Project files: sss.zip

In this tutorial we are going to use the ilrBssrdfShader to create some nice Sub Suface Scattering (SSS) effects on a planet. The planet is lit from behind, but since we want some kind of gas planet, the light should bounce around in the atmosphere. We aim for something like in the image below.

Introduction

Load the scene and do a rendering. No SSS is applied, and everything is basically set to default. The only light in the scene comes from behind the planet. It is a directional light with Ray Traced Shadows turned on.

Apply SSS

Start by assigning the ilrBssrdfShaders to the objects, the s_planet shader to the polyobject p_planet and s_ring to p_ring. Then choose the Material Preset ketchup on both the ilrBssrdfShaders. Also increase the Intensity to 5.0 (set the HSV Value to 5.0). If we render now we will start to see some light shining through.

This is a good start, but the scene require some tweaking. We begin to tweak the planet shader, and then apply this, slightly modified, to the ring. The light scatters around, but the colors are not very good.

The SSS effect is controlled by a number of settings. The first we want to change is the attribute Diffuse Reflectance. This controls the color of the light that bounces around inside the model. We will change the Diffuse Reflectance color to something yellowish (RGB: 0.2, 0.2, 0.0)

The Scatter Length controls how far the different color values will scatter (measured in world units). The planet has a radius of approximately 2.0 and the scatter length for Red is far greater than that. Try to decrease it to [1.0, 0.1, 0.1].

The reason it turns all red is that the other color components don’t scatter long enough to reach the far side of the planet.

Now that the planet looks good, apply the same values to the s_ring shader. The only difference between the shaders is that we want the ring to be illuminated almost all the way around, Increase the Scatter Length Scale to 0.2.

The specular highlight is over exposed on the ring. We need to change a few attributes on the s_ring shader to remove it. Set Eccentricity to 0.1, Specular Roll Off to 0.01 and Specular Color to (RGB: 0.05, 0.05, 0.05).

Using blocking geometry

Blocking geometry is used to absorb light from the sss object. For example this can be used to simulate a skeleton in a hand. Open the scene blocking from the tutorial project.

Blocking geometry are just placed inside the SSS object and the effect is controlled by the Absorption Color, Absorption Intensity and Absorption Exponent attributes. The Absorption Exponent attribute control how fast the blocker contribution disappears with the distance. You can have any numbers of blockers, and they can be animated.

Setting a negative intensity will make the blocker emit light instead of absorbing it. This can be used to create a neon light effect.