The planetary bodies wouldn't be complete without their rings. Whilst Saturn has the most spectacular system, Jupiter, Uranus and Neptune also possess systems of their own. I chose to start with Saturn, and Figure 1 shows an initial screenshot.
Figure 1. Initial rendering of saturn ring system.
I had to consider the following points, and will discuss each in turn:
- Mesh
- Texture & Lighting
- Size & Orientation
- Shadows
Mesh
I implemented a simple algorithmic disc mesh. For the planetary bodies I can use a single mesh, scaled appropriately. However for each instance of a ring system I generate a seperate mesh since both inner and outer radii can change. In the future I may look at using a single mesh and using vertex displacement in the vertex shader to size the ring appropriately.
Texture & Lighting
I chose to start by lighting the ring equally from each side, though in reality the rings look different from above and below. Using a diffuse, per-pixel shader gives a term such as the following, where the absolute value gives the same lighting from above and below.
float diffuse = saturate(abs(dot(direction, normal)));
In order to be able to see through the gaps in the ring system, I blended the rings using the following render states:
SourceBlend = Blend.SourceAlpha;
DestinationBlend = Blend.InverseSourceAlpha;
There are numerous textures available for Saturn's rings. I took a thin slice for a texture map, and added an alpha channel corresponding to the ring transmittence.
Size & Orientation
I added a further xml file for the physical data on a ring system, and used it to size the ring mesh and apply the appropriate texture.
In order to actually see any light on the ring system, I also had to add the correct obliquity (axial tilt) to the planet, available from the NASA JPL Horizons system. This parameter also needed to be applied to the positions and orbits of the planet's moons.
Shadows
The final piece for my first iteration on the ring system was to add shadows. Shadows are cast from the planet onto the rings, and also from the rings onto the planet. For the former, when rendering the ring I implemented a ray-sphere intersection algorithm to add shadow whenever a ray fired toward the sun intersected the planet. For the latter, when rendering the planet, I implemented a simple ray-plane algorithm to add shadow whenever a ray fired from the planet intersected the ring.
