Question

I followed the tutorial on this page:

http://iphone-3d-programming.labs.oreilly.com/ch01.html

I got down to the part where it says, “Compile and build and you should now see a solid gray screen. Hurray!” However, when I ran the program, I just get a black screen.

These are what the files look like:

HelloArrowAppDelegate.h

#import <UIKit/UIKit.h>
#import "GLView.h"

@interface HelloArrowAppDelegate : NSObject <UIApplicationDelegate> {
    UIWindow *m_window;
    GLView* m_view;
}

@end

HelloArrowAppDelegate.mm

#import "HelloArrowAppDelegate.h"

@implementation HelloArrowAppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.
    CGRect screenBounds = [[UIScreen mainScreen] bounds];

    m_window = [[UIWindow alloc] initWithFrame: screenBounds];
    m_view = [[GLView alloc] initWithFrame:screenBounds];

    [m_window addSubview: m_view];
    [m_window makeKeyAndVisible];

    return YES;
}

- (void)dealloc {
    [m_view release];
    [m_window release];
    [super dealloc];
}

@end

GLView.h

#import <UIKit/UIKit.h>
#import <OpenGLES/EAGL.h>
#import <QuartzCore/QuartzCore.h>
#import <OpenGLES/ES1/gl.h>
#import <OpenGLES/ES1/glext.h>

@interface GLView : UIView {
    EAGLContext* m_context;
}

-(void) drawView;

@end

GLView.mm

#import "GLView.h"

@implementation GLView

- (void) drawView
{
    glClearColor(0.5f, 0.5f, 0.5f, 1);
    glClear(GL_COLOR_BUFFER_BIT);

    [m_context presentRenderbuffer:GL_RENDERBUFFER_OES];
}

+ (Class) layerClass
{
    return [CAEAGLLayer class];
}

- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        CAEAGLLayer* eaglLayer = (CAEAGLLayer*) super.layer;
        eaglLayer.opaque = YES;

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

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

        // OpenGL Initialization
        GLuint framebuffer, renderbuffer;
        glGenFramebuffersOES(1, &framebuffer);
        glGenFramebuffersOES(1, &renderbuffer);

        [m_context
         renderbufferStorage:GL_RENDERBUFFER_OES
         fromDrawable: eaglLayer];

        glFramebufferRenderbufferOES(
                                     GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES,
                                     GL_RENDERBUFFER_OES, renderbuffer);

        glViewport(0, 0, CGRectGetWidth(frame), CGRectGetHeight(frame));

        [self drawView];
    }
    return self;
}

- (void)dealloc {
    if ([EAGLContext currentContext] == m_context) {
        [EAGLContext setCurrentContext:nil];
    }

    [m_context release];
    [super dealloc];
}

@end

Answer 1

The initWithFrame is incorrect. You want to generate a framebuffer and a renderbuffer and link the two. Instead you generate two framebuffers and completely ignore one. You should also keep the references to them (the variables ‘renderbuffer’ and ‘framebuffer’) in your class, as you’ll need to delete them later unless you want to leak memory.

Without fixing the second issue, I recommend:

- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        CAEAGLLayer* eaglLayer = (CAEAGLLayer*) super.layer;
        eaglLayer.opaque = YES;

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

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

        // these should be in the class so that we can release them later,
        // this will leak resources
        GLuint framebuffer, renderbuffer;

        // generate and bind a framebuffer
        glGenFramebuffersOES(1, &framebuffer);
        glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);

        // generate a colour renderbuffer; this example doesn't seem to want
        // e.g. a depth buffer, but if it did then you'd generate and add one
        // of those here also

        // generate and bind
        glGenRenderbuffersOES(1, &renderbuffer);
        glBindRenderbufferOES(GL_RENDERBUFFER_OES, renderbuffer);

        // get storage from the layer
        [m_context
         renderbufferStorage:GL_RENDERBUFFER_OES
         fromDrawable: eaglLayer];

        // link to the framebuffer
        glFramebufferRenderbufferOES(
                                     GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES,
                                     GL_RENDERBUFFER_OES, renderbuffer);

        glViewport(0, 0, CGRectGetWidth(frame), CGRectGetHeight(frame));

        [self drawView];
    }
    return self;
}

Put framebuffer and renderbuffer somewhere you can get to them again at the relevant moment and you should also:

- (void)dealloc {

    if(renderbuffer) glDeleteRenderbuffersOES(1, &renderbuffer);
    if(framebuffer) glDeleteFramebuffersOES(1, &framebuffer);

    if ([EAGLContext currentContext] == m_context) {
        [EAGLContext setCurrentContext:nil];
    }

    [m_context release];
    [super dealloc];
}

I’ve tested this against the code you provide. I get the grey screen. Changing the call to glClearColor changes the colour of the screen, so clearly the GL context is working.