[vox-tech] OpenGL: Frustums

[Peter Jay Salzman]

I'm not understanding the Redbook's description of frustums at all.  I keep
feeling like there's a whole lot that it's leaving out.

In version 1.4 of the Redbook, it says (pg 129):

   void glFrustum(
      GLdouble left,
      GLdouble right,
      GLdouble bottom,
      GLdouble top,
      GLdouble near,
      GLdouble far
   );

   The frustum's viewing volume is defined by parameters:
   (left, bottom, -near) and (right, top, -near) specify the (x,y,z)
   coordinates of the lower left and upper right corners, respectively...


First of all, why "-near" and not "near"?  That's such a strange definition
I can't help but feel it's a typo.

Second of all, what is the significance of the near clipping plane?  So in
other words, if I hold an apple REALLY REALLY close to my face, if the apple
isn't past the near clipping plane, I won't see it?  Or does the scene get
projected onto the near clipping plane (if so, why is it called a "clipping
plane")?

Also, I'm kind of wondering - if the frustum defines clipping planes, what
happens if the frustum doesn't coincide with the direction of the camera
(set by, say, gluLookAt)?   So by default, the camera looks down the
negative z axis.  Why even bother defining a camera direction if we set a
viewing volume which, also, seems to define a camera direction?

Thanks,
Peter

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