Rendering OpenGL in draw call back

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Brian Corrie (bcorrie++at++cs.anu.edu.au)
Tue, 16 Feb 1999 17:16:13 +1100

I have a pfProblem. We have some code, written in OpenGL, that does some
fairly sophisticated rendering (mulit-pass reflections etc). Rather than
re-writing the code in Performer, I want to integrate it (as OpenGL) into the
Performer scene graph. Although I haven't found any documentation on this type
of thing, things like the Performer Volumizer node do this. Some of Angus's
stuff at dorbie.com do similar things but not in the scene graph environment...

My idea was that I would subclass a pfGeode (with no geometry) and insert the
OpenGL rendering code in the draw call back. Thus my OpenGL would get called
with the appropriate transformations etc. from the scene graph applied. Seemed
to make sense to me... I have gone one step further and implemented it as a
database loader so I can use the node in any Performer app. Brilliant I
thought! (like most of my brilliant ideas, things are looking a bit dim at the
moment 8-)

My question/problem is twofold:

1) The above doesn't work... yet. I have a simple geometry that I am using as
a test (a simple room environment). My shading doesn't work. I have tried
everything that I can think of (although I am not a pfExpert), but in three
different Performer apps (perfly, my own code, and Avocado (a VE environment
from GMD in Germany)) I get three different results. I can't seem to turn off
the Performer geostate/lightmodel/whatever so that whatever app I use seems to
have some sort of an effect on my OpenGL. I have tried creating a GeoState in
my subclassed node and doing a gstate->setInherit(0) and then set up the
GeoState that I want. I have tried setting up the state in the draw callback
with OpenGL. Nothing seems to work. I can't seem to get rid of the Performer
state in its entirety. I would like to say something like:

draw() {
  pfGoAway()
  do_opengl()
  pfComeBack()
}

I have tried using pfPushState, glPushAttrib, and the appropriate
glEnable/glDisable calls in the Draw callback. I am confused 8-( So am I doing
something fundamentally wrong? Or am I missing something simple (the most
likely case 8-)? Has anyone done such a thing before? I think it is pretty
clear that I don't have a firm grasp of the entire Performer state machine and
how shading info is inherited. What am I missing???

2) Even if I do get the above going, will I ever be able to do multipass
things in the draw callback with OpenGL (sounds dangerous!!!)? Am I asking too
much? I didn't think so and it seem like such an elegant solution to my
problem.

Any comments or suggestions would be greatly appreciated!!!

Thanks in advance,

        Brian

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