Can anyone explain what volume medium is ? Tweet

[coilbook]

Hi
I've been using octane for a while but still cannot tell the difference between medium radius (the sun roll out) and density (volume medium texture). They both control thickness of the fog What is the difference? Thanks

[bepeg4d]

Hi,
we have already told about Medium radius here, for example:
viewtopic.php?f=24&t=55567&p=284654&hilit=coilbook#p284654
Even if it seems to you to have the same impact of the Density value, they are different.
About Medium Radius, imagine that there is a virtual fog sphere around your camera, in this way the fog is limited to a specific dimension, in general greater than the scene dimensions, giving the illusion that the fog is covering the scene till the horizont.
If the fog sphere is smaller then the scene dimensions, the fog is less present, but can also gives unwanted results, with God Rays for example. The value is outside the Medium node, and independent from the values inside it.
On the other side, the Density is a multiplier of absorption and scattering values of the Medium node chosen, and it is relative to the kind of medium the light hhas to pas through. For air/fog, you should use a very low density value like 0.05, as a starting value. Since the Medium node can be used also with materials, the density needs to be adjjusted accordingly.
In short:
Medium Radius = scene/camera dependent
Density = material dependent

Happy GPU Rendering,
ciao Beppe

[coilbook]

Hi,
we have already told about Medium radius here, for example:
viewtopic.php?f=24&t=55567&p=284654&hilit=coilbook#p284654
Even if it seems to you to have the same impact of the Density value, they are different.
About Medium Radius, imagine that there is a virtual fog sphere around your camera, in this way the fog is limited to a specific dimension, in general greater than the scene dimensions, giving the illusion that the fog is covering the scene till the horizont.
If the fog sphere is smaller then the scene dimensions, the fog is less present, but can also gives unwanted results, with God Rays for example. The value is outside the Medium node, and independent from the values inside it.
On the other side, the Density is a multiplier of absorption and scattering values of the Medium node chosen, and it is relative to the kind of medium the light hhas to pas through. For air/fog, you should use a very low density value like 0.05, as a starting value. Since the Medium node can be used also with materials, the density needs to be adjjusted accordingly.
In short:
Medium Radius = scene/camera dependent
Density = material dependent

Happy GPU Rendering,
ciao Beppe

Thanks a lot for explaining!
By the way anyway to have this added in the future, From the camera to whatever distance you choose like a buffer zone where medium volume wont kick in so upclose objects look nice and rich without changing there color by the medium volume. I know you can add a sphere around the camera but then there is a sharp line where you can see the beginning of the fog. Like in fining nemo upfront objects were very nice and colorful and had no color change and about 20-30 feet after fog started

[bepeg4d]

Hi,
the Environment medium offers you ‘simple’ fog/god rays effect, for more detailed effects, better to use a Volume with ramp, to differently attenuate scatteting and absorption where you want.
ciao Beppe

[coilbook]

Hi,
the Environment medium offers you ‘simple’ fog/god rays effect, for more detailed effects, better to use a Volume with ramp, to differently attenuate scatteting and absorption where you want.
ciao Beppe

Thanks!
By the way do I need to add anything else I am trying to make foreground without scatter but the ramp does not do anything

[coilbook]

It seems that volume with ramp does nothing. is it not working?

[promity]

For me this question is also an old headache.

[haze]

It seems that volume with ramp does nothing. is it not working?

Make sure that the Max grid value is accurate. If it is too big, then the ramp will only affect a very small portion of the density spectrum.

[coilbook]

It seems that volume with ramp does nothing. is it not working?

Make sure that the Max grid value is accurate. If it is too big, then the ramp will only affect a very small portion of the density spectrum.

Hi I am not using any VDB. Just octane sun with volume medium added. Will scattering ramp even work with it, And how do I know what value I need? Thanks

[haze]

It seems that volume with ramp does nothing. is it not working?

Make sure that the Max grid value is accurate. If it is too big, then the ramp will only affect a very small portion of the density spectrum.

Hi I am not using any VDB. Just octane sun with volume medium added. Will scattering ramp even work with it, And how do I know what value I need? Thanks

OK I understand. Volume medium has some extra settings which are actually only used with Volumes, so if you attach a volume medium to your environment, then the step length and ramps are not used. Same for the step length in the scattering medium and absorption medium.

To come back to the original question, I think there are 3 options for achieving the effect you are looking for:
1. Increase the scale of the scene in the direction away from the camera, so that the absorption effect seems to start beyond the point you want, or
2. Create a custom sphere around the area with a medium that you want to have attenuated light
3. Create a VDB with the shape you want, with densities lower at the edges, then use the ramps to change how it behaves near the edges where densities are lower

I would try option 1 and experiment with the Medium radius in the environment, before trying the other options.

If you can send us a scene, we can look into it too.