[Paraview] How to do volume rendering of image data?

[Moreland, Kenneth]

Uh, what lookup table?  The data file you posted has no LOOKUP_TABLE entry.  Even if it did, the VTK format specifies that the LOOKUP_TABLE is applied to a SCALARS entry whereas the molar.vtk file has data stored as COLOR_SCALARS.  Even if the RGB values were listed under a SCALARS entry (which is an error in itself), the lookup table cannot be applied to a vector.  VTK specifically defines a lookup table as something that converts scalars to colors.  I'm not sure what the interface would look like for a lookup table that operated on vectors in either the VTK API or in a GUI.

As David DeMarle suggested in a separate email, it makes more sense to derive an alpha field and apply it to the color data.  I'm still not sure ParaView supports directly volume rendering RGBA color values (the majority use case is to analyze general scalar fields), but perhaps it should.

-Ken


On 7/19/10 11:53 PM, "Steve Huntley" <stephen at xhuntley.net> wrote:

The alpha values are in the lookup table.  Each line in the lookup table
has four values: red, green, blue, opacity.  That's my understanding of
a properly formatted lookup table in a vtk file.  I used:
http://www.vtk.org/VTK/img/file-formats.pdf as a reference.

All opacity values are set to 1.0, except black which is set to 0.0.
But when the file is opened in ParaView and volume representation
chosen, it appears that not only the opacity values but the entire
lookup table is ignored.

Assuming homogeneous opacity is the furthest thing from what I want.  I
want to find a way to control opacity without destroying the colors in
my image data file via mapping to the default color scale.  Getting away
from homogeneous opacity is what I can't figure out how to do.  I want
to see through the black background pixels to see the full color pixels
of my 3D object.

--Steve H.

Moreland, Kenneth wrote:
> The data you sent has RGB values but no alpha values that I can see.
>  Volume rendering is generally not very useful if you do not have some
> representation of opacity.  If you were to assume a homogeneous
> opacity, you would get a mostly black box with a blob of color in the
> middle if you looked hard enough.  How exactly you expect the volume
> renderer to behave?
>
> -Ken
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   ****      Kenneth Moreland
    ***      Sandia National Laboratories
***********
*** *** ***  email: kmorel at sandia.gov
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