[Paraview] Question about volume viz algorithm(s)

[Comer Duncan]

Utkarsh,

Thanks very much for your reply.  I appreciate it.


> 1. I would like to know more about the algorithm that is used in Paraview
> to
> > do volume viz.  I have used Paraview to viz a cartesian grid data set
> with
> > reaonable success.  I can not seem to locate in the documentation much at
> > all about just which algorithm is being used.  And how come there is a
> need
> > to stuff the data through tetrahedralize first?
>
> What is your data type? If it's a vtkImageData then you shouldn't need
> to tetrahedralize. If it's vtkStructuredGrid then you'll need to
> tetrahedralize since ParaView currently only supports volume rendering
> for regular cartesian grids i.e image data and unstructured grids.
>

My data is a 31x31x31 array of doubles in a 3d cartesian grid.  The vtk file
header plus a couple of numbers is:

# vtk DataFile Version 2.0
brillset_a6_n64_dx0p15625_gxx.dat
ASCII
DATASET STRUCTURED_POINTS
DIMENSIONS 31 31 31
ORIGIN 0 0 0
SPACING 0.412500 0.412500 0.412500
POINT_DATA 29791
 SCALARS gxx float
LOOKUP_TABLE default
1.00006867
1.00006955
...
...
...

so, it's structured. The min is .953... and the max is 1.030... so it covers
a pretty modest range.  I have viewed this data with Paraview and thought I
had to stuff it through tetrahedralize first...was wondering how come?
The volume viz of it shows most of the action is in the middle of the
volume. Have played around with diverging and other maps. Have run
isosurface on it with success.

I am just trying to understand what methods are being used in Paraview in
order to better learn how to do better viz.



>
> For image data, ParaView uses fixed-point-ray-cast mapper (software)
> or a gpu-based raycaster. For details for the two algorithms used look
> at
> *
> http://www.vtk.org/doc/nightly/html/classvtkFixedPointVolumeRayCastMapper.html#_details
>
> For unstructured grid, we have:
>
> * ZSweep -
> http://www.vtk.org/doc/nightly/html/classvtkUnstructuredGridVolumeZSweepMapper.html#_details
> * Projected Tetrahedra  -
>
> http://www.vtk.org/doc/nightly/html/classvtkProjectedTetrahedraMapper.html#_details
> * Sofware Raycaster (Bunyk) -
>
> http://www.vtk.org/doc/nightly/html/classvtkUnstructuredGridVolumeRayCastMapper.html#_details
> * HAVS Mapper -
> http://www.vtk.org/doc/nightly/html/classvtkHAVSVolumeMapper.html#_details
>
>
Thanks very much for the pointers. Will take a look.


>
> > 2. I have recently taken a look at the vtk examples for volume viz.  They
> > are nice.  However, I decided to take a look at modifying
> SimpleRayCast.py
> > to viz my 31^3 data set i doubles but seem to find that the routine which
> > does the heavy lifting does not take doubles.  What do I need to do first
> to
> > make use of this example?
>
> You may want to use something like vtkImageShiftScale
> (http://www.vtk.org/doc/nightly/html/classvtkImageShiftScale.html) to
> transform your data to the appropriate type.
>
>
I guess not, as my data isn't an image.


Thanks again for taking the time to answer my questions.

Comer
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