Huge drop in fps with transparent EAGLView

Question

I have a Cocos2d project and I want a constant background throughout the app. In the applicationDidFinishLaunching method of its delegate, I have replaced the line:

I have changed the pixelFormat of glView from kEAGLColorFormatRGB565 to kEAGLColorFormatRGBA8. When I make that change, glView becomes transparent and I can see through it, but the fps drops dramatically. If I don't make that change, the view doesn't become transparent, but I don't see the huge drop in fps. I'm talking about a significant drop in fps, from 59.0-60.0 to about 35.0-42.0.

I am using this code right below the addSubview line above to make the view transparent:

director.openGLView.opaque = NO;

The whole applicationDidFinishLaunching method looks like this:

- (void) applicationDidFinishLaunching:(UIApplication*)application
{
    // Init the window
    window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    // Try to use CADisplayLink director
    // if it fails (SDK < 3.1) use the default director
    if( ! [CCDirector setDirectorType:kCCDirectorTypeDisplayLink] )
        [CCDirector setDirectorType:kCCDirectorTypeDefault];


    CCDirector *director = [CCDirector sharedDirector];

    // Init the View Controller
    viewController = [[RootViewController alloc] initWithNibName:nil bundle:nil];
    viewController.wantsFullScreenLayout = YES;

    //
    // Create the EAGLView manually
    //  1. Create a RGB565 format. Alternative: RGBA8
    //  2. depth format of 0 bit. Use 16 or 24 bit for 3d effects, like CCPageTurnTransition
    //
    //
    EAGLView *glView = [EAGLView viewWithFrame:[window bounds]
                                   pixelFormat:kEAGLColorFormatRGBA8    // kEAGLColorFormatRGBA8
                                   depthFormat:0                        // GL_DEPTH_COMPONENT16_OES
                        ];

    // attach the openglView to the director
    [director setOpenGLView:glView];
    glView.opaque = NO;
//  // Enables High Res mode (Retina Display) on iPhone 4 and maintains low res on all other devices
    if( ! [director enableRetinaDisplay:YES] )
        CCLOG(@"Retina Display Not supported");

    //
    // VERY IMPORTANT:
    // If the rotation is going to be controlled by a UIViewController
    // then the device orientation should be "Portrait".
    //
    // IMPORTANT:
    // By default, this template only supports Landscape orientations.
    // Edit the RootViewController.m file to edit the supported orientations.
    //
#if GAME_AUTOROTATION == kGameAutorotationUIViewController 
    [director setDeviceOrientation:kCCDeviceOrientationPortrait];
#else
    [director setDeviceOrientation:kCCDeviceOrientationLandscapeLeft];
#endif

    [director setAnimationInterval:1.0/60];
    [director setDisplayFPS:YES];


    // make the OpenGLView a child of the view controller
    [viewController setView:glView];

    //Required in iOS6, recommended in 4 and 5
    [window setRootViewController:viewController];

    // make the View Controller a child of the main window, needed for iOS 4 and 5
    [window addSubview: viewController.view];

    [window makeKeyAndVisible];

    // Default texture format for PNG/BMP/TIFF/JPEG/GIF images
    // It can be RGBA8888, RGBA4444, RGB5_A1, RGB565
    // You can change anytime.
    [CCTexture2D setDefaultAlphaPixelFormat:kCCTexture2DPixelFormat_RGBA8888];


    // Removes the startup flicker
    [self removeStartupFlicker];

    // Run the intro Scene
    [[CCDirector sharedDirector] runWithScene: [Intro scene]];
}

Any ideas as to why this is happening? I can provide more code if need be.

I forgot to mention one other code change that I did. In CCDirector.m setGLDefaultValuesI changed this line from this:

glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

To this:

glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

Answer 1

Anytime you add an alpha channel with transparency, the OpenGL engine is going to have to perform some form (albeit default) alpha blending. This is math per pixel that it wasnt having to do in the non-RGBA (i.e. RGB 565) case. Anytime you add to the math needed by the engine frame rates are going to drop.

Answer 2

This is to be expected. The RGBA8888 framebuffer uses twice as much memory and the GPU needs to perform more work to render into this framebuffer. This is why RGB565 is the default format.