object render order

Yves Starreveld ystarrev at macserver.irus.rri.on.ca
Wed Sep 15 10:02:22 EDT 1999

Hi Tim, Bryan,
I ran into this also. There is no per-polygon sorting which takes  
opacity into account (unless you use ATI board on a Mac - there is an  
agl_ extension to solve it there)
The depth buffer draws the frontmost, transparent polygon, but then  
not the deeper one you should be able to see behind it.
I hacked around this by manually sorting vtkActors in a vtkRenderer  
such that the most transparent are drawn last.
This will not work on complex actors where one of two slightly  
translucent actors is not always either in front or behind the other.

Yves Starreveld
Robarts Research Institute,
London, Ontario, Canada

> As I understand it, transparency is not properly implemented in  
OpenGL, I
> too have seen odd behaviours. I don't know how to get round this, or 
> whether it is generally regarded as a bug.
> At 13:31 14/09/99 -0500, Bryan VanDeVen wrote:
> >
> >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  
> >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,  
> >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.
> >
> >TIA,
> >
> >--
> >Bryan Van de Ven
> >Applied Research Labs
> >University of Texas, Austin
> >

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