ParaView 5.6 / OSPRay volume rendering: inconsistent defaults for scalar opacity unit distance

We had a recent bug report where a user was seeing significantly different OSPRay volume rendering opacities for different scaled resolutions of the same data set. These had the same physical grid projections, so no difference is expected.

This was tracked down to differing default values for the parameter “scaled opacity unit distance”:

The XDMF reader and ExtractSubset filters produce different defaults for this value (where the ExtractSubset value appears to be as expected)
ParaView 5.6 vs 5.5.2 XDMF readers have different defaults (where the 5.5.2 value was as expected)

More detailed discussion of this issue, including screenshots, available in the OSPRay Github issue:

https://github.com/ospray/ospray/issues/308

Can defaults for this value be made consistent?

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