OpenGL ES difference for iOS and Android

Question

I have an OpenGL ES app that works both on iOS and Android. Most of the code was written ages ago by another person and now I have to maintain it. OpenGL usage seems fairly simple (the game is 2D and uses only textured sprites in a simple manner). But I see two major differences in graphics code realization for iOS and Android:

1) iOS code contains this code:

    glGenFramebuffersOES(1, &m_defaultFramebuffer);
    glGenRenderbuffersOES(1, &m_colorRenderbuffer);
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, m_defaultFramebuffer);
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, m_colorRenderbuffer);
    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, m_colorRenderbuffer);

and Android's one does not.

2) When Android app goes to background, all OpenGL textures are destroyed (glDeleteTextures) and EGL is shutdowned using eglTerminate. When app returns from sleep, EGL is re-initialized and textures are re-created.

iOS code does not do these things. It just pauses rendering loop by calling [m_displayLink setPaused:YES];

Other OpenGL-related code is the same for iOS and Android.

Everything works well on both platforms, but I want to have a full understanding of what's going on. Can anybody explain me a rationale behind these two differences?

Answer 1

1) This is just a difference in the APIs. On iOS, you create your own framebuffer to render in to when the App starts. On Android the framebuffer is created automatically in GLSurfaceView, so the App doesn't need to create its own.

2) On iOS, when your App goes to the background, the OpenGL context is preserved, which means all your textures and buffers are still there when you return it to the foreground. Older versions of Android had only a single OpenGL context, so it was destroyed whenever your App went to the background (so that other Apps could then make use of it). Later versions of Android do have the option to behave more like iOS by calling setPreserveEGLContextOnPause. However for this to work, the Android version has to be 3.x or above (API 11) and the device must support it also. When it is not used or supported, the App must delete and re-create all it's OpenGL resources when going between background and foreground, which is what your App appears to be doing.