Re: pfEarthSky with off-axis viewing frustra

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Christopher R Volpe (volpe++at++ash.crd.ge.com)
Fri, 11 Apr 1997 11:58:21 -0400

Gan Wang wrote:
>
> On Apr 9, 1:14pm, Christopher R Volpe wrote:
> > Subject: pfEarthSky with off-axis viewing frustra
> > The man page for pfEarthSky says it doesn't work with off-axis viewing
> > frusta, but that is what I need. Can someone tell me what the nature of
> > the problem is and if there is a workaround?
> >
>
> PfEarthSky only works with "Z-up" coordinates. So you have make sure that your
> rendering coordinates have Z-up. If you scene graph is built in a non-Z-up
> coordinate system, you have to take care of the coordinate transformation
> (rotation and maybe translation if the two origin do not coincide) from your
> coordinates to the rendering coordinates. This can be done by adding a DCS on
> the top of your scene graph. Hope this help.

The database I'm using has Z being in the up direction. As for
"rendering coordinates" having Z-up, I suspect this means (correct me if
I'm wrong), that the plane of my physical display device must be
vertical, and the view-up within that plane must also be vertical. I.e.,
I can have a screen straight ahead of me, and another one off to the
left or right, angled toward me or not, and I can have a screen above me
that's not angled down towards me, but if I have a screen above me
that's angled down towards me, I'm in trouble because the "up direction"
in viewing coordinates is not parallel to the "up direction" in world
coordinates. In fact, this would cause me trouble even if the frustum
for that screen were completely on-axis (i.e. even if the vector from my
eyepoint to the center of this screen up above me were perpendicular to
the screen). Furthermore, if the display were "rolled" rather than
"pitched", I'd still have a problem because even though the plane of the
display device is vertical, the view up vector is not. So the disclaimer
about off-axis viewing frusta is incorrect and should be a disclaimer
about viewing planes with pitch and roll offsets.

Is this a correct description of the situation?

thanks,
Chris

--


Chris Volpe			Phone: (518) 387-7766 
GE Corporate R&D		Fax:   (518) 387-6560
PO Box 8 			Email: volpecr++at++crd.ge.com
Schenectady, NY 12301		Web:   http://www.crd.ge.com/~volpecr
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