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

[Scott M. <mcm...@ca...>]

In answer to a previous question: This is an OpenGL question and
not a Performer one.

I have been having problems with the approach below on one piece
(so far) of NT/PC hardware:

   the AccelGalaxy card and/or its OpenGL driver.

The culprit is the following piece of code that I am not sure is valid
OpenGL code:

   glGetIntegerv( GL_DEPTH_RANGE, z_vals ) ;

The OpenGL 1.1 manual says to use glGetFloatv for this enum, and
AccelGalaxy drive truncates the floating point values to ints and
reports 0 and 1 for near far (where as an SGI O2 would report 0 and
something close to MAX_INT).

The way I did get this to work is to sample the depth buffer as
GL_FLOAT (instead of int) and use the results of 

   glGetFloatv(GL_DEPTH_RANGE, z_vals);

I still need to do timing to see if sampling the depth buffer as
GL_FLOATs incurs an extreme penalty (not sure I have much choice
though as I need not only a cross platform solution but a cross
OpenGL driver solution).

scott


Stephen J Baker wrote:
> 
> > 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?
> 
> Yes. That's correct.
> 
> > > 2) Could you explain the nz/fz equations more clearly...
> 
> Here is what I use in an actual Performer application:
> 
> --------------------------- CUT HERE ---------------------------
> /*
>   This routine calculates the terms:
> 
>               fp*np(fz-nz)
>      z_top =  ------------
>                  fp-np
> 
>   and
>               (fp+np)(fz-nz)   fz+nz
>      z_bot =  -------------- + -----
>                  2(fp-np)        2
> 
>   So the actual per-pixel z-to-range math can be boiled down to:  z_top / ( z - z_bot )
> */
> 
> void read_z ( pfChannel *chan )
> {
>   float z_top ;
>   float z_bot ;
>   float np, fp ;
>   float nz, fz ;
>   int width  = ...whatever... ;
>   int height = ...whatever... ;
>   int top, bot, left, right = ...whatever... ;
>   unsigned long z_result [ height * width ] ;
>   float result [ height * width ] ;
>   GLint z_vals[ 2 ] ;
> 
>   /* Set up the math... */
> 
>   pfGetChanNearFar ( chan, & np, & fp );
>   glGetIntegerv    ( GL_DEPTH_RANGE, z_vals ) ;
> 
>   nz = (float) z_vals [ 0 ] ;
>   fz = (float) z_vals [ 1 ] ;
> 
>   z_top = -fp*np *(fz-nz) /  (fp-np) ;
>   z_bot = (fp+np)*(fz-nz) / ((fp-np) * 2.0 ) + (fz+nz) / 2.0 ;
> 
>   /* Read the Z buffer... */
> 
>   glReadBuffer ( GL_BACK ) ;
>   glReadPixels ( left, bot, right-left+1, top-bot+1,
>                  GL_DEPTH_COMPONENT, GL_INT, z_result ) ;
> 
>   /* Convert to real-world units... */
> 
>   for ( ...each Z value to convert... )
>     result[...whatever...] = z_top / ( (float) z_result[...whatever...] - z_bot ) ;
> }
> ---------------------- CUT HERE ------------------------------
> 
> Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
> Work: sj...@ht...            http://www.hti.com
> Home: sjb...@ai...       http://web2.airmail.net/sjbaker1
> 
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-- 
Scott McMillan                        mailto:mcm...@ca...
Cambridge Research Associates         http://www.cambridge.com
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