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tne long wavelength red light can penetrate this deeply To simulate ht scattering properties of skin the mental ray Fast SSS Skin shade!

cattered Hgnt from three layers to surface shader components The SSS components are for the epidermis subderm.s and backscatter {um penetrating all the way through the skin).

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Blood

Hair Follicle

Sweat Gland

Blood

Epidermis

Dermis

Hair Follicle

Sweat Gland

Back scatter radius

Epidermal (Epidermis) (skin layers/no blood)

Fat layer (not shown)

Fatty Tissue -

Nerve

Epidermis

Dermis

Fatty Tissue -

Nerve

110 ESSENCE The Face

Real skin

S^b-surtace scattering

Q Step 2: Getting the scale right

AH of the SSS shaders scatter light in real-world units so you have tc aet the scale right for your skin shader to look real If your mooel = too small, then light will scatter through it too much and t * I look <e some miniature transparent toy. Get the scale too arge and There * b*

t no scattering and your model will look massive or ust plain fake almost

There is a Scale Conversion Factor mat *>n let you scale everyir ng the shader to match the scale of your model. As a start, apply the default SSS Fast Skin shader and try a range of Sea e Conversion Factors Use the width of the nose as one unit for a guide Here are the results of a range of scales, from too small to too large _

ESSENCE The Face t

Real skin

Sub-surfa f :>ta iter

O s,eP 3: Back scatter

SSS shaders have more components than you are probably used to. In addition to the normal diffuse and specular components, there are three sub-surface scattering components Here, we see the output of the back scattering component This scene is lit with one backlight, and a large area light on the model's left This and the following component renders are using the mental ray's SSS Fast Skin shader with default parameters. Here, the Back Surface Scatter weight is 1.0. Unscattered Diffuse. Epidermal, Subdermal, and Overall Specular weights are all set to 0.

t p.nracian skin the epidermal layer scatters a creamy color mostly in^the blue component Although we have a bump map applied

%Totthe t st renders. ,t does no. attect the SSS components Thls s because h Nght entering beneath the sk.n and scattered is softened L„t so much that no bump detail is visible at al In this image, all the component weights except Epidermal layer are set to 0.

O Step 5: Subdermal layer

Here, all of the blue and much of the green components are missing resulting in a deep orange scattered light. This layer is even softer than the epidermal, but not as soft as the back scatter. In this image, all the weights except Subdermal layer are set to 0.

O Step 6: Specular #1

Skin specularity can be approximated with two specular components of roughly equal weight. A broader sheen, and a sharper, oily reflection. Here, we see the result of the broad sheen specular component. This is tinted blue as most of the red from the incident light penetrates the skin The blue hardly penetrates at all, and is dominant in the reflected specular component. In this image, all the weights except Overall Specular and Specular Color #1 are set to 0.

112 ESSENCE The Face

Real skin

Sub-surface scattering

Q Step 7: Unscattered diffuse color

This would be your normal diffuse component in a regular. non-SSS shader. For pale skin, it is only a minor component—most of the look of skin comes from SSS and specular light. As the skin is quite translucent to the longer wavelengths (red), the surface diffuse map needs to have red and to a lesser degree green removed from it.

Q Step 8: All but the diffuse multiplier

This is a scary shader. It looks like a soft silicone skin mask We are missing surface detail. Not all of the skin is equally translucent. There are freckles, moles, dirt, makeup, and hair that are not very translucent. For this reason, the SSS shader has an Overall Diffuse multiplier. First, the shader adds the Unscattered Diffuse Color, Epidermal and Subdermal color components. These make up the diffuse component of the shader. This is then multiplied by the Overall Diffuse Coloration multiplier map. You should use a very desaturated map with high contrast for this so you don't affect the skin tone too much. You should also introduce all those facial features like freckles and makeup

Unscattered Diffuse

Q Step 9: The first rough SSS render

Turning it all on with the recommended weights gives us our first rough skin render. The default weights for the various components are: Unscattered Diffuse 0.3; Epidermal 0.5; Subdermal 0.4; Back Surface 0 5 Overall Specular 1.0; Specular 0.3; Specular #2 0.3. It's not a bad start and is probably what you will get with your first attempt with the shader We need to tweak the maps and lighting now to get closer to reality.

O Step 10: Improved bump and a bit of ambient

With all the components on, we have lost bump detail. I was also using a bump map without the hair in it so I switched to using our full-color map for bump. I also added a little ambient light in the shader. I like to develop shaders with a low angle, but soft (large area) side light. This shows up the shader performance under a harsh, but realistic lighting condition. It also gives me a feel for real render times for cinematic quality work. Now it's time to start refining the lighting as we tweak the shader.

ESSENCE The Face

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