[SDL] [SDL1.3 hg] Sharing a SDL_GLContext between two SDL_Windows

interim_descriptor interim.descriptor at gmail.com
Sun Dec 19 13:38:03 PST 2010

Thanks for your suggestion!

In fact, I had indeed been previously using one very large window that spans
all displays (with a single rendering thread).

The issue that made me depart from this method was vsync and framerate: if I
turned on vsync, I couldn't tell OSX to which display I wanted to vsync my
multi-display-spanning window. This resulted in *really* strange behavior,
where I was getting 30 fps for 1 second, then 60 fps for the next, back and
forth like clockwork, for no apparent reason! I couldn't resolve this, so I
went to 1 window per display, which allowed me to maintain 60fps, with vsync
only applied to the 2nd display.

On Sun, Dec 19, 2010 at 6:12 AM, Alex Barry <alex.barry at gmail.com> wrote:

> You could align one window to be as big as 2 or three monitors (the OS
> should make this fairly automatic).  From there, query the size of each
> monitor and divide the window into the three sizes (SDL_Rect would come in
> handy).  Next, create three seperate threads and a mutex (to be shared) -
> each thread locks the mutex, draws on it's portion of the window, then
> unlocks the mutex - this will probably be less hardware hungry.  You'd have
> to make sure that the OS "aligns" each monitor to the right, or something
> like that, though - it's very dependant on how the monitors are setup.
> Does that help at all?
> -Alex
> PS: Otherwise, your implementation is more independant on the monitor
> setup, and probably isn't that hardware intensive - especially if you're
> using a nice gfx card/cpu (which most VJs should have, I'd assume).
> On Sun, Dec 19, 2010 at 2:31 AM, interim_descriptor <
> interim.descriptor at gmail.com> wrote:
>> Hi.
>> I'm currently sharing a single SDL_GLContext between two SDL_Windows (one
>> on each display, the 2nd of which has vsync enabled).
>> I create a 3rd invisible window, which has dimensions equal to the greater
>> dimensions of either window, and then call SDL_GL_CreateContext() on its
>> windowID.
>> Each frame, I:
>>    1. Render the 1st window
>>    2. Call SDL_GL_SetSwapInterval(false) disabling vsync
>>    3. Call SDL_GL_SwapWindow()
>>    4. Call SDL_GL_MakeCurrent() for the 2nd window
>>    5. Render the 2nd window
>>    6. Call SDL_GL_SetSwapInterval(true), enabling
>>    7. Call SDL_GL_SwapWindow()
>> This seems to work pretty well, but I'm wondering if there's a better way
>> to do what I want to do, or if there are any performance implications I
>> should be aware of...
>> The reason I'm sharing a glContext is because I'm writing VJ software the
>> plays back multiple video simultaneously, so I'd like to only update one set
>> of glTextures.
>> Has anybody else solved this sort of problem in a way they believe to be
>> more efficient?
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