Opengl Context Loss

Question

So interestingly enough I have never had an Opengl context lost (where all buffer resources are wiped) until now. I currently am using OpenGL 4.2, via SDL 1.2 and GLEW on Win7 64, also my application is windowed without the ability to switch to fullscreen while running (only allowed on start up).

On my dev machine context never seems to be lost on re-size, but on other machines my application can lose the OpenGL context (it seems rare). Due to memory constraints (I have alot of memory being used by other parts of the application) I do not back up my gl buffer contents (VBOs, FBOs, Textures, etc) in system memory, oddly this hasn't been a problem for me in the past because the context never got wiped.

Its hard to discern from googling under what circumstances an OpenGL context will be lost (where all GPU memory buffers are wiped), other than maybe toggleing between fullscreen and windowed.

Back in my DX days, context lost could happen for many reasons, and I would be notified when it happened and reload my buffers from system memory backups. I was under the assumption (and I was perhaps wrong in that assumption) that OpenGL (or a managing library like SDL) would handle this buffer reload for me. Is this in any way even partially true?

One of the issues I have is that losing context on a resize, is pretty darn inconvenient, I am using ALOT of GPU memory, and having to reload everything could pause the app for while (well longer than I would like).

Is this a device dependent thing or driver dependent? Is it some combination of device, driver, and SDL version? How can a context loss like this be detected so that I can react to it?

Is it standard practice to keep system memory contents of all gl buffer contents, so that they may be reloaded on context loss? Or is a context loss rare enough that it isn't standard practice?

Answer 1

Context resets (loss) in OpenGL are ordinarily handled behind the scenes completely transparently. Literally nobody keeps GL resources around in application memory in order to handle a lost context because unless you are using a very new extension to OpenGL (robust context) there is no way to ever know when a context reset occurs in OpenGL in order to handle lost state. The driver does all that for you ordinarily, but you can receive notifications and define behavior related to context resets as described in heading 2.6 - "Graphics Reset Recovery".

But be aware that a lost context in OpenGL is very different from a lost context in D3D. In GL, a lost context occurs because some catastrophic error occurred (e.g. shader taking too long or memory access violation) and are most useful in something like WebGL, which has stricter security/reliability constraints than regular GL. In D3D you can lose your context simply by Alt + Tabbing or switching from windowed mode to fullscreen mode. In any event, I believe this is an SDL issue and not at all related to GL's notion of a context reset.

Answer 2

You're using SDL-1.2. With SDL-1.2 it's perfectly possible that the OpenGL context gets recreated (i.e. properly shut down and reinitialized) when the window gets resized. This is a known limitation of SDL and has been addressed in SDL-2.

So either use SDL-2 or use a different framework that's been tailored specifically for OpenGL, like GLFW.

Or is a context loss rare enough that it isn't standard practice?

OpenGL context's are not "lost". They're deallocated and that's what SDL-1.2 is doing in certain conditions.