[Octave-bug-tracker] [bug #44781] Qt (but not FLTK) figure draws empty axes before doing the actual plot

[Dan Sebald]

Follow-up Comment #5, bug #44781 (project octave):

Hmm, I can't duplicate that here, but OpenGL is very driver dependent.  My
system ends up using the X11 software implementation, which is kind of slow,
so may not behave exactly the same as others.

I see the OpenGL code for drawing objects in libinterp/corefcn/gl-render.cc. 
I don't quite understand how the canvas is accessed.  Is there something to
the OpenGL context and its initialization that directs which window/viewport
it should act on?  Brainstorming, one thing I'm wondering about is that these
OpenGL commands appear to be executed in the Octave thread while the Qt canvas
is instantiated in the main GUI thread. (As you pointed out.)  If so, perhaps
there needs to be a mutex keeping the GUI from acting on the Qt window while
Octave core via OpenGL is drawing in the canvas.  Has that sort of thing been
worked out?

On the other hand, there would still be a window in which the GUI thread could
act on the window if the semaphore is taken, empty axes are drawn, the
semaphore freed, [GUI comes in], the semaphore taken again, the long draw
done, the semaphore freed again.  The Octave thread would have to not let go
of the semaphore.


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