Re: [Plib-devel] Ancient history....depth buffer queries

[Steve B. <sjb...@ai...>]

Scott McMillan wrote:

> I was going through some very old info-performer archives...

*VERY* old!

> ...on depth/range calculations and I came across your formula again...

Actually the formula came from SGI - but it seems to be right for
other systems.

> I have a question.  Here's what you wrote (I think...as formatting
> has garbled the equation):
> 
> > This comes up about every 6 months....here is the stock reply...
> >
> >   The 'official' SGI solution to this is...
> >
> >     z = value in z buffer after rendering (input)
> >     range = distance to pixel in database units (output)
> >
> >     np = distance to near clipping plane
> >     fp = distance far clipping plane
> >
> >     nz = near-clip z value
> >     fz = far-clip z value
> >                 (as read from glGetIntegerv ( GL_DEPTH_RANGE,...) for OpenGL)
> >
> >     For each Z-buffer value:
> >
> >                      fp*np(fz-nz)
> >                      ------------
> >                         fp-np
> >     - range = --------------------------
> >                   (fp+np)(fz-nz)   fz+nz
> >               z - -------------- - -----
> >                      2(fp-np)        2
> >
> 
> My questions are:
> 1) has this formatting been restored correctly?

Hmmm - not sure.  I'd have to be at work to answer that one.
I'll have to let you know sometime tomorrow.

> 2) Could you explain the nz/fz equations more clearly...

Sorry - it's not my equation - I just regurgitated it from
a previous email from Micheal Jones - who no longer works
at SGI.

I'll try to dig out some actual code where I used the
equation and email it to you tomorrow.  Are you actually
using Performer or is this really an OpenGL problem?

If it's OpenGL, then I wrote a FAQ about Z buffer values
here:

  http://web2.airmail.net/sjbaker1/omniv.html

...that has the inverse equation (for OpenGL - not Performer).

-- 
Steve Baker                  http://web2.airmail.net/sjbaker1
sjb...@ai... (home)  http://www.woodsoup.org/~sbaker
sj...@ht...      (work)