Defining Scattering

 
 
 

The parameters discussed in this section are based on the Fast Subsurface Scattering Phenomena. Although much of the discussion applies when creating any fast subsurface scattering effect.

The scattering options for Fast Subsurface Scattering effects control how light is scattered on the shaded object's front and back surfaces.

Because the front and back surfaces shift, depending on the position of the camera, the scattering effect is extremely view-dependent. As a rule, scattering on either surface is most visible when that surface is lit directly.

NoteOne side effect of fast subsurface scattering's view dependency is that the scattering effect is computed only enough to display properly when seen from the camera's point of view. As a result, scattering on the back surface of the object often contains artifacts when seen in a reflective surface (a mirror behind the camera, for example).

Front Scattering

Front scattering is created when light passes just below an object's surface, underneath the diffuse shading, and scatters.

Front scattering is controlled by four parameters:

  • Color defines the color of the front-scattered light. The brighter the color value, the more intense the front scattering appears.

  • Weight controls the degree to which front scattering contributes to the overall effect.

  • Radius controls how far light is scattered along the front surface.

  • Falloff controls the falloff profile for front-scattered light. Higher values create a sharper falloff and vice-versa.

    If the falloff value is set high enough, it may cause the radius to appear shorter. If this is the case, increase the radius to smooth the front scattering.

A

The cow in this image is shaded with light blue front-scattering, red back-scattering, and no diffuse lighting.

B

Reducing the front scattering Radius causes light to scatter less in the object's front surface, creating a sharper transition between front and back scattering.

C

Increasing the Falloff creates a sharper falloff profile for the scattered light, causing the front scattering to blend less smoothly with the back scattering. This is particularly apparent on the neck.

Back Scattering

Back scattering is scattering that becomes visible when the shaded object's back surface is lit, and the light scatters deep enough within the object that it becomes visible on the front surface.

Unless a large amount of back scattering, and a very bright back light have been defined, back scattering is generally only visible on the edges, or particularly thin parts of backlit objects.

Back scattering is controlled by four parameters:

  • Color defines the color of the back-scattered light. The brighter the color value, the more intense the back scattering appears.

  • Weight controls the degree to which back scattering contributes to the overall effect.

  • Radius controls how far light is scattered along the back surface.

  • Depth controls how deeply light scatters within the object. The higher the value, the more likely it is that back scattering will be visible from the camera's point of view.

    NoteAs you increase the Depth, you'll probably need to increase the number of lightmap samples to smooth out the effect. See Lightmap Samples for details.

    Normally, Radius and Depth should be set to the same value.

   

The back scattering Color is bright red, and the Weight is fairly high (0.75). However, a low Depth value (5) makes the back scattering visible only on the edges of areas where the object is brightly backlit.

Increasing the Depth to 50 makes the back scattering visible through much more of the object.

   

Increasing the Depth and brightening the Color value's red component brightens the back scattering further.

Finally, increasing the Weight to 1 makes the back scattering far more prominent wherever it is visible.

The Scattering Bias

The Bias parameter allows you to adjust the subsurface scattering effect to favor either forward scattering or backward scattering, depending on whether the bias value is positive or negative.

  • Forward scattering means that more light from the back surface will scatter forwards. Positive Bias values favor forward scattering.

  • Backward scattering means that more light from the front surface will scatter backwards. Negative Bias values favor backward scattering.

Typically, small values (in the -0.1 to 0.1 range) are used to subtly nudge the final scattering effect in one direction or another.

A

The cow in this image is shaded with blue front-scattering and red back-scattering, both with a Weight of 1. There is no diffuse lighting. With the Bias set to 0, the scattering is uniform.

B

Setting the Bias to 0.5 causes more light from the back to scatter forward, which makes front more prominent.

C

Setting the Bias to -0.5 causes more light from the front to scatter backward, which makes back scattering more prominent.

Lightmap Samples

The Lightmap Samples parameter controls the number of times the lightmap is sampled for each rendered ray. This helps to smooth the scattering effect when speckling artifacts appear.

For subtle scattering, this value need not be particularly high (128 or lower), but for intense back scattering, you may need to increase the value quite a bit. Increasing the number of samples does increase render times, although you shouldn't see much of an impact until you start using relatively high values (1024 or greater). For best results, increase the number of samples by powers of two (32, 64, 128, 256, and so on).

A

The cow in this image is shaded with purple front-scattering and yellow back-scattering, both with a Weight of 1. There is no diffuse lighting. Lightmap Samples is set to 64, which is obviously too low.

B

Setting the Lightmap Samples to 256 smooths the result somewhat, but there is still quite a bit of visible speckling.

C

Setting the Lightmap Samples to 1024 produces a sufficiently smooth result, though there is still a touch of speckling in the cow's head.

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