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Cloud is a material available from Render's matter panel.
Allows to make all voxels of the selected color to behave as a cloud in the scene.

Interface 0.99.5a render matter-cloud

Density

Range from 0 to 100, unitless. Can be changed using the related slider or by entering a number manually.

Phase mix

Range from 0 to 100, unitless. Can be changed using the related slider or by entering a number manually.

Phase 0 & 1

Phase 0 and phase 1 range from -0.90 to 0.90, unitless. Can be changed using the related slider or by entering a number manually.

Cloud material is available on Cube, Sphr and Cyli shapes.

Clouds a deep dive

How do clouds work?

Clouds have 3 main components.. ...Cloud(density) ...Mix(Phase mix) ...Phase 0/1.

Understanding density is quite simple. Cloud density is determined by the slider from 0-100 and the more you add the more dense the material becomes. Clouds at 1 are an almost invisible haze, but clouds at 100 will be as dense as sponge.

This was easy enough. How to use mix? To understand mix, you first have to understand how Phase works.

Phase determines how light rays scatter in the cloud material. The Phase value -0.90 to 0.90 specifies if rays scatter "backwards"(negative values make rays scatter backwards) or "forwards"(positive numbers make rays scatter forwards).

What does that mean?

Cloud Interface - Arrow Phase 0
Backwards Scattering means that if a light ray hits the cloud more of the light will get reflected and scatter back.

This makes the cloud brighter because more light hits the camera. Setting the value to -0.90 will make the cloud look almost like cotton or snow.

Forward scattering means if a light ray hits the cloud most of the light will penetrate the cloud material and less light will be reflected, making the cloud darker. Setting the phase value to 0.90 will make the voxel almost completely black.

How does Mix affect the clouds appearance? Instead of seeing the mix value as slider from 0-100 you need to look at it as a scale. Why? Because Mix determines how much of each scatter occurs in the cloud material.

Thats why 2 Phase values exist. If you set Phase 0 to -0.90 and Phase 1 to 0.90 and set mix to 50 you will get a 50/50 split of each scatter type. If you set mix to 0 you will only use the value in Phase 0(the left number)and if you set mix to 100 you will only use the value in Phase 1(the right number).

Usually you don't need to change the phase value,they are set by default to represent realistic clouds. You can usually get realistic looking clouds by tweaking the mix and density values.

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