From: Yves Strube (czys++at++ocag.ch)
Date: 10/30/2001 05:16:03
Hi,
I oberserve some strange effects in my application when changing the
field of view to zoom in and out of the scene. If I zoom in, objects
which I see with e.g. a FOV of 90 degrees are clipped. I observe that at
first the parts of the objects which are further away from the point of
view are clipped and so on until the objects are no more visible at all.
This looks like the back clipping plane comes closer if I zoom into the
scene (for whatever reason) and is pushed farther away if I zoom out. So
I displayed the back clipping distance with Channel::getNearFar() and
wondered that the back clipping distance is reported to be constant!?
If I adjust the back clipping plane with pfChannel::setNearFar()
dynamically to be farther away when zooming into the scene and come
closer when zooming out everything looks well. But then I have to put in
values which have nothing to do with reality. I have to set the back
clipping plane to a value of e.g. 3.0e6 for very small FOVs to see
objects which are only 100000 units away. Otherwise these objects
disappear. So the units given for the back clipping distance are not the
same as the units given for object positions.
And it is even worse that this behaviour is not linear. I.e. if I zoom
into the scene objects which I see with an FOV of 90 degrees disappear
for an FOV of 70 degrees and appear again for an FOV of 64 degrees. This
effect can only be avoided by setting the back clipping plane to an
astronomically large value (e.g. 3.0e6) with the negative effect that my
z buffer resolution gets worse.
Since I know that the objects are in the scene and observe how they are
cut by something (must be the back clipping plane) I am really curious
how this behaviour can be explained?
I thought that the FOV has nothing to do with the front and back
clipping plane!?
I use Performer 2.4.2 (same effect with 2.4) on a Linux machine and
would be glad to get any help or a note from someone who observes the
same effect.
Thanks,
Yves
-- Oerlikon-Contraves AG Yves Strube, S-EMI Birchstr. 155 Email: czys++at++ocag.ch CH-8050 Zurich Phone: +41 1 316 2675 Switzerland Fax: +41 1 316 2032
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