[Paraview] Question about volume viz algorithm(s)

[Utkarsh Ayachit]

> 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.

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


> 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.

Utkarsh