From: Angus Dorbie (dorbie++at++sgi.com)
Date: 01/11/2000 15:58:35
Yair Kurzion wrote:
>
> Hello Steve !
>
> Say you put a DRAW callback on a node A, and node A has children B, C, D, E.
> Performer will make sure that this callback is called before drawing ANY of the
> children of this node.
The pre draw is called before all children the post draw is called after
all children.
>..... If Performer decided to draw some children interleaved
> with other geometry (e.g. B, C, g1, D, E where g1 is some other geometry), it
> will call the draw callback multiple times - once for each time its starts
> drawing some children of A (In the example, the DRAW callback gets called
> before drawing B and before drawing D).
I'm confused, draw callbacks turn sorting off bellow the node so how
could the above scenario happen?
Cheers,Angus.
>
> Here are cases when Performer decides to interleave the drawing of some
> children of a node with other geometry:
>
> 1. The children of a node contain transparent and opaque geometry. Performer
> always starts by drawing ALL the opaque geometry and then draws the
> transparent geometry. This is true on low end machines where
> multi-sample-alpha is not available, or on high-end machines when you
> request alpha-blended transparent geometry.
>
> 2. If some of the children of a node are light sources. Performer traverses
> the light sources separately and calls the relevant draw callbacks on
> each visible light source.
>
> In the case of 'perfly -m 0 enterprise.flt', the pfScene node is a parent of
> the model enterprise.flt and of a light source. I think enterprise.flt has no
> transparent geometry so if you place the DRAW callback on sceneGroup, it will
> be called only once.
>
> -yair
>
> > I've been on a "tangent of understanding" for the past several hours
> > after noticing (from printf statements) that my (pfChanTravFrunc'd)
> > DRAW callbacks are being called twice per frame - even though I'm
> > using only one channel, there's only one parent, etc. I'd like to
> > know what's going on.
> >
> > Trying to whittle the problem down to it's simplest form, I registered
> > DRAW callbacks with the pfScene itself using vanilla perfly (C
> > version). I added to perfly.c:initSceneGraph() the following:
> >
> > pfNodeTravFuncs( scene, PFTRAV_DRAW, scenePreDraw, scenePostDraw);
> >
> > and then created the functions scenePreDraw, scenePostDraw with
> > diagnostic output. For example, scenePreDraw contains the following:
> >
> > printf("scenePreDraw - %d (node=%p,chan=%p)\n", pfGetFrameCount(),
> > pfGetTravChan(trav), pfGetTravNode(trav));
> >
> > return PFTRAV_CONT;
> >
> >
> > When run (e.g., perfly -m 0 enterprise.flt) the printf is executed
> > twice per frame - with identical node and channel pointers. E.g.,
> >
> > scenePreDraw - 61 (node=401dfdcb00,chan=401dd46990)
> > scenePostDraw - 61
> > scenePreDraw - 61 (node=401dfdcb00,chan=401dd46990)
> > scenePostDraw - 61
> >
> > This is identical to what is happening with my nodes deeper in the
> > scene graph.
> >
> > One glimmer of hope was thinking that one iteration was during display
> > list compilation and the other during display list execution, but I
> > believe -m 0 eliminates display list generation (I also tried
> > CULLoDRAW).
> >
> > I'm pretty sure my scene isn't being drawn twice ;), but I'm a little
> > concerned the code in my drawbacks is being executed twice.
> >
> >
> > (pf 2.2.3, 6.5.5m)
> >
> > There must be a logical explanation for this ..
> >
> > Any takers?
> >
> > Steve
> > --
> > stephen.maher++at++gsfc.nasa.gov (301) 286-3368 fax:(301) 286-1776
> > http://holodeck.gsfc.nasa.gov/vr/vr.html
> > http://svs.gsfc.nasa.gov http://www.digitalearth.gov
> > NASA Goddard Space Flight Center
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> >
>
> --
> \_________ \_____ \__ \__ \_____ Yair Kurzion
> \_________ \_____ \__ \__ \_____ yair++at++sgi.com
> \__ \__ \____\__ \__ http://reality.sgi.com/yair
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-- "Success is the ability to go from one failure to another with no loss of enthusiasm." - Winston Churchill.Performer + OpenGL examples and tutors: http://www.dorbie.com/
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