[vtkusers] Strange effect with transparent layers

[Gerald Dalley]

I haven't used transparency yet in VTK, but often people find that they need
to use the vtkDepthSortPoly to get transparency to work correctly.

--Gerald Dalley
  dalleyg at dma.org

-----Original Message-----
From: vtkusers-admin at public.kitware.com
[mailto:vtkusers-admin at public.kitware.com]On Behalf Of Eric Sokolowsky
Sent: Tuesday, 30 October 2001 6:48 PM
To: vtkusers at public.kitware.com
Subject: [vtkusers] Strange effect with transparent layers


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