I finished a batch of renders from 3dsmax with Corona (v2 release candidate 4)
Density had to be set to 2.5 to match TGs output.
With GI enabled, I finally had some success to get something that looks close to TG. Corona simulates anisotropic scattering, by using values from 0.6 to 0.85 (forward scaterring) it finally looked more like clouds instead of a white or gray mass. It was surprising to see that not even 10 GI bounces were enough to get a convincing look so I left it the default limit which is 25. A render with
constant scattering direction at 0.825 is attached.
Using the vdb density channel as scattering input instead of constant and a custom gradient on the output curve I came very close to the GI render from TG but it took to long to render, I'll try to play around with the options another time.
vdb_calib_01_black_cloud_no_shadows---3dsmax_Corona---Density-2.5.jpg[attachimg=1]
vdb_calib_01_shadow_of_black_cloud---3dsmax_Corona---Density-2.5.jpg[attachimg=2]
vdb_calib_01_white_cloud_with_gi---3dsmax_Corona---Density-2.5_ScatterDir-0.825.jpg[attachimg=3]
For comaprison - with isotropic scattering direction (uniform light scattering in all directions) the volume looks all white:
vdb_calib_01_white_cloud_with_gi---3dsmax_Corona---Density-2.5_ScatterDir-0.0.jpg[attachimg=4]
In conclusion, I don't see a reason why the density exported from TG should be changed, we all seem to get good results with it and looking at how differently the density is interpreted depending on the renderer it's hard to say whether the density from TG is off or not.
Matt, would it be possible to get a vdb file with more detail?
Thanks for the interesting challenge btw, it helps to understand how different media affect scattering direction in light, very interesting stuff.