This parameter blends between full diffuse reflection (0) and full subsurface scattering (1). Note that, if you're using a standard surface or OpenPBR shader, SSS won’t appear to have any effect if metalness or transmission is set to 100% since those components replace diffuse and SSS components.

The head on the left side has full diffuse reflection. The Subsurface values are 0, 0.5, and 1.

You can define a target output color for the subsurface scattering effect. For example, if you choose a green color, the material mainly absorbs the incoming light’s blue and red parts of the spectrum. The green parts pass through the object and create the color that is finally visible.

The heads in the following image have equal subsurface properties and only color changes.

Radius multiplied by Scale represents the average distance a light ray can travel through the object before it’s scattered inside the object’s volume for each color channel. This is a material-specific value known as mean free path.

Since Radius controls how rays are scattered in the RGB spectrum, the values may also affect the material’s overall color. The final color effect, however, strongly depends on the adjusted Scale value, and the object’s size and thickness. With 0 (black), there won’t be any scattering. Small values mean that light is quickly scattered and you’ll get a more opaque appearance. Higher values create more translucency.

Radius multiplied by Scale is a physical parameter measured in scene units. In MaterialX you can define how far the light travels for each component of the RGB spectrum to get more complex scattering. You don’t have to enter numerical values, and you can use the associated color field. This color is called mean free path color. The dots contain the Radius colors that were used for the images.

This is a global multiplier for all Radius values. An object’s size influences its appearance: With identical SSS properties, a small object is more transparent to light than a huge object.

Let’s assume your scene scale is 1 m and you have a marble block with a size of 0.1 m (= 10 cm) in each direction. With the shader’s default Scale value of 1 and a purely white Radius, an average light ray can travel 1 m before it’s scattered.

For a correct result, Scale should be in relation to the size of the marble block and consider its dimensions. In order for the rays to be scattered, Scale must be decreased, for example to 0.005. If all Radius values are 1, the rays will be scattered after 0.005 m (= 5 mm). As a global rule of thumb, however, you can say that higher Scale values create a softer and more translucent look.

We also want to take a look at an (exaggerated) example - again with a scene scale of 1 m. Imagine an object with a size of 2 m that needs to be downscaled to 17 cm. The factor for this translation is 17 cm / 200 cm = 0.085. Now we also assume that you've found a subsurface Scale of 0.35 for the original object. To get the compensated Scale value, multiply both values:

• 0.35 * 0.085 = 0.02975

The Scale values in the images are 0.01, 0.1, and 1.

If you have a back light and want the light shine through an object more strongly, you need to increase Anisotropy and use positive values. This effect is called forward scattering. With negative values, you can achieve backward scattering and the object appears less translucent. Anisotropy distributes energy in different directions.

A value of 0 describes isotropic scattering where light is scattered evenly. The limits are -1 and 1, but we don’t recommend using the minimum and maximum values. In most cases the values will range between -0.5 and 0.5. For illustrative purposes, the Anisotropy values for the objects below are -0.5, 0, and 0.5. There’s a strong area light behind the heads to provoke the effect.

Candle wax is certainly one of the most often described materials in conjunction with subsurface scattering. The final appearance of the wax depends on the type of wax itself. Beeswax and soy wax tend to have a dull appearance with a touch of velvet. Natural beeswax has impurities like pollen that create a warm yellow or light orange tone.

Paraffin wax, on the other hand, is made of crude oil, and has a shiny or glossy look. Stearin candles are a mixture of stearic acid and palmitic acid. This type of wax can have a milky appearance with a less glossy surface. Pure paraffin and stearin are white substances, but can be dyed with any color.

A candle’s material also affects the SSS properties because the various types of wax have different densities. The differences might appear subtle, but can have an impact on the shader’s Scale parameter. The densities are:

• Beeswax approx. 0.96 kg/m^3

• Stearin wax approx. 0.94 kg/m^3

• Paraffin wax approx. 0.90 kg/m^3

The candle in this example has a radius of 0.04 m (= 4 cm) with a height of approximately 0.15 m (= 15 cm) and is made of stearin. An additional Area Light LOP with Type set to Sphere, and a Color Temperature of 2000 mimics the candle flame.

The following table contains the shader’s Subsurface parameter values with descriptions. Note that you might also want to change the values of the Specular tab to give the candle a less glossy look, for example with a specular Roughness of 0.25.

The image series below illustrates how the look changes if you keep the subsurface values, but make the candle bigger. The first image shows the original candle. The following three images use Global Scale transformation values of 0.5, 1, and 2.

Orange juice is a complex suspension of water, sugars like glucose and fructose, organic acids such as citric acid, and other compounds. You can also find varying amounts of fruit pulp in orange juice. The fact there’s not that one type of orange juice gives you some freedom in terms of color and SSS values.

The glass has a height of 0.13 m (= 13 cm), and the upper edge uses a radius of 0.043 m (= 4.3 cm). Inside the glass, there’s a solid geometry that represents the juice. The juice itself also has slightly transparent characteristics and you can play with the shader’s Transmission parameters and colors. For Transmission, you can try a value around 0.1.

As before, here’s a table with the SSS values that were used for the image.

The milk example uses the same setup as the orange juice scene. In terms of shading you typically have to differentiate between whole milk and low-fat milk. Whole milk has a slightly yellow touch due to its higher fat content. This tint shouldn’t be too prominent and only visible in more translucent areas.

If you choose a high value for Scale, it often happens that the milk has a thin and watery look similar to starch dissolved in water. If the value is very small, the fluid might have a more creamy and heavy look. The left image has a Scale of 0.01, on right image the value is 0.0005.

Similar to wax, milk shows strong forward scattering. However, an Anisotropy of 0 is acceptable if this value creates the desired look. You should also consider working on the shader’s Specular tab. The table contains the values for whole milk shown in the render.

Many mushrooms also show SSS properties as the ones in this example. The largest mushroom has a height of 0.1 m (= 10 cm) and serves as a reference for the shader’s Scale parameter. The mushrooms have a slightly translucent cap, but there’s also a certain amount of diffuse reflection. Therefore, the Subsurface parameter’s value should be smaller than 1. The Color and Radius settings strongly depend on the mushroom’s own coloring. In this scene, the predominant colors are shades of brown or dark orange.

In contrast to other scenes of this guide, the mushrooms are illuminated by a Spot Light LOP with a Color Temperature of 5500 Kelvin. From the camera’s perspective, the spotlight is located to the left of the object.

The choice of the object’s Scale parameter is certainly the most difficult part due to the low Subsurface value. With strong illumination, the thinner parts near the edges of the cap appear semi-translucent and often show a typical “halo”. A positive Anisotropy value can help to amplify this effect.

Skin shaders can become very complex, especially if you want to consider different skin layers like the epidermis. The MaterialX surface shaders don’t support layers like shallow, medium, or deep you might know from other material systems.

Skin material would normally have a higher red Radius value since red light penetrates deepest into the skin compared to green and blue parts of the RGB spectrum. A possible combination of Radius values is 1, 0.15, 0.1.

You can also consider other features, for example blood circulation, hair, skin type (dry, oily, rough), skin irregularities, pores, or freckles. Large parts of the skin are also covered with thin hair and create an effect similar to peach fuzz. Depending on the thickness on the skin, the SSS properties can vary for different parts of the body. For example, if you look at the head, the skin on the cheeks is typically thicker than the skin on the forehead. A map will consider these differences.

Other lobes like diffuse and specular reflection also have huge impact on the final look.