Subsurface Scattering (SSS)
Subsurface Scattering (SSS) is one of the hottest recent developments in texturing. Some materials, such as flesh, wax, or alabaster, allow light to pass partially though them before it bounces back. The effect is a kind of shallow translucence that is characteristic of many organic materials. Especially in the case of skin, the lack of SSS gives an opaque look that destroys any pretense of realism and warmth.With normal rendering techniques, even objects you might expect to be translucent, like paper, can render as completely opaque, with no light passing through them. Four new SSS shaders, available only with the mental ray renderer, address this shortcoming (Figure 3.25). Two are especially customized for skin.Figure 3.25. The Deviled Egg rendered with one of the new SSS shaders in mental ray. The light from behind shows through the thin areas of the model, such as the arms and ears.
 An important limitation of these SSS shaders is that they will not coordinate with Blend, Composite, Shellac, Top/Bottom, and Double Sided materials. To overcome this limitation and allow a single model to appear to have multiple materials, you must use bitmap images to represent shading.Note | When learning how to control the SSS shaders, first exaggerate the translucent lighting effect. Then make minor adjustments to achieve a more realistic rendering. |
Playing with SSS
In the following exercise, we will explore some of the parameters that enable an exaggerated SSS effect.
| 1. | Open the file sss start.max from the DVD. The scene opens with a Torus object in the Camera01 viewport. An omni light in the center of the torus provides lighting that will scatter through the surface of the object. Within the Material Editor, the first slot is assigned to the torus. | | 2. | Render the Scene from the Camera viewport (Figure 3.26).Figure 3.26. With Standard as the Material type, the scene renders the object dark where the light does not hit the surface in front of the torus.
 | | 3. | You will need to assign an SSS shader to the torus. In the Material Editor, change the Material from Standard to SSS Fast Material (mi) and render the scene.Hmmm, you can't see much difference in the rendering. At these default values, it can be difficult to tell which parameters will exaggerate the scattering effect. | | 4. | Within the Diffuse Subsurface Scattering rollout, change both the Front surface scatter color and the Back surface scatter color to white. | | 5. | Change both the Front and Back surface scatter weight to 1.0.The color of the scattering is now white. Increasing the weight makes the effect brighter and more obvious (Figure 3.27).Figure 3.27. The SSS Fast Material (mi) applied to the torus.
[View full size image] | | 6. | In the SSS Fast Material (mi) Parameters rollout, set the Number of Samples to 1000. This will increase rendering time, but not by much. The higher the Number of Samples, the less grainy the image will render. | | 7. | In the Advanced Options rollout, set the Scale conversion factor to 0.002 (Figure 3.28).Figure 3.28. The Number of Samples affects graininess. The Scale conversion factor connects the size relationship of the translucence effect to the size of the model.
 The Scale conversion factor, set to 0.002, is 500 times smaller than its original value of 1.0. If this value is set too small, then the scattering effect will be evenly spread all over the image, and the surface will appear flat instead of 3D.Lowering the Scale conversion factor forces the model to calculate smaller. It is as if you had modeled the torus 500 times smaller! The Scale conversion factor is also highly dependent on the units you've chosen to use in 3ds max 7. You will need to first adjust the Scale conversion factor to see what works best in each individual scene.Setting the Scale conversion factor to 0.002 is not too realistic for rendering skin. But it does work well for backlit, fresh-cut juicy fruit like grapes. Once you become proficient with the parameters, try setting up different colored lights to see a colorful scattering effect.Tip | If an object is modeled larger, the Scale conversion factor will need to be lowered even more to achieve the same exaggerated scattering effect. If the value needs to go below 0.001, set the Spinner Precision higher than three decimal places. The Spinner Precision is set within the Preference Settings dialog under the General tab. |
| | 8. | Render the scene to see the new changes (Figure 3.29). To compare the final results, open the file sss finish.max from the DVD.Figure 3.29. The light is traveling through the front area of the torus object as it would through a backlit grape or other translucent fruit.
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