iOS OpenGL ES Logical Buffer Loads

Question

Slogging through the list of OpenGL API usage performance warnings given by the Analyze instrument, I am finding that we are generating several logical buffer loads per frame - places where we are not clearing a buffer because a draw call completely overwrites it.

Counterintuitively, introducing glClear() calls for these cases simply moves the location of the warning to the glClear() calls. Apple implement GL_EXT_discard_framebuffer, however using this on its own is also not sufficient to stop the warning. A glDiscardFramebufferEXT() followed by a glClear() does stop the warning, and improves performance significantly, but the glClear() call itself takes time to clear the buffer, which in our use case is a redundant operation.

Do others also find they need to call both functions to avoid the reload cost or am I missing something? Is there a cheap way of hinting to OpenGL that the contents of the framebuffer is about to be completely overwritten and so does not need to be reloaded into tile memory?

Answer 1

The documentation implies that a fullscreen glClear() sets some magical flag, which is consistent with what I've seen while debugging the same issue. I wouldn't worry about doing a redundant glClear(), as this is the intended usage pattern from what I can tell.

Update: You may also be experiencing the same bug I had, where I was clearing the colour and depth buffers, but forgot to set glDepthMask(GL_TRUE) before calling glClear(). This resulted in a Logical Buffer Load warning.