[SDL] can't do OpenGL inside of timer function

[Brennan]

Ok, it seems like the best way for me to code what I'm
trying to do is to just put a delay inside of the
infinite while loop and call my update_universe (I
eventually want to add some physics in there) after
the delay statement.  I guess I'll have to avoid using
the timer function all together.
--- David Olofson <david.olofson at reologica.se> wrote:
> On Friday 05 October 2001 06:38, Brennan wrote:
> > I am learning both SDL and OpenGL and am having
> some
> > difficulty.  Can someone please help me out?
> >
> > I copied the example from
> > http://sdldoc.csn.ul.ie/guidevideoopengl.php and
> > modified it so that I had a timer function and so
> it
> > was running in a window.  (see attached)
> >
> > Why is it that I can not make any OpenGL calls or
> > SDL_PollEvent from inside the timer callback?
> 
> Because neither SDL nor OpenGL are thread safe by
> design - and if you 
> have an OpenGL implementation that is thread safe
> (ie is aware of 
> preemptive context switches), SDL doesn't support
> it. You'd need to use 
> special (wgl, glx or whatever) calls to "bind"
> threads to OpenGL contexts 
> for that to work.
> 
> 
> > Along similar lines I have noticed that the main
> event
> > loop:
> >
> > while(1) { process_events(); draw_screen(); }
> >
> > causes an infinite loop.
> 
> Of course it does - SDL_PollEvent() does *exactly*
> what the name implies. 
> 
> Use SDL_WaitEvent() instead.
> 
> 
> > Shouldn't an event loop block in some way?
> 
> No, at least not on events, in your average single
> threaded game. (In 
> fact, you can't use a blocking event API in a single
> threaded game, 
> unless it's ok that the game freezes when there's no
> user input.)
> 
> 
> > Incoming events should not have to
> > be polled.  They should be handled through
> callbacks.
> > And the draw_screen should be done in a timer
> > callback.
> 
> I don't know about *should* here. It's a matter of
> taste and needs.
> 
> Besides, rendering should block only of SDL_Flip(),
> which should perform 
> a retrace synchronized hardware flip to the next
> buffer in a chain of 
> three. Unfortunately, retrace sync as well as triple
> buffering is only 
> possible on a few platforms, and triple buffering is
> not supported at all 
> by SDL.
> 
> 
> > Is there a way to implement what I'm trying to do?
> 
> Yes, but you have to shift things around a bit. Do
> the rendering from 
> within your main thread, and move the event loop
> into a thread of it's 
> own. Make sure that you *never* perform OpenGL
> calls, or SDL calls other 
> than the event related ones, from the event thread
> context.
> 
> 
> > Is there a reason why I can't do OpenGL or SDL
> inside
> > of the timer callback?  Is it that SDL can't do
> that
> > or something is wrong on my end?
> 
> (See above.)
> 
> The specific reason why it doesn't work at all on
> Win2k is probably that 
> Win2k has some ideas about threading... You need
> some special wgl calls 
> to tell OpenGL that you'll be calling from another
> thread than the main 
> application thread.
> 
> 
> > I'm compiling under Cygwin under Windows 2000.
> 
> Compiler shouldn't make a difference, but the code
> would probably work on 
> some other platforms...
> 
> 
> //David Olofson --- Programmer, Reologica
> Instruments AB
> 
> .- M A I A
> -------------------------------------------------.
> |      Multimedia Application Integration
> Architecture      |
> | A Free/Open Source Plugin API for Professional
> Multimedia |
> `---------------------------->
> http://www.linuxdj.com/maia -'
> .- David Olofson
> -------------------------------------------.
> | Audio Hacker - Open Source Advocate - Singer -
> Songwriter |
> `------------------------------------->
> http://olofson.net -'
> 
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