[vtkusers] Different renderings between vtkVolumeTextureMapper3D and vtkVolumeRayCastMapper

[Weiguang Guan]

Hello,

Shall I expect vtkVolumeTextureMapper3D produces similar renderings as 
vtkVolumeRayCastMapper does? I have a 256 cubed data of low dynamic 
range (from 0 to 35). When I make a opacity a step function (1 if >=0, 
or 0 otherwise), I see a cube as expected. But the weird thing is that 
these two mappers gives a completely different looks when using a step 
function (1 if >=4, or 0 otherwise) --- the vtkVolumeTextureMapper3D 
doesn't render low-intensity voxels while vtkVolumeRayCastMapper does.

What could it be the z data spacing that makes the difference (I don't 
know what is the effect z spacing would have on rendering)? I am sure 
that I specify exactly the same spacing for both mappers.

All suggestions are appreciated. Thanks.

Best,
Weiguang