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Re: [vox-tech] OpenGL: Frustums
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Re: [vox-tech] OpenGL: Frustums



On Sun, 13 Feb 2005, Peter Jay Salzman wrote:

> On Sun 13 Feb 05,  4:58 PM, Mark K. Kim <lugod@cbreak.org> said:
> > On Sun, 13 Feb 2005, Mark K. Kim wrote:
> >
> > > On Sun, 13 Feb 2005, Peter Jay Salzman wrote:
> > > [snip]
> > > > First of all, why "-near" and not "near"?  That's such a strange definition
> > > > I can't help but feel it's a typo.
> > >
> > > Not sure.  My v1.2 redbook says the same thing.
> >
> > Oh, I think I figured out why it's -near.  I think it's because the Z axis
> > comes out of your screen, so the direction you're looking at is in the -Z
> > axis direction.  "near" is in the direction you're looking at, therefore
> > the plane you're defining is -near relative to the axis.
> >
> > -Mark
>
> I don't think I buy that.  It's true that the default viewing matrix is at
> the origin looking down the -z axis, but that's just one special case.  I
> can't believe that the designers would implement a fundamental definition
> based on a default viewing angle?   So if the camera turns around and faces
> the positive z axis...
>
> Most of the time in a 3D world, you first draw the scene, then you position
> and point the camera.  So most of the time the camera really isn't going to
> be at the origin anyhow.  (I think.)

That depends on many factors.  Because the graphics pipeline is a bunch of
matrix operations (among others), the order of operations in the pipeline
(as well as the matrix operation order) sometimes makes things come out
exactly the opposite of how one thinks things should be.

My knowledge of OpenGL is limited -- You might wanna just try it out and
see what happens.  -near vs. near should be easy to test.  Then try
changing the camera's view and see what happens.  Also try changing the
camera's view THEN setting the frustum -- I don't think that'll make any
difference but it might shed some light on how the pipeline works as far
as the frustum is concerned.

-Mark

-- 
Mark K. Kim
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