What does absorption mean ?

[Paul Oblomov]

Like at the end of isotropic scattering we can create kinda exit color ? So, it is not Rayleigh scattering ?

[kalin.milenkov]

Hello,

The absorption parameter is used to determine the per color component (R,G,B) extinction coefficients of the volume, despite the name it doesn't tackle with absorption coefficient directly. The final extinction is the result from modulating the extinction corresponding to the primary opacity source with a factor derived from the absorption color. The extinction determines how the light is attenuated by the volume (it includes the effects from both scattering and absorption). So the three color components will be attenuated at different rate.
Roughly speaking the absorption color defines the "transparency" of the volume at specific thickness (the mean free path length defined by the primary opacity) or say the color of a white ray after traveling the same distance inside the volume.
The effect from the attenuation is visible as coloring of the shadows cast by the volume or tint in the color of the objects visible through it.

The change of the extinction coefficient will reflect in a change of the scattering coefficient as well (the ratio between the scattering and extinction coefficients is given by the 'Smoke Color'). The density of the scattering events along the ray might be different for the different color components if the absorption color parameter is not gray. This can result in the volume (or some parts of it) having a color complementary to the one given by the absorption color parameter.

The Rayleigh scattering defines a wavelength dependent scattering coefficient. The rendering though works with the three primaries only. The wavelength dependence is thus approximate. The second characteristic of the Rayleigh scattering is that it's "predominantly elastic" that is there is no absorption involved. This is equivalent to setting the 'Smoke Color' to white. The problem though is that the color control is inverted, it does show the complement of the color one might expect to see for Rayleigh scattering (that is the values of the color components to be proportional to the rate of scattering).

[Paul Oblomov]

Ok.
1) do I now able to create planetary atmosphere, without bad cropped volumetric gizmos, hard blended with anything ?
2) we have disney hyperion clouds asset - this new parameter will anyhow help me getting that great cloud look or it is more of something strange artistic effects?

[Svetlin.Nikolov]

Yo Paul,

Colored absorption basically means the light changes colors while traveling through the volume, instead of just becoming darker.

Not sure how this would help (1) or (2).

Next big thing on our todo list is custom scattering phase function so you can make clouds that forward-scatter and smoke that back-scatters, which will make them look much more like the real things.

Cheers!

[Paul Oblomov]

Sad, I thought my time finally has come

What if I compute density field out of the cloud and then do something with it ? To mimic forward scattering.

[Svetlin.Nikolov]

Hang in there! It's taking long because we need to go back and change or fix some of the old stuff so the new one can step on top, and this goes for CPU/GPU, grid volumes and particle shader... With time it's getting ever easier for us to add new stuff though, so I hope it won't be long.