Re: [Freeglut-developer] Need help with offscreen rendering

[Sven P. <Sve...@ae...>]

Am Donnerstag, 11. Mai 2006 22:37 schrieb Orion Poplawski:
> [...]
> I think the initial reshape callback is unexpected by the code and
> throws things out of whack.  Now, I'm not sure it is reasonable for the
> code not to be able to handle multiple calls to the reshape callback,
> but there it is.  It would however be more efficient if there was only
> one call.
>
> Also, it throws off code like I sent before which tries to enforce
> limits on the size of the window and calls glutReshapeWindow() from the
> callback.  In this case it would get the (300,300) call which would then
> set the size back to (300,300).  Uncomment the two lines in changeSize()
> to see that in action.
>
> Thoughts? [...]

Some remarks:

 * In general, all changes to the window sizes and positions done in a program 
are only *requests* for the window manager. It might honor them exactly, 
modify them or ignore them completely. This is up to the window manager 
policy, and has nothing to do with GLUT or freeglut. Therefore your program 
should work with all possible window sizes/positions, as there can't be any 
guarantees.

 * In GLUT/freeglut, almost nothing is done immediately. More or less all 
calls only set some internal state, and those changes are only processed when 
control has been relinquished to the main loop. When e.g. glutReshapeWindow 
is called, only the requested size is remembered internally, along with the 
fact that the window manager should be informed about the request when the 
main loop has control again. When the latter will actually happen is 
undefined, there might be other changes waiting etc.

 * Don't make any assumptions about how often and when callbacks are called, 
this is again mostly up to the window manager. One of the few things which 
are guaranteed is that the reshape callback is called at least once before 
the display callback is called for the resized window (it would not make 
sense the other way round).

 * To set the initial window size use (not surprisingly) glutInitWindowSize, 
and not glutReshapeWindow before the main loop has been entered. This is a 
border case, so we should check what GLUT does here, but anyway, using 
glutInitWindowSize is aesthetically nicer and more portable.

Cheers,
   S.