Fast Simple (misss)

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Category: Illumination

Shader Family: Material Phenomenon

Output: Custom

In the real world, many materials are translucent to some extent, and do not immediately reflect light at their surface. Instead, light penetrates the surface and is scattered inside the material before it is either absorbed or transmitted. This effect called "subsurface scattering" greatly enhances the realism of a wide variety of rendered materials, even when used sparingly.

This shader simulates the appearance of subsurface scattering, but not in a physically correct way. As such, it allows you to tune the scattering effect by eye rather than requiring real-world scattering and absorption coefficients (not to mention photon lighting) as does the misss_physical_phen shader.

The Fast Subsurface Scattering shader supports bump mapping and displacement mapping. Although these ports are not exposed in the shader's property editor, they appear in the shader's render tree node, allowing you to connect bump and/or displacement shaders.

Note that if you connect a bump map to the bump map port, the bumping affects only the diffuse lighting component. This is because the various levels of scattering take place below the object's surface, and are not affected by bump mapping.

For more information and images about subsurface scattering, see Fast Subsurface Scattering Effects.

Note

To simulate human skin, you can use the Fast Skin (misss), which is a specialized and more complex variant of this shader.
Note that fast subsurface scattering effects are not included in the maps generated by the RenderMap tool.

Surface

Ambient Color

Controls the color of the ambient illumination component. The ambient component is included in the lightmap, and is scattered like other light.

Overall Color Multiplier

The diffuse component and both scattering components are multiplied by this color value to produce the final scattering-effect color. This can be used in several ways, among them:

To brighten or darken the effect overall.
To tint the final result.
To connect maps that attenuate light. For example, dirt on an object's surface would block most of the scattered light.

Diffuse

Color	Defines the color of the diffuse illumination component. This is normally driven by the object's main surface texture.
Weight	Controls the weight of the Diffuse illumination component when it is blended with the other components (ambient, scattering, etc.).

Specular

Color	Defines the Specular surface color in the illuminated area of the object. The value is blended with the object's Diffuse and Ambient values.
Specular Decay	Defines the rate at which the specularity decays outward.

Scattering

Front Scattering

Color	Controls the color for the front surface scattering. The front surface is the side of the object facing the camera, and so front surface scattering is most visible when that side is well lit.
Weight	Controls the weight of the front scattering surface when it is blended with the other lighting components (back scattering, diffuse, etc.).
Radius	Controls the distance that light scatters along the front surface. This value is measured in scene units, but is divided by the Scale value on the Advanced Settings tab.
Sample Falloff	Defines the falloff profile for light scattered along the radius. Higher values cause a sharper falloff. Lower values cause a smoother falloff but cause the scatter distance to appear shorter, though you can increase the Radius value to compensate.

Back Scattering

Color	Controls the color for the back surface scattering (the light going through the object). The Back surface is the side facing away from the camera. Back scattering is most visible when the object is backlit to some extent, or when light passes directly through the object.
Weight	Controls the weight of the back scattering when it is blended with the other lighting components (front scattering, diffuse, etc.).
Radius	Controls the distance that light scatters along the back surface. This value is measured in scene units, but is divided by the Scale value on the Advanced Settings tab. Normally, the back scattering radius should be set to the same value as the back scattering Depth parameter.
Depth	Controls the back scatter depth. The higher the value, the more deeply light is scattered within the object. In effect, raising the depth makes the object appear more translucent when backlit. Generally speaking, higher depth values require an increased number of lightmap samples in order to remove speckling in the rendered image. Normally, the back scattering depth should be set to the same value as the back scattering Radius parameter.

Scatter Controls

Lightmap Samples

Specifies the number of lightmap samples considered by each rendered ray. Generally, this should be set to a power of two (16, 32, 64, 128, etc.).

Subtle scattering effects typically require few samples. However, as you begin to increase the front and back scattering radii, or the back scattering depth, you will need to increase the number of samples to avoid speckling in the rendered image.

Bias

Allows you to bias the scattering in the rendered image in favor of either front scattering (positive values) or back scattering (negative values).

Advanced Settings

Indirect Illumination

Radiance

Set the RGB Radiance color values to control the amount of indirect illumination (such as that created by photon lighting or final gathering) that is sampled and stored in the lightmap and scattered over the object's surface. Although this can increase render times significantly, it can add a touch of extra realism to a scattering effect.

Also, you can texture this parameter to reveal an indirect lighting effect on a given location on the object.

Lightmap

Group Name	The name of the "scatter group" that the shaded object belongs to. All objects that can scatter light into one another should belong to the same scatter group.
Size as % of output resolution	Specifies the size of the lightmap, expressed as a percentage of the final rendered image resolution. Normally, this value should not need to be higher than 50%. However, should the rendered image still contain artifacts, you can set it higher than this.
Sampling Gamma	This is a gamma curve for the light stored in the lightmap. Values of less than 1 cause light to spread perpendicularly to the lighting direction. Values greater than 1 cause light to concentrate in areas facing the light source itself.

Miscellaneous

Scale

This is a conversion factor used to convert the scene's distance units into the shader's distance units. This is useful when you reuse one object's material on another object that was built using different units.

Use Screen Compositing for Colors

Normally, all of the light layers computed by the shader are added together to produce the final effect. Turning this option on blends the layers together using a Screen compositing mode, which creates a softer result.

This is especially useful when the combination of the lighting layers creates too bright a result, creating overexposed white patches on the object.

Render Tree Usage

The fast subsurface scattering shader is a material phenomenon - a predefined combination of shaders packaged into a single node. It is plugged directly into the Material node's Material port, which prevents the material node from accepting other connections. You can extend the phenomenon's effect by driving its parameters with other shaders. Note that this shader's render tree node has bump and displacement inputs that have no corresponding parameters in the property editor. Bump and displacement shaders can be connected to these ports to add bump and/or displacement mapping to the object's surface.

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