From: Yong-joo Cho (ycho++at++evl.uic.edu)
Date: 05/17/2001 12:50:56
Let's do it at 3...
Yongjoo Cho
On Thu, 17 May 2001, Radomir Mech wrote:
> phowe++at++csc.com.au wrote:
> >
> > Hi,
> >
> > I'm wanting to read back values from the Z-buffer using glReadPixels and
> > I have found I'm getting different values on an Octane2 compared to an
> > Oynx2 and an O2. What I'm doing is using gluProject to convert a point
> > in world space to screen space, and compare it with what gets read back
> > from the Z buffer (to determine if the point is occluded by another
> > object).
> > On an Oynx2 and O2 the values match (more or less), but on an Octane2
> > they are completely different. Now I have determined that the cause of the
> > issue is due to the fact that we change the projection matrix to
> > orthographic
> > before the call to glReadPixels. If the call to glReadPixels is made before
> > the projection matrix is changed, the values match.
> >
> > Does anybody know what is happening here, does the glReadPixels call
> > use the projection matrix for converting the Z value somehow in openGl 1.2
> > verses 1.1 ? Is there away to avoid this?
> >
> > The man pages mentions on option GL_DEPTH_COMPONENT24_SGIX,
> > which I tried using but it gave me the same result.
> >
> > The tech spec on the Octane2 mentioned it has a24bit eye space Z buffer.
> > What does that mean? Is that different to the Oynx2?
> >
> > Any Information would be appreciated.
> >
> > Thanks
> >
> > Peter
> >
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> .H 3 "Depth Buffer in Eye Space"
>
> Hi Peter,
>
> Release notes on Ocane2 say:
> ----
> The Octane2 VPro graphics system stores eye Z values in the depth buffer
> instead of device Z values as many other graphics chips do.
> Specifically, it stores eye Z divided by eye W. Let us call this divided eye Z.
> In addition, the eye Z values are stored in the depth buffer in an internal
> floating point representation.
>
> Note: Eye coordinates are obtained by multiplying the object coordinates
> by the modelview matrix. Eye coordinates multiplied by the projection matrix
> yield
> clip coordinates. Clip coordinates divided by their W coordinates (clip W)
> result in
> device coordinates.
> ----
> This should answer your question about the difference between Octane 2 and an
> Onyx2.
>
> When reading from the depth buffer, the OpenGL driver needs to convert divided
> eye Z values stored in the depth buffer to device Z values the user expects. To
> do this it needs to access the projection matrix, to determine the distance to
> the near and far clip plane from the matrix and to apply these distances to
> convert divided eye Z values to device Z values. If you change the projection
> matrix, you will receive incorrect values.
>
> Hope this helps,
> Radomir
> -----------------------------------------------------------------------
> List Archives, FAQ, FTP: http://www.sgi.com/software/performer/
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>
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