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

[Stephen J B. <sj...@ht...>]

> 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 ------------------------------


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