Requiring some explanations about VDBs, attributes, translations, resampling, and maybe some vray rendering

[Wolrajh]

Hello good people!

So I have a few questions about VDB, mostly about things that I never cared about - until I had to.

- Why are there translations on the fields of a VDB ?
Most render engines do not seem to care, except for Vray. Vray apparently requires all those translatations to be the same for some reason - or else it will generate huge bounding boxes in Maya, and maybe even wrongly sample velocity. Resampling fields will "freeze" those transformations but I never had to do it with Arnold, RS, 3delight or Mantra.
For extra giggle, putting a simple point wrangle with @P=0 apparently emulates how Vray see things.
 

 

- Why can't I get rid of the "dopobject" and Cd attributes when saving to file ?
The dopobject attribute is only added when importing the fields with a dopimport fields and not with an object merge. But with the dopimport it just keeps coming back from god knows where, and it makes no sense.

 

- With Vray, am I stuck with having to use the same voxel size for all fields, including Vel, which can in turn creater HUGE files ?
Doc indicates Vray can't handle vector fields and I can drop a vdb vector split, but there is nothing about voxel size. I have to accumulate tons of tricks to try and lower the file size. A 250mb VDB file with vel resampled to twice the voxel size can turn into a 1gb file with vel at the original voxel size - and because of how vray handles preview in the viewport, this is unbearable. Alternative is throwing the volume quality through the window to try and hope to reduce file size.

 

I'll include a simple scene file with a few scenarios and oddities which just illustrate those quirks.
As I said I usually have no issues but complaints from the Maya Vray departement upon long loading/refresh times ( even for mere 250mb files ) or motion blur not working forced me to approach things very differently...

VDB_savingToFile.hip

Edited June 22, 2021 by Wolrajh
Just ... redid it all.

[Noobini]

i scrolled thru to see how long the 'question' is....didn't bother to read any  of it, tbh.

[Wolrajh]

19 minutes ago, Noobini said:

i scrolled thru to see how long the 'question' is....didn't bother to read any  of it, tbh.

THanks, I redid it entirely. Probably not a good idea to post under the effects of frustration and COVID vaccine.

To be fair the first question was only a few lines into the post, but I definitely put too much context and pictures.