[vtkusers] Why data value range of volume affects its rendering

[Weiguang Guan]

Hi all,

I recently notice an issue (maybe a bug) with volume rendering algorithms 
(vtkVolumeTextureMapper3D and vtkVolumeRayCastMapper), and cannot find a 
posting reporting the issue in the mailing list.

I have a a scalar volume of vtkImagedata whose value ranges from 0 to 
7035. By thresholding it at 255, I generate another volume with value 
range [0-255]. Then, I do volume renderings (both vtkVolumeTextureMapper3D 
and vtkVolumeRayCastMapper) of them seperately with the same opacity 
transfer function:

vtkPiecewiseFunction opacityTransferFunction
     opacityTransferFunction AddPoint  50   0.0
     opacityTransferFunction AddPoint  255  0.2
     opacityTransferFunction ClampingOff

Should I expect to have exactly the same renderings? Rendering with
vtkVolumeTextureMapper3D produces a great difference while rendering with 
vtkVolumeRayCastMapper produces a slightly difference but still 
noticeable. Can anybody explain why?

I have a tcl script to reproduce the "bug", and can let you have it if 
you're interested.

Weiguang