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

[Steve B. <sjb...@ai...>]

Scott McMillan wrote:

> If I only had a blue book.....

If you have an O2, you don't need the Blue Book - the SGI 'man' pages
are the exact same thing as the corresponding Blue Book entries.

> However on the 02 I don't get what
> you state above.  I get something slightly different:
> 
>    (0,2147482496)  - not MAX_INT

Yep - there is a reason for that.  It's because of the way glPolygonOffset
works - they need a little extra depth range beyond far-Z to allow for
glPolygonOffset and accumulated roundoff error in iterating Z across
a polygon.

They made a mistake about that on the old VGXT machines and it could
cause *MAJOR* hassles.  The slightly curtailed depth range is a kludge
to get around that....I *think*...  :-)

> If I am misunderstanding your last statement please clarify with
> some math.

...if you understand what's going on - you are one step ahead of
me!   I just regurgitate that equation on demand.
 
> When they want a depth buffer image, you don't have too many
> choices.  I was wondering if there was an extra penatly incurred
> by reading floats instead of ints.  I am not really sure where
> to go to read up on technical aspects of the depth buffer and
> read backs.

Well, you can be sure the hardware is 'int' deep down inside -
so it'll certainly take some extra math to convert it to float...
but then you'd have to convert it to float anyway - so you might
as well let OpenGL do it - just on the offchance that you might
one day find a machine with a floating point Z buffer and save
yourself a ton of time.

I think the conversion time will be small compared to the readout
time.

> P.S. should we take this discussion off-line...it is straying a
> bit from PLIB development?

Maybe...This list isn't exactly overrun with techy stuff recently.
I don't think we'll get too many complaints - and there is an
off-chance that someone out there will *know* what they are
talking about!

-- 
Steve Baker                  http://web2.airmail.net/sjbaker1
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