Swap Buffers and Clear Back using windowing system

Question

When swapping between back and front buffers the content in the back buffer becomes undefined. I´d like to define it using the "windowing system", such as GLX, EGL, WGL. Using "native" renderer such as OpenGL (glClear) is my backup plan, don´t bother mentioning it. The reason it is backup is because I don´t want to mess with native rendering contexts. I´ll stick to X/GLX for this question but if you feel inclined to describe how to do it in other Environments then go ahead.

From the Xlib documentation (http://www.x.org/docs/X11/xlib.pdf) I find an operation, XClearWindow, for clearing window with "background pixel" (awesome name by the way... not).

1. Does XClearWindow clear front/back or both buffers? I guess back buffer makes sense but I can´t figure it out from Xlib documents alone... And if anyone asked about triple buffers, it wasn´t me!
2. Is it synchronous to OpenGL rendering, or do I have to synchronize myself by for example calling glxWaitGL before the operation?
3. Is the command blocking, ie halts until complete? Implementation dependent?

Other suggestions as how to clear the back buffer after a swap using windowing system (GLX) is appriciated.

Cheers!

Answer 1

When swapping between back and front buffers the content in the back buffer becomes undefined.

Yes, and that is a good thing.

I´d like to define it using the "windowing system", such as GLX, EGL, WGL

Why? Apart from that doing this is just as undefined, as the background after a swap nothing good will come from it.

At best it will just degrade performance if the OpenGL DDX is aware of the XClearWindow it will sync. At worst you're introducing a race condition between which the results are unpredicatable.

Other suggestions as how to clear the back buffer after a swap using windowing system (GLX) is appriciated.

Use the proper OpenGL operation: glClear(…).

Answer 2

After some research I may have found a solution. The documents seem to be in order but I have not had the chance of testing this in practice. I´ll update the answer with code once I got something working.

Does XClearWindow clear front/back or both buffers?

X does not have the concept of double buffers. Whenever interacting with X against a double buffered window, both buffers are affected. The exception being reading operations such as XGetImage that only operate on the front buffer.

X is however extended with a double buffer concept through the X Double Buffer Extension, or xdbe: http://www.x.org/releases/X11R7.6/doc/xextproto/dbe.html#dbeswapbuffers

xdbe provide the XdbeSwapBuffers operation similar to glxSwapBuffers provided by GLX. There are some important differences:

• XdbeSwapBuffers does not manage any synchronization with any client API like glxSwapBuffers do. The user has to do this manually. Luckily, GLX provide great synchronization operations (glxWaitGL and glxWaitX) that won´t stall execution. That pretty much answers my second question.
• glxSwapBuffers performs an implicit flush (glFlush) for the current context. XdbeSwapBuffers does not. When to flush or not is an application designer decision.
• XdbeSwapBuffers may swap many windows in one call.
• XdbeSwapBuffers may have different behavior when swapping: 'Undefined', 'Background', 'Untouched', 'Copied'. Read the link for details.

For clearing after a swap to a predefined color the 'Background' swap behavior is the way to go. What to clear to can be configured through X and may be a pixmap or single color (background pixel).

Is the command blocking, ie halts until complete? Implementation dependent?

An application using X have to provide its own synchronization mechanisms in many situations. This would indicate an asynchronous execution pattern, but the standard itself does not require it. I´d go with "implementation defined" with a strong suggestion that most commands for most platforms are asynchronously executed.