Question

The standards allude to rendering starting upon my first gl command and continuing in parallel to further commands. Certain functions, like glBufferSubData indicate loading can happen during rendering so long as the object is not currently in use. This introduces a logical concept of a “frame”, though never explicitly mentioned in the standard.

So my question is what defines this logical frame? That is, which calls demarcate the game, such that I can start making gl calls again without interefering with the previous frame?

For example, using EGL you eventually call eglSwapBuffers (most implementations have some kind of swap command). Logically this is the boundary between one frame and the next. However, this calls blocks to support v-sync, meaning you can’t issue new commands until it returns. Yet the documentation implies you can start issuing new commands prior to its return in another thread (provided you don’t touch any in-use buffers).

How can I start issuing commands to the next buffer even while the swap command is still blocking on the previous buffer? I would like to start streaming data for the next frame while the GPU is working on the old frame (in particular, I will have two vertex buffers which would be swapped each frame specifically for this purpose, and alluded to in the OpenGL documentation).

Answer 1

OpenGL has no concept of “frame”, logical or otherwise.

OpenGL is really very simple: every command executes as if all prior commands had completed before hand.

Note the key phrase “as if”. Let’s say you render from a buffer object, then modify its data immediately afterwards. Like this:

glBindVertexArray(someVaoThatUsesBufferX);
glDrawArrays(...);
glBindBuffer(GL_ARRAY_BUFFER, BufferX);
glBufferSubData(GL_ARRAY_BUFFER, ...);

This is 100% legal in OpenGL. There are no caveats, questions, concerns, etc about exactly how this will function. That glBufferSubData call will execute as though the glDrawArrays command has finished.

The only thing you have to consider is the one thing the specification does not specify: performance.

An implementation is well within its rights to detect that you’re modifying a buffer that may be in use, and therefore stall the CPU in glBufferSubData until the rendering from that buffer is complete. The OpenGL implementation is required to do either this or something else that prevents the actual source buffer from being modified while it is in use.

So OpenGL implementations execute commands asynchronously where possible, according to the specification. As long as the outside world cannot tell that glDrawArrays didn’t finish drawing anything yet, the implementation can do whatever it wants. If you issue a glReadPixels right after the drawing command, the pipeline would have to stall. You can do it, but there is no guarantee of performance.

This is why OpenGL is defined as a closed box the way it is. This gives implementations lots of freedom to be asynchronous wherever possible. Every access of OpenGL data requires an OpenGL function call, which allows the implementation to check to see if that data is actually available yet. If not, it stalls.

Getting rid of stalls is one reason why buffer object invalidation is possible; it effectively tells OpenGL that you want to orphan the buffer’s data storage. It’s the reason why buffer objects can be used for pixel transfers; it allows the transfer to happen asynchronously. It’s the reason why fence sync objects exist, so that you can tell whether a resource is still in use (perhaps for GL_UNSYNCHRONIZED_BIT buffer mapping). And so forth.

However, this calls blocks to support v-sync, meaning you can’t issue new commands until it returns.

Says who? The buffer swapping command may stall. It may not. It’s implementation-defined, and it can be changed with certain commands. The documentation for eglSwapBuffers only says that it performs a flush, which could stall the CPU but does not have to.