Shader parameters

Hi There,
I’m experimenting with “smoke” vs “smoke/scattering” shader.
Can someone give me a better explanation about the difference between the two?
Smoke has Scattering albedo and Extinction coefficent , and the extinction seems to control the “opacity/alpha”, while the other other seems driven by the scattering coefficient…
The colors too seems to react in the opposite way.
Sorry for the dummy question, I’m just a compositor…

Hi Simone,

So the basic idea is that the “Smoke - scattering/absorption” shader is giving you direct control over the physical scattering and absorption coefficients, which is what the renderer actually uses internally, whereas the “Smoke” shader is trying to provide some easier to use and more intuitive parameters. Both shaders have the same capabilities and can specify the same materials, so I’d recommend using the “Smoke” shader as the default choice.

As you noticed the extinction coefficient controls the opacity. Extinction is actually just the same as scattering+absorption, which means that in the “scattering/absorption” shader whenever you change either of the scattering or absorption coefficients, the opacity will change also. This can make it hard to use, since the opacity will be constantly changing when you are adjusting the scattering properties that control the smoke appearance. On the other hand, the “Smoke” shader lets you set the extinction directly, and then you can modify the other visual properties independently without affecting the opacity.

The scattering albedo parameter should be between 0 and 1, and it controls the ratio of scattering to absorption. So an albedo of 1 will give a material that scatters all the light falling on it and absorbs none of the light. An albedo of 0 will give a material that only absorbs light, and does not reflect any light.

One other thing to bear in mind is how the colours will work. The scattering colour works pretty much as you would expect, it controls the colour of the light that is scattered. The absorption/extinction colour works a little differently. Since it is controlling the colour of the light that is absorbed, the remaining light will be the complementary colour. So if the absorption colour is blue, the remaining light will be yellow.

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