strange artifact when rendering volume

Question

I'm using a volume scatter shader to render an atmosphere for a planet. To save on time I used a spherical gradient and ran it through a color ramp to get white only on the rims of the sphere. This was then fed into the density value of the volume scatter. When rendered it is behaving as if there are more, smaller spheres of volume inside of it. I'll attach images of my nodes and the results. Note: The atmosphere and planet are on separate render layers so I can apply a blur to the atmosphere without affecting the planet.

here is the artifact across the entire planet

here is the artifact up close

here is the artifact without the planet

here is the node setup for my atmosphere

here is my compositor node setup

Solution found: Including my directional light in my atmosphere render layer fixed the issue, although I still don't know why. Does anyone know why having them separate would result in that artifact?

Answer 1

The artifact is probably due to the Color Ramp transition method Ease and the fact that your ramp control points are really close to each other. Even though this is inside the planet, it impacts your final render because of the 2 step compositing. I suggest trying B-Spline which should produce smoother transition.

An alternative is to use RGB Curve and play with the curve shape which may give you more control than the ramp for this purpose. Note the Vector Handle vs Auto Handle which impact the shape of the control point, respectively sharp and smooth.

Answer 2

As a bonus answer just to help you with animating atmospheres of planets, you actually do not need to use a volume shader if you are using blur in the compositor. Volume shaders work if you are not, however, because they achieve the blurred look of an atmosphere. If you want to use the compositor for an atmospheric look,, on the other hand, you can use a transparent node set to a bluish color, and then render it on the second layer as you did with the volume atmosphere shader. Then in the compositor just, use a little blur on that layer, and maybe a few other nodes to alter it's position or effect on the planet's look. This should increase your final render speed by a lot.