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geomview transparency

  • To: wanderst at yolanda.hper.indiana.edu, software
  • Subject: geomview transparency
  • From: munzner
  • Date: Wed, 8 Jun 94 11:20:46 -0500
  • In-Reply-To: "William Anderst"'s message of Tue, 7 Jun 1994 23:08:04 -0500 <9406072308.ZM11505 at yolanda.hper.indiana.edu>

Transparency just plain does not work right. I think we need to be
more clear about this in the manual. We don't sort polygons
back-to-front before drawing them, so while transparency occasionally
looks almost acceptable with just one object in general it's
guaranteed to be wrong. It's one of the things on our list of possible
future improvements that *might* get done in the next year.

We haven't committed the resources to doing transparency right yet in
Geomview because we just use RenderMan when we want a nice-looking
transparent still frame or sequence of frames for a movie. The
controls on the Materials panel allow you to set up the scene and
specify opacity levels for objects interactively. As you've noticed,
things look wrong in Geomview. If you just hit the Transparent button
and then don't use the opacity slider, the opacity information you
built in to your object will be correctly handled by RenderMan when
you save the scene as a RenderMan snapshot and render it.

RenderMan is a commercial product from Pixar which costs a fair amount
(several hundred dollars?) for an SGI license, but is bundled for free
with the NeXTStep operating system. 

 >Do the vertices in the triangle have to be listed in counterclockwise
 >order for the transparency to work (some graphics libraries I have
 >used require this)?
No, as I said above transparency won't work right in Geomview no
matter what. However, the vertices should theoretically be in
counterclockwise order so that the normals point in the right
direction. Because by default normals are flipped to face the camera,
you don't usually need to get this right to see the right thing. 

 >Does the lighting effect transparency (how)?

Although it's probably not the answer you wanted to hear, hope this

Tamara Munzner          ((555) 555-5555
munzner at geom.umn.edu    The Geometry Center  

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