Skin Rendering

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Skin Rendering

Wanho Choi

(wanochoi.com)

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Skin

• Very

complicated structure

https://wtamu.edu/~cbaird/sq/2015/11/23/how-does-the-outer-layer-of-skin-cells-on-my-finger-detect-when-i-am-touching-an-object/ http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.226.4533&rep=rep1&type=pdf

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Most Materials

• They have

two main components.

• Reflection =

specular reflection + diﬀuse reflection

https://blog.selfshadow.com/publications/s2015-shading-course/hoffman/s2015_pbs_physics_math_slides.pdf

view dependent

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Subsurface Scattering (SSS)

• Light goes beneath the skin surface,

scatters and gets partially absorbed, and then exits somewhere else.

• Beneath the skin surface, the incoming light quickly becomes

diﬀuse as it scatters.

https://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf

incident light

surface reflection

(specular)

subsurface scattering

(=diﬀuse)

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General Diﬀuse vis-a-vis SSS

https://eo-college.org/courses/echoes-in-space/lessons/geometry/topic/the-scattering-mechanisms/

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BRDF

• Bidirectional Reflectance Distribution Function

http://www.codinglabs.net/article_physically_based_rendering_cook_torrance.aspx

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BSSRDF

• Bidirectional Surface Scattering Reflectance Distribution Function

• BRDF is a

special case of BSSRDF

(BRDF assumes light enters and exits at the

same point.)

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Subsurface Scattering (SSS)

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Comparison

https://slideplayer.com/slide/4491049/

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Single vs Multiple Scattering

• Single-layer scattering: dipole model (milk, marble, or ketchup, etc.)

• Multi-layer scattering: multipole model (skin)

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Multi-layer Skin Model

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Skin Surface Reflectance

• Direct reflection: about 6%

• Fresnel reflection with the topmost oily layer

• Reflects directly without being colored (

white specular color)

• Not a perfect mirror-like reflection duet to the fine-scale roughness (

BRDF)

• Kelemen/Szirmay-Kalos model > Blinn-Phong model

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Kelemen/Szirmay-Katos Model

• Specular surface reflectance model

• Analytic BRDF

• Approximation of Torrance/Sparrow model

• The

Phong model fails to capture increased specularity at grazing angles.

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Varying Specular Parameters

• Diﬀerence is subtle but apparent

• Two-channel map that specifies and .

m

ρ

_s

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Skin Subsurface Reflectance

• The input light

exits the surface in a 3D neighborhood surrounding the point of entry.

• Scattering: partially absorbed & acquiring color

• Two or

three-layer model is enough

• The

narrow scattering of the epidermis on top of the broad scattering dermis

• Sometimes light travels completely through

thin regions such as ears.

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SSS as Diﬀusion

• For highly scattering media, the distribution of light loses tends to

isotropy.

• It means that the entered light is randomly likely to end up coming out in

any direction.

• This eﬀectively makes the scattering sort of a "

blurring" function.

• Therefore, the

subsurface scattering can be approximated as a diﬀusion phenomenon.

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Simple Laser Pointer Experiment

• Consider a flat and very thin surface in a dark room with a white

laser beam illuminating it.

• We will see a glow

around the center point where the laser beam is striking the surface.

• Light

disappears smoothly in proportion to the distance from the laser center.

• It is same in all directions, so it can be described as

1D diﬀusion profile curve.

• The profile is

color dependent: red light scatters much farther than green and blue.

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Expansion to Skin

• A very narrow patch of incoming light creates a larger, colored patch of outgoing light.

• Every region on the surface needs to do this and all the overlapping,

colored patches sum to give a

translucent appearance.

https://tryingtobeananimator.wordpress.com/2017/03/20/subsurface-scattering/

=

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BSSRDF Approximation

L(x

_o

, ω

_o

_{) = ∫}

Ω

S(x

i

, ω

i

; x

o

, ω

o

) L(x

i

, ω

i

) (n

i

⋅ ω

i

) dω

i

L(x

_o

, ω

_o

_{) = ∫}

Ω

R(∥x

_{diﬀusion profile}

i

, − x

o

∥) L(x

i

, ω

i

) (n

i

⋅ ω

i

) dω

i

BRDF

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How to Use Diﬀusion Profile

• Collect all incoming light for each location.

-

Sum all incident lights ignoring the directions,

except for an N・L term for each light and Fresnel transmittance terms.)

- Only the

total amount of incoming light is important

• Spread it around into neighboring locations based on the profile.

: equally in all directions

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Analytic Formula for Diﬀusion Profile

• Dipole model [Jensen et al. 2001]

• Multipole model [Donner and Jensen 2005]

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Analytic Formula for Diﬀusion Profile

• Dipole model [Jensen et al. 2001]

• Multipole model [Donner and Jensen 2005]

http://graphics.snu.ac.kr/class/graphics2011/materials/paper02_fast_skin.pdf

layer 1

layer 2

to satisfy the boundary condition,

we have to mirror that dipole!

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Sum-of-Gaussians

• Skin’s

diﬀusion profile as a sum of several Gaussians.

R(r) ≈ =

_∑

k

i=1

w

_i

1 2πv

e

−r

2 _/(2v)

https://developer.nvidia.com/gpugems/gpugems3/part-iii-rendering/chapter-14-advanced-techniques-realistic-real-time-skin

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Six Gaussians for Skin

• Using

six Gaussians allows accurate rendering with the three-layer skin model.

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Texture Space Diﬀusion

• Texture-space diﬀusion (several

blurred irradiance textures)

• One texture is computed for each Gaussian function used in the diﬀusion profiles.

• Therefore,

six oﬀ-screen blur textures

• Convolution operation (=blur) to the texture

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Texture Space Diﬀusion

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UV Distortion Correction

• Generally, skin surface is

not flat.

• On curved surfaces,

distances in texture distances on the mesh

• We compute a

stretch-correction texture that modulates the spacing of convolution.

• Horizontal and vertical stretching usually diﬀer significantly.

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Comparison

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Pre-Scatter Texturing

• First perform all scattering computations

without any coloring.

• Then diﬀuse color texture is

multiplied after scattering.

• All

high-frequency texture detail is maintained

• No color bleeding

• However, the diﬀuse map

already has natural color bleeding

because it came from photographs of real skin

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Comparison

pre-scattering

post-scattering

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Modified Translucent Shadow Maps

• For the

transmission through thin surface regions such as ears

• Translucent shadow maps to compute depth through the surface

• It

connects shadowed regions to locations on the light-facing surface

• Multiple convolutions of irradiance are available by accessing the same textures computed

for local scattering.

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Comparison

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Reference

• https://developer.nvidia.com/gpugems/gpugems3/part-iii-rendering/chapter-14-advanced-techniques-realistic-real-time-skin

• https://therealmjp.github.io/posts/sss-intro/

• https://computergraphics.stackexchange.com/questions/81/what-is-the-dipole-approximation-for-subsurface-scattering

• https://www.ci.i.u-tokyo.ac.jp/~hachisuka/dirpole_tr_slides.pdf

• https://slideplayer.com/slide/4491049/http://graphics.snu.ac.kr/class/graphics2011/materials/

paper02_fast_skin.pdf

• http://graphics.snu.ac.kr/class/graphics2011/materials/paper02_fast_skin.pdf

• https://www.evl.uic.edu/sjames/cs525/project3.html