[vtkusers] Volume Rendering Problem

[Timur Aksoy]

Dear VTK users,
I have been a user of VTK for one year and stuck in a problem about 
Volume Rendering.
Problem:
I have a Signed Short Distance Volume of an object where object's inside 
has negative, outside has positive and boundaries have 0 level set 
scalar values.  I want to project this volume with Minimum Intensity 
Projection onto an image plane in off screen mode.  And then I want to 
convert the resulting render window into vtkimage data and finally to 
ITK image data for further processing.  That is the only way I can 
obtain MinIP in ITK, I believe.     However I want to preserve the 
negative values inside and 0-level set on the boundaries on 2D rendered 
image.  I have tried FixedPointRayCastVolume Mapper since it is the only 
Ray caster that accepts signed data types.  However in the setColor 
method of Volume Property, I can map volume intensities only to positive 
RGB values with color transfer function.  Therefore the negative values 
and 0-levelset are not preserved after rendering.   I have tried 
vtkPiecewise Function for gray level mapping to both negative and 
positive  intensities in SetColor method, but it did not produce an 
acceptable rendered image.   Is there a way I can project the negative 
intensities of the volume with MinIP and still get negative intensities 
in the render window.   I have also tried other VTK raycast functions 
but did not work.
The second related problem I have is that when I use vtkPiecewise 
Function to map gray intensity values to positive scalars in SetColor 
method, the rendered image is still unacceptable.  I was wondering why I 
cannot map volume color with vtkPiecewise Function.
Last error I get is that I cannot export VTK image data to signed short 
ITK image type.  I could export to unsigned short image type in ITK  
though.
My related code is with vtkPiecewise Function is attached.
Thank you for your time.
Sincerely,
Timur Aksoy

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