OpenGL ES 2.x: How to Discard Depth Buffer glDiscardFramebufferEXT?

Question

I read iOS OpenGL ES Logical Buffer Loads that a performance gain can be reached by "discarding" your depth buffer after each draw cycle. I try this, but it's as my game engine is not rendering any longer. I am getting an glError 1286, or GL_INVALID_FRAMEBUFFER_OPERATION_EXT, when I try to render the next cycle.

I get the feeling I need to initialize or setup the depth buffer each cycle if I'm going to discard it, but I can't seem to find any information on this. Here is how I init the depth buffer (all buffers, actually):

    // ---- GENERAL INIT ---- //        
    // Extract width and height.
    int bufferWidth, bufferHeight;
    glGetRenderbufferParameteriv(GL_RENDERBUFFER,
                                 GL_RENDERBUFFER_WIDTH, &bufferWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER,
                                 GL_RENDERBUFFER_HEIGHT, &bufferHeight);

    // Create a depth buffer that has the same size as the color buffer.
    glGenRenderbuffers(1, &m_depthRenderbuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, m_depthRenderbuffer);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24_OES, GAMESTATE->GetViewportSize().x, GAMESTATE->GetViewportSize().y);

    // Create the framebuffer object.
    GLuint framebuffer;
    glGenFramebuffers(1, &framebuffer);
    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                              GL_RENDERBUFFER, m_colorRenderbuffer);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT,
                              GL_RENDERBUFFER, m_depthRenderbuffer);
    glBindRenderbuffer(GL_RENDERBUFFER, m_colorRenderbuffer);

And here is what I'm trying to do to discard the depth buffer at the end of each draw cycle:

    // Discard the depth buffer
    const GLenum discards[]  = {GL_DEPTH_ATTACHMENT, GL_COLOR_ATTACHMENT0};
    glBindFramebuffer(GL_FRAMEBUFFER, m_depthRenderbuffer);
    glDiscardFramebufferEXT(GL_FRAMEBUFFER,1,discards);

I call that immediately following all of my draw calls and...

[m_context presentRenderbuffer:GL_RENDERBUFFER];

Any ideas? Any info someone could point me to? I tried reading through Apple's guide on the subject (where I got the original idea), http://developer.apple.com/library/ios/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/WorkingwithEAGLContexts/WorkingwithEAGLContexts.html, but it doesn't seem to work quite right for me.

Answer 1

Your call to glDiscardFramebufferEXT(GL_FRAMEBUFFER,1,discards) is saying that you are discarding just 1 framebuffer attachment, however your discards array includes two: GL_DEPTH_ATTACHMENT and GL_COLOR_ATTACHMENT0.

Try changing it to:

glDiscardFramebufferEXT(GL_FRAMEBUFFER, 2, discards);

In fact, you say that you are discarding these framebuffer attachments at the end of the draw cycle, but directly before [m_context presentRenderbuffer:GL_RENDERBUFFER];. You are discarding the colour renderbuffer attachment that you need in order to present the renderbuffer - perhaps try just discarding the depth attachment, as this is no longer needed at this point.

You only need to initialise your buffers once, not every draw cycle. The glDiscardFramebufferEXT() doesn't actually delete your framebuffer attachment - it is simply a hint to the API to say that the contents of the renderbuffer are not needed in that draw cycle after the discard command completes. From Apple's OpenGL ES Programming Guide for iOS:

A discard operation is defined by the EXT_discard_framebuffer extension and is available on iOS 4.0 and later. Discard operations should be omitted when your application is running on earlier versions of ioS, but included whenever they are available. A discard is a performance hint to OpenGL ES; it tells OpenGL ES that the contents of one or more renderbuffers are not used by your application after the discard command completes. By hinting to OpenGL ES that your application does not need the contents of a renderbuffer, the data in the buffers can be discarded or expensive tasks to keep the contents of those buffers updated can be avoided.