Creating an atmosphere with volumetric clouds

In this part, we will put volumetric clouds on the planet.

First, open the default world and save it with another file name.

WARNING: To obtain the best results, you should set the sunlight color to white with an intensity of 1.0.

First, open the default world and save it with another file name.

Then, open the sky editor. Delete all the clouds layer as we will not use them. Replace the current atmosphere by a "Multi Atmos" one. Expand the "Atmosphere" control stack. Set the "Atmosphere 1" to "KH Atmos".

The picture at the left shows the new atmosphere in the RTR, at the right in the photo realistic renderer . Looks like the sun halo is somewhat exagerated. We will fix if below.

Expand the "Atmosphere 1" control stack.

The first three parameters  "Atmosphere", "Aerosols" and "Ozone Layer" are sub control stacks that are described below.
The "Cloud Layer" 1 to 4 are sub plugins that controls volumetric clouds.
The "Primitive" sub plugin specify on which primitive object the atmosphere is applied. Only KH primitives may be selected. If no primitive is selected, the atmosphere is applied on the planet itself.

The "Atmosphere" sub control stack controls the Rayleigh scattering. This is what gives the blue color to the sky. 

The "Enable checkbox" offers the choice to disable completely the Rayleigh scattering.
The "Height" parameter specifies the standard thickness of the atmosphere. This is the equivalent height if the density was constant.
The "Density" parameter controls the atmosphere density. This parameter may be driven by a texture or a function.
The "Single Scatter" parameter determines the amount of first order Rayleigh scattering.
The "Multi Scatter" parameter is a rought approximation of the other orders of Rayleigh scattering.
The "Clouds Factor" controls the impact of cloud absortion on Rayleight scattering.

The "Aerosols" sub control stack controls the Mie scattering. This is what gives the sun halo. It is due to the aerosols contained in the low part of the atmosphere.

The "Enable checkbox" offers the choice to disable completely the Mie scattering.
The "Height" parameter specifies the standard thickness of the aerosols layer. This is the equivalent height if the density was constant.
The "Density" parameter controls the aerosols layer density. This parameter may be driven by a texture or a function.
The "Halo" parameter controls the sun's halo size.

Lowering the density gives a more acceptable halo.

The "Ozone Layer" sub control stack controls the glow of the higher layers of the atmosphere. Without it, the sunset sky color would be greenish.

The "Enable checkbox" offers the choice to disable completely the ozone layer.
The "Height" parameter specifies the thickness of the ozone layer. This is the effective thickness of the layer.
The "Density" parameter controls the ozone layer density.
The "Altitude" parameter specifies the altitude of the middle of the ozone layer.

Choose a cloud sub plugin for the cloud layer 1. At this time, there is only one king of clouds plugin: "KH Clouds".  This plugin produce fractals based volumetric clouds. Future plugins may add different kinds of clouds, like Coriolis Force driven vortextes.

Expand the just created sub plugin control stack.

The "Quality" parameter controls the quality of the rendered clouds in the photorealistic renderer. The higher the value, the higher the quality of the clouds and longer it takes to render them. A value of 0 disables the rendering of the clouds. Values up to 20 may be set. However, in most situations, a value of 2 is adequate and values higher than 5 should never be required.
The "RT Quality" parameter does the same for the real time renderer. The clouds density is slightly modified by the quality parameter. Therefore, it may be useful to be able to preview it in the RTR. However, using high values for this parameter will result in a serious slowdown of the MW user interface responsiveness. Here too, the 0 value disables clouds rendering. Avoid to set it to a high value if you don't own a very powerful computer.
The "Thickness" parameter specifies the clouds layer thickness in meters.
The "Density" parameter controls the actual shapes of the clouds. The default value of 1.0 results in an homogenous stratus layer. It should definitely be replaced by a texture or a function.
The "Altitude" parameter specifies the altitude (in meters) of the middle of the cloud layer above the planet or object base level. Do not confuse this with the altitude over the sea level.
The "Assymetry" parameter controls the how the light is scattered by the clouds. With a value of 1, the light is scattered relatively homogeneously in two symetrical lobes. With a value of 0, the light is scattered in a very narrow single lobe.
The "Illumination" parameter indicate the amount of scattered light.
The "Vertical profile" parameter controls the overall clouds density along the layer thickness. Whatever the curve, the cloud layer is truncated according to the "Altitude" and "Thickness" parameters.

Now, it's time to really play with the clouds. Drive the clouds' density with a texture. The default new texture is not adequate for first try clouds.It is particularly slow at rendering. Very constrasted clouds render faster. This may be achieved by changing the "Output Control" curve. In this case, Input limits are set to 0.4 and 0.6. The output limits sre set to 0.0 and 5.0. The curve itself is modified by forcing the end points to have horizontal tangents.

Now you are ready to play yourself with the parameters and recreate all the clouds patterns you are dreaming of.