Re: Z-Buffer Resolution ?

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Angus Dorbie (dorbie++at++sgi.com)
Wed, 03 Nov 1999 14:10:46 -0800

The 'compression' is a proprietary storage scheme which actually uses a
pseudo floating point hardware representation to effectively spend more
of the precision in the distance instead of squandering it all at the
near plane. So, the kinds of scenarios you are talking about are exactly
where you win with an iR depth buffer and why we say it's the equivalent
of a higher precision simpler storage method.

It's probably advisable to measure the discrete increments in depth at a
given range using readback on an actual iR. As explained the storage
scheme is not the usual z precision depth scheme and is not something
SGI has published.

Draw a polygon, readpixels the depth move the polygon closer by some
small amount draw and read again, repeat until the value you read
changes, the overall delta between TWO measured points of change is the
discernable depth separation at the polygon range.

Cheers,Angus.

Rudi Schmidt wrote:
>
> Hi pfFolks!
>
> I'm running a gunnery simulation on an Onyx2 InfiniteReality, with a
> viewing range from 1m to 12km.
> In order to select the correct visual, I need to understand more about
> the actual implementation of the Performer's Z-buffer on an
> InfiniteReaity, and especially its resolution.
> The IR's visuals give me a choice between 15 bit, 16 bit or 23 bit
> resolution.
> QUESTION: Can somebody explain the mapping of these three versions of
> Z-buffer to my viewing range as stated above (near=1.0m, far=12000m),
> depending on the distance of an object in the scene (e.g. what is the
> Z-resolution in 6 kilometres distance from the observer ?
>
> Also, I did read -in a document called "Onyx2 Technical Report" about
> the fact that IR's z-buffer would be compressed, and that an actual 15
> or 16 bit on an IR are euqivalent to a traditional 24 bit z-buffer, as
> well as IR's 23 bit Z should be the same as conventional 32 bit.
> QUESTION: Can somebody explain this "Z-compression" ?
>
> pfThanks in advance.
>
> -- Rudi Schmidt, KMW
> --------------------
> Rudolf Schmidt
> Geltendorf (Munich)
> mailto:Rudolf++at++Schmidt.net (home)
> mailto:Rudolf.Schmidt++at++KMWeg.de (office)
> -----------------------------
>
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 no loss of enthusiasm."    -    Winston Churchill.


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