[vtkusers] vtkVolumeTextureMapper3D behaves differently between v5.1 and v6.2

[Weiguang guan]

Hello VTK users,

We develop a viz system using vtkVolumeTextureMapper3D for volume 
rendering. Recently, we've moved up from v5.1 to v6.2. After that, we 
notice our viz system has a significant slow-down in rendering speed 
(compared on the same computer) when volume size is large, say LxWxH ~= 
512^3. It can always render small volumes at interactive rate.

I know that vtkVolumeTextureMapper3D needs to resample large volume to 
fit in 128*256*256 (BTW, our data is regular volume data with a single 
component). But this down-sampling process is not new in 6.2. Are there 
other changes that cause the performance issue? Does anyone experience 
the same issue? Thanks.

Best,
Weiguang
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