object render order

Bryan VanDeVen bryanv at arlut.utexas.edu
Tue Sep 14 14:31:01 EDT 1999

I seem to be having some troubles with object order.  I have an
application that displays an isosurface of sonar signal excess along
with the underlying bathymetry (a warped grid).   At certain view
angles, the surface can be seen (correctly) above the basin actor.
However, when a user starts interacting, things run afoul.  Some of the
polygons that make up the warped grid basin begin to "pop up front" in
front of the isosurface as the scene is rotated.  In fact starting from
a view straight down with all of the basin correctly behind the
isosurface, just zooming straight out will produce this behaviour where
the basin starts to appear "above" the isosurface.

I'm willing to believe that perhaps the basin and the surface aren't as
precisely lined up as could be, but it seems to me scenes should still
render consistently (right or wrong) through rotations and zooming,
maintaining initial relative positions even if they are off.

This happens whether the basin is opaque or semi-transparent, stripping
or decimating the grid does not seem to give any change, and the order
in which I add the actors does not seem to matter either.

Someone please tell me I am overlooking something simple and stupid :)
Any and all advice is very welcome.


Bryan Van de Ven
Applied Research Labs
University of Texas, Austin

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