Re: transparency

• To: Stuart Wier <wier@xxxxxxxxxxxxxxxx>
• Subject: Re: transparency
• From: "From: Bill Hibbard <hibbard@xxxxxxxxxxxxxxxxx>" <billh@xxxxxxxxxxxxx>
• Date: Fri, 13 Jul 2001 16:59:23 -0500 (CDT)

Hi Stu,

This happens because correct rendering of transparent
triangles (iso-surfaces are made of lots of little
triangles) depends on sorting the triangles in depth order.
However, that order changes as the display is rotated in
3-D. This is not feasible within VisAD, although the Java3D
folks are thinking of doing something for this in a future
version. This applies to alpha blending transparency. You
can get better results with screen door transparency, but
the last time I tried making VisAD use Java3D's screen
door transparency it didn't work. Maybe I'll take another
crack at it. In general, I recommend against transparency
for iso-surfaces. We give the same advice with Vis5D, which
has the same problem (except that screen door transparency
does work in OpenGL).

Cheers,
Bill
----------------------------------------------------------
Bill Hibbard, SSEC, 1225 W. Dayton St., Madison, WI  53706
hibbard@xxxxxxxxxxxxxxxxx  608-263-4427  fax: 608-263-6738
http://www.ssec.wisc.edu/~billh/vis.html


On Fri, 13 Jul 2001, Stuart Wier wrote:

> Transparency is working in 3d idossurfaces by making a 
> ScalarMap to Display.RGBA and providing a color-alpha table
> of the form float [4][length], with the last element being alpha.
> Resetting the color table with a new value of alpha changes
> the transparency as expected.
> 
> The surface is transparent to the VisAD wireframe box, 
> some lines in black (contours of other data for example), 
> and to other isosurfaces. It's cool seeing one isosurface
> inside another one.
> 
> However the surface is always opaque to map background lines 
> and pseudo-colored data surfaces on a level under the 3D surface.
> For these objects the isosurface at full transparency verges
> to an opaque VisAD background color, black for example (unless
> the background color is changed from black).
> 
> Anyone one have any idea what is going on?
> 
> Stu
> -- 
> 
> Stuart Wier                              UCAR Unidata Program
> wier@xxxxxxxxxxxxxxxx                           P.O. Box 3000
> http://www.unidata.ucar.edu/staff/wier      Boulder, CO 80307
> 

• References:
  • transparency
    • From: Stuart Wier