    glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
    [context renderbufferStorage:GL_RENDERBUFFER_OES fromDrawable:(CAEAGLLayer*)self.layer];
    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, viewRenderbuffer);

    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);

    if (USE_DEPTH_BUFFER) {
        glGenRenderbuffersOES(1, &depthRenderbuffer);
        glBindRenderbufferOES(GL_RENDERBUFFER_OES, depthRenderbuffer);
        glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_DEPTH_COMPONENT16_OES, backingWidth, backingHeight);
        glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, depthRenderbuffer);
    }

    if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES) 
    {
        return NO;
    }

    return YES;
}

This is pretty much boilerplate code, as generated by the OpenGL ES Application template in XCode. Perhaps by not initializing things before calling glMatrixMode(), you're getting a crash.

Also, why are you doing OpenGL drawing in applicationDidFinishLaunching:? Wouldn't a view or view controller be a more appropriate place for OpenGL calls than your UIApplicationDelegate?