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

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

Steve:

I was going through some very old info-performer archives on
depth/range calculations and I came across your formula again, and
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?
2) Could you explain the nz/fz equations more clearly...
   Did you really mean glGetIntegerv, or do I do something
   like the following?:

   float depth_range[2];
   glGetFloatv(GL_DEPTH_RANGE, depth_range);
   nz = near_clip - depth_range[0];   // indices
   fz =  far_clip - depth_range[1];

   With integers I get 0,2147482496 (not MAX_INT)
   With floats   I get 0,1

3) What format should I read the depth buffer in in the first place?

Note that I am querying the depth buffer as floats with the following:

GLfloat *depthData3 = (GLfloat *) malloc(sizeX*sizeY*sizeof(GLfloat));
glReadPixels(0, 0, sizeX, sizeY,
             GL_DEPTH_COMPONENT, GL_FLOAT, depthData3);

Any help is greatly appreciated,
scott

-- 
Scott McMillan
mailto:mcm...@ca...

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