Visual imperfections while rendering with scattering atmosphere I've been experimenting with a photorealistic animation of an Earth-like planet, but I've been having a couple of rendering issues regarding the atmosphere volume material:


Visible red areas on the planet's night side, where it should be black. This is much less noticeable when switching from computing with my GTX 980 to a CPU compute, but still visible at certain angles.
The rendered volume appears to be as if it was rendering with a flat shading, as the edges of the geometry can be seen from the shaidngs of the areas close to the scattering area.


Here is a screnshot of the render result and the atmosphere setup. The planet doesn't have a night lights texture.



I'm clueless what could be the reason why this happens, as I followed the steps from here and from here. My best guess would be that I need to tweak the rendering settings a bit since it seems like the red areas are scattering miscalculations.
The problem with the 'red' scattering is due to how you are using the Volume Scatter shader - this should not generally be used on its own and should typically be combined with a Volume Absorption shader, otherwise it will producce unrealistic effects.

The 'scatter' node produces the result of the scattering in a volume whereas the 'absorption' node attenuates the light as it passes through to emulate losses due to actual absorption and also the losses due to scattering. Without combining the absorption with a Density at least as high as that used for scattering you will be breaking the usual laws of conservation of energy - effectively generating light from nowhere.

You should combine the absorption as follows :



The example is produced by two spheres - one with a diffuse shader and another (slightly larger) sphere with the volumetric material shown. Note that the density is much smaller than in your example - and ensure the volumetric 'Step' size is small enough to produce enough steps through the volume of your atmosphere.

See also https://blender.stackexchange.com/a/92327/29586 for a more realistic atmosphere (one that falls off realistically in relation to altitude).