[vtkusers] Strange effect with transparent layers

[Eric Sokolowsky]

I am using vtk to do some visualizations of cloud cover data.  I wanted to use
transparent layers of polygons for this purpose.  I am using VTK 3.1.

As a test, I use vtkSetScalar on the vertices of the polygon to set the transparency
value, with values near 0 for layers further from the surface of the earth, and
values of 1 for layers nearer the surface of the earth.  I have about 8-10 layers.
However, it looks like the transparency values are not accumulating as I expected.
In places where the topmost layer is visible, only that transparency value is used,
and so you can see the base (opaque) layer all the way through all the other layers.
It's like they are completely transparent where they occlude, instead of accumulating
transparency values as I would expect.  In places not occluded by some of the
higher layers (with lower transparency values), and where lower layers occlude the
base layer, the opacity is greater, but it still does not accumulate.  This
produces a bizarre effect (see the attached image for an example).

Has anybody else seen this effect?  Is it a problem in VTK?  I realize that I am using
a relatively old version (3.1); has this problem been fixed in a more recent version?
Or is this an OpenGL problem?  I am running this code on an SGI Octane and an SGI
IR2 with similar results.

-- 
   ____   __    Eric Sokolowsky            NASA Goddard Space Flight Center
  / __/__/_/__  Visualization Programmer   Scientific Visualization Studio
 / __/ _/ / _/  esok at cosimo.gsfc.nasa.gov  Code 935.0  Bldg 28  Rm S121
/___/_//_/__/   301.286.3751               Greenbelt, MD 20771
-------------- next part --------------
A non-text attachment was scrubbed...
Name: vtkbug.png
Type: application/octet-stream
Size: 21911 bytes
Desc: 
URL: <http://www.vtk.org/pipermail/vtkusers/attachments/20011030/a0fa23cc/attachment.obj>