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Volume Shader Density Changes with Scale

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jlapre
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I'm working on visualizing Super Nova remnants (or nebula), using Houdini VDBs.
These phenomena grow in size over insane spatial and temporal scales.
e.g. from the scale of the Solar System to 10 light-years over 100s of years!
To keep the resolution of the VDBs manageable, I'm trying to scale the VDBs up at the object level.
However, the constant smoke shader, appears to increase in density as the VDBS are scaled up.
I'm seeing this in both Mantra and Karma, so it's not render-specific.
Please find attached:
  • Mantra render scale 1 (red)
  • Mantra render scale 10 (red denser)
  • Karma render scale 1 (green)
  • Karma render scale 10 (green denser)
  • Simplified non-commercial example hip file
I tried to counter-animate the density of the shader, but the correlation does not appear to be linear. i.e. I get fluctuations in density appearance, when I want it to remain constant, while the VDB is scaling up.
Any insight would be much appreciated.
Sincerely,

Attachments:
DensityAndScale.v001.hipnc (662.9 KB)
MantraScale1.png (233.1 KB)
MantraScale10.png (246.0 KB)
KarmaScale1.png (220.3 KB)
KarmaScale10.png (291.6 KB)

Jeroen
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tamte
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it's quite normal behavior
imagine a fog of uniform density, the further the object from your POV the more occluded by the fog it will be
so at 10m it will be 10x more occluded tha in 1m so scale definitely plays role as density accumulates over distance

jlapre
I tried to counter-animate the density of the shader, but the correlation does not appear to be linear. i.e. I get fluctuations in density appearance, when I want it to remain constant, while the VDB is scaling up.
the relationship should be linear inverse
so in your test scene put this into your material's smoke density
1/ch("/obj/null1/scale")
Tomas Slancik
CG Supervisor
Framestore, NY
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jlapre
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tamte
it's quite normal behavior
imagine a fog of uniform density, the further the object from your POV the more occluded by the fog it will be
so at 10m it will be 10x more occluded tha in 1m so scale definitely plays role as density accumulates over distance

jlapre
I tried to counter-animate the density of the shader, but the correlation does not appear to be linear. i.e. I get fluctuations in density appearance, when I want it to remain constant, while the VDB is scaling up.
the relationship should be linear inverse
so in your test scene put this into your material's smoke density
1/ch("/obj/null1/scale")

Hi Tomas,
thanks for the explanation.
I'll give your counteract method another try.
Thanks
Jeroen
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