[Paraview] how does ScalarOpacityUnitDistance affect volume rendering?

[Moreland, Kenneth]

Note that 1 is not always full transparency.  You probably have an object with small spatial bounds so that 1 unit in space passes through the entire model and then much more.

-Ken

   ****      Kenneth Moreland
    ***      Sandia National Laboratories
***********
*** *** ***  email: kmorel at sandia.gov
**  ***  **  phone: (505) 844-8919
    ***      web:   http://www.cs.unm.edu/~kmorel

From: Jeff Mauldin <jamauld at sandia.gov<mailto:jamauld at sandia.gov>>
Date: Tue, 2 Aug 2011 15:28:41 -0600
To: Kenneth Moreland <kmorel at sandia.gov<mailto:kmorel at sandia.gov>>, "paraview at paraview.org<mailto:paraview at paraview.org>" <paraview at paraview.org<mailto:paraview at paraview.org>>
Subject: Re: [Paraview] how does ScalarOpacityUnitDistance affect volume rendering?

Ken is right.  0 is full opacity and 1 is full transparency.

Moreland, Kenneth wrote:

The correct expression for radiative transform defines light attenuation
per unit length.  The Scale value allows you to set what that unit length
is.  The larger the unit length, the further light has to travel to
attenuate the same amount.

Your description of the behavior seems backward.  If you set the unit
distance to 0, you should see full opacity.  It means that when light
travels 0 distance it's attenuated a finite amount.  Thus, any finite
distance should attenuate all light.  The larger you make the unit
distance, the more transparent the rendering should become.  This is the
behavior I get with disk_out_ref.  Are you sure you didn't get it
backwards?

-Ken

   ****      Kenneth Moreland
    ***      Sandia National Laboratories
***********
*** *** ***  email: kmorel at sandia.gov<mailto:kmorel at sandia.gov>
**  ***  **  phone: (505) 844-8919
    ***      web:   http://www.cs.unm.edu/~kmorel




On 8/2/11 11:21 AM, "Jeff Mauldin" <jamauld at sandia.gov><mailto:jamauld at sandia.gov> wrote:



I am working with volume rendering on a particular data set.  I am doing
projected tetra volume mapping.  When I bring up the Color Scale Editor
dialog (which I have used extensively), there is an entry called "Scale"
that I haven't been able to find documentation to.  In playing with the
python tracing, I see that it alters a value called
ScalarOpacityUnitDistance.  The effect appears to be that setting this
parameter to zero causes the rendering to be completely invisible (alpha
zero everywhere, presumably) and setting it to 1 causes the rendering to
be completely opaque.  I'm guessing it's some kind of multiplier to the
opacity value in color transfer function, and possibly some kind of
multiplier applying to how much each volume ray segment contributes to
the overall opacity of a pixel.  What exactly is this scale value doing
(so I can understand what I'm looking at)?


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