zbuffer/fog problems

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Nils Lande (nils.lande++at++ks-t.no)
Mon, 21 Jun 1999 12:18:17 +0200

        Hi,

        I'm working on a visualisation for a submarine periscope simulation
and have som problems related to z-buffer resolution.

        The scene consists of a terrain database (based on our own loader),
a dome for the sky and a sea disc. The problem is that the peiscope is close
to the sea, so I need the near clip-plane to be close. On a clear day you
can see far out at sea so I need the far clip-plane to be at a long
distance.

        I'm currently using near=0.1 and far=40 000. This gives me two
problems, z-fighting in the terrain and poor fogging.

        The poor fog is my main problem. Up to a distance there is no fog,
everything beyond that distance has uniform fogging, that is, the fog
doesn't vary with distance. If I move the near clip-plane to near=10.0 this
alleviates the problem. Changing the fog type does not seem to help.

        Is this an unsolvable conflict (small near and large far values)?

        I'm running on an Octane with IMPACTSR graphics. What puzzles me is
that if I run the code on an Onyx ("KONAL" graphics) the fog is perfect,
even if the number of depth buffer bits are the same.

        The sky dome consistes of a pfGeoSet with a pfGeoState which is
attached to the scene graph. If I didn't need to worry about the sky dome I
could move the far clip-plane closer (far =15-20 000). Could I somehow draw
the sky geoset in a draw callback with depth testing disabled so that the
sky would be in the background even if the far clip-plane is moved closer.

        Any help on this would be greatly appreciated.

        Cheers,
        Nils

        Some machine and version info:

        IRIX64 periskop 6.5 6.5.3m 01221553 IP30 (the Octane)
        IRIX64 vinoren 6.5 6.5.2m 11051731 IP27 (the Onyx)

        Performer 2.2.0

-------------------------------------------------------
 Nils Lande
 Company: Kongsberg Defence & Aerospace AS
           CCIT/Simulation & Training
           P.O Box 1003, 3601 Kongsberg
 Phone : +47 32 28 69 00 Direct: +47 32 28 69 41
 Fax : +47 32 28 69 01
 Email : nils.lande++at++kongsberg.com
-------------------------------------------------------

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