[vtkusers] Volume Rendering Performance With Two Volumes

[ianl]

Andras Lasso wrote
> Slicer 4.8.1 still uses the OpenGL1 backend. We've noticed slowdown with
> OpenGL2 backend in Slicer 4.9, but solved it by tuning adjustment of
> sampling settings.
> 
> See the full discussion here:
> https://discourse.slicer.org/t/volume-rendering-slow-in-latest-nightlies-for-macos/2231/21

Interesting that 3DSlicer 4.8.1 is still using the old back end and I am
seeing a similar issue. This somewhat disproves my suspicion that there
might be an issue with multiple volumes introduced with OpenGL2. However,
the effect is certainly worse in our code with the new back end.

I have tried your fix to set LockSampleDistanceToInputSpacing. This does not
seem to make much difference for us as we have
InteractiveAdjustSampleDistances set on, which presumably overrides it. As
it looks like 3DSlicer is also adjusting ImageSampleDistance from
observation, this may be a difference. I will look into perhaps varying that
too in interactive modes.

Interestingly, I have just noticed that if I have just two VTK windows with
volumes in, performance is much better and consistent with and without
LockSampleDistanceToInputSpacing. If I have other VTK windows, MPR views for
example, performance drops off on one of the volumes.


Andras Lasso wrote
> As a side note, these are very large volumes for a modest GPU such as AMD
> R7 200.

Granted, the card is a few years old, but does have 2GB of graphics memory
and renders volumes of this size and larger very nicely with the old OpenGL
back end.




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