David Megginson writes:
> Jim Wilson writes:
>
> > And the question that brings to mind is, how will we be able to set
> > the z axis in a way that it can handle the panel? In other words,
> > the scale is so large now that it'll disappear just like airplane
> > model components do when viewed too closely. Can we have two
> > different scales in the same tree/graph?
>
> Actually, I'm not sure -- any advice from the PLIB gurus?
I'm not sure about your definition of the z axis ... in this case your
question makes sense if we define the z axis to be along the center of
projection.
This really is more of an opengl issue than a plib issue.
Opengl defines the view volume to be a frustum (just like a pyramid
with a bit of the top lopped off.) You can refer to the picture here
for an example:
http://www.cs.ucf.edu/~moshell/CAP5725/CAP5725.week4.html
So if a portion of the scene is closer to us than the near clip plane,
it will be clipped out. That is what happens when you get the
external view point too close to the aircraft. Similar wierdness if
you set the far clip plane too far away and then get too close to the
ground.
The good news is that you can reset the clip planes at any time during
the rendering process meaning you can render a portion of the scene,
then move one or both clip planes, and redraw another portion of the
scene, continuing this as much as needed.
The bad news is that certain driver optimizations can get screwed up
if you move the clip planes (ie. the fog tables in some versions of
mesa). Or the driver may be forced to recompute things like fog
tables and other values when you change the position of the clip
planes. Thus, there can be visual artifact and or performance
implications with moving the clip planes.
Also, be aware that the precision of the depth buffer is *very*
sensitive to the position of the near clip plane:
http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html
The conclusion is that for best depth buffer precision we want to push
out the near clip plane as far as possible. The position of the far
clip plane is mostly irrelevant, although it has to be beyond the
furthest object we want to see. :-)
So the point of all this is that we can draw the world, then move the
near clip plane in, and then draw the panel.
This would work best if we put the panel into a different scene graph
from everything else:
setclipplanesforworld();
ssgCullAndDraw(world);
setclipplanesforpanel();
ssgCullAndDraw(panel);
Regards,
Curt.
--
Curtis Olson IVLab / HumanFIRST Program FlightGear Project
Twin Cities [EMAIL PROTECTED] [EMAIL PROTECTED]
Minnesota http://www.menet.umn.edu/~curt http://www.flightgear.org
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