lighting/opacity question

[Charles Law]

Jim,

>From your description, it sounds like you are setting the alpha values of
your scalars but still getting occlusion from zbuffering.  Try setting the
Opacity of the actors property to 0.99 ( anything less than 1.0 disables
the zbuffer).

Charles.

At 08:35 AM 6/24/99 -0400, Will Schroeder wrote:
>Hi Jim-
>
>The problem is this: opacity is implemented using alpha compositing in
hardware. Alpha compositing only works when the polygons are rendered in
the right order: from back to front. In your data, in one direction the
polygons are ordered correctly, in other directions, they are not.
>
>One solution is to sort the polygons based on camera position using a BSP
tree or something similar. Some folks on this list (I believe) have played
around with this, maybe you can get an implementation from them.
>
>Another solution is to use something like RenderMan or BMRT (free) that
manage the rendering of translucent primitives correctly. Of course, these
are much slower than hardware.
>
>Will
>
>At 10:08 AM 6/23/99 -0700, you wrote:
>>
>>Hi,
>>
>>I'm not understanding something about lighting/opacity, at least I'm not
>>getting the results I'm expecting. I have forty data files representing a
>>volume changing over time. ( a timestep per file). I read the data files
>>in order, extract three surfaces using marching cubes, assign each surface 
>>a color and an opacity so I can see thru the object, and display these three
>>surfaces. I works fine if I just read and display the data looking down
the z
>>axis. What I'd like to do is to move around the object. The approch I've
been
>>trying is to move the camera every time step. I have a light defined. I
set the
>>light's position to the camera position after moving the camera. I also
>>recompute the viewplane normals after the camera and light moves.When I'm on
>>the "front" of the object I can see thru the object .e.g., the object is
sort of
>>like a cone, wher there is one surface it is light blue, where there are two
>>surfaces it is a darker blue. But when I'm on the back of the object I
see only
>>light blue or a single surface. Also at 90 and 270 degrees there is a
mottled
>>look to the surfaces.
>>
>>
>>I have a mpeg file if the above discription isn't clear. 
>>
>>
>>Any ideas??
>>
>>
>>Thanks,
>>Jim
>>
>>
>>---------------------------------------------
>>Jim Harsh  E-Mail: harsh at lanl.gov
>>Los Alamos National Laboratory, MS P940
>>Los Alamos, NM 87545
>>505-665-0485, FAX - 505-665-3359
>>Date: 23-Jun-99,Time: 09:38:44
>>---------------------------------------------
>>
>>
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>
>--------------------------------
>Dr. William J. Schroeder
>Kitware Visualization Solutions / vtk Commercial Support
>469 Clifton Corporate Parkway
>Clifton Park, NY 12065
>will.schroeder at kitware.com
>1-518-371-3971 (Phone & Fax)
>
>
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