Physically based skin rendering

Universitat Politecnica de Catalunya

Physically-based rendering of human skinMaster in Innovation and Research in Informatics

Roger Hernando

Advisors:Antonio ChicaPere-Pau Vazquez

Motivation Implemented skin rendering methods Proposed extensions Developed framework Results Conclusions & future work

Outline

1 Motivation

2 Implemented skin rendering methods

3 Proposed extensions

4 Developed framework

5 Results

6 Conclusions & future work

Roger Hernando Physically-based rendering of human skin


Motivation

Motivation

Enhance the rendering of human scanned characters:



Introduction

Skin rendering

Human perception is highlyspecialized.

Very sensitive to theappearance of humanskin.

Skin is a multilayeredtranslucent material:

Light scatters throughskin.Light interacts with eachlayer.

Described in terms of theBSSRDF not in terms of theBRDF.



Introduction

BRDF vs BSSRDF



Introduction

Skin rendering

Skin simulation:Subsurface scattering.Forward scattering.



Subsurface scattering


BSSRDF S relates the outgoing radiance L0(x0,−→ω0) at a point

x0 with the incoming radiant flux Φi (xi ,−→ωi ) at a point xi :

dL0(x0,−→ω0) = S(xi ,

−→ωi ; x0,−→ω0)dΦi (xi ,

−→ωi ) (1)

8D function:

S(xi ,−→ωi ; x0,

−→ω0) = S(xi , yi , σi , φi ; x0, y0, σ0, φ0) (2)





Simplified using the diffusion approximation R(r):





Radiant exitance M at a point (x , y):

M(x , y) =∫ ∫

E (x ′, y ′)R(r ′)dx ′dy ′ (3)

Expressed as 2D convolution:

M(x , y) = E (x , y) ∗ R(r) (4)

For real time rendering, R(r) is approximated by a set of 1Dseparable convolutions.



Objective

Objective

Implement state-of-the-art methods to render the skin(subsurface scattering, forward scattering).Propose some extensions to the methods.Implement a test-bed application and also other PBRtechniques.Test and compare the implemented methods.



Outline

1 Motivation




5 Results




Implemented Methods

Methods

Subsurface scattering:Screen space subsurface scattering.Separable pre-integrated subsurface scattering.Separable artistic subsurface scattering.

Forward scattering:Real-Time realistic skin translucency.



Skin rendering methods


Subsurface scattering:Screen space subsurface scattering.Separable pre-integrated subsurface scattering.Separable artistic subsurface scattering.

Screen-space methods.Mimicking the skin diffusion profile.Subsurface scattering should be applied only to the diffuselighting.




First method: Screen space subsurface scattering

Jimenez et al. used d’Eon and Lubeke approximation R(r) asa sum of gaussian 6 functions:

Rd (r) =k∑

i=1wiG(vi , r) (5)

Convolution performed in screen space.12 1D convolutions (expensive).




Screen space subsurface scattering Overview




Second method: Separable pre-integrated subsurfacescattering

Approximate R(r) with just one separable convolution.Assuming the irradiance E is additively separable, theseparable kernel is defined as follows:

A(r) = 1||ap||1

ap(r) (6)




Second method: Separable pre-integrated subsurfacescattering

Stages of the algorithm:Compute the diffusion kernels ap using a Monte Carlosimulation.

Kernel computation is slow.Precompute the kernels for later use.

Uneven energy distribution, more energy near the origin.More samples are taken near the origin.




Third method: Separable artistic subsurface scattering

Known limitations:Separable pre-integrated subsurface scattering is not an artisticfriendly method.

Kernel controlled with various parameters.Based on the sum of gaussians diffusion.





Parameters:Weight (w): filter width.Strength (s): amount of light which penetrates the skin.Falloff (f ): amount of light travelling through the skin.

A(r) = p[ r ∗ w

0.001 + f

]∗ s + δ(r) + (1− s) (7)





Highly configurable:




Forward scattering

Forward scattering:Real-Time realistic skin translucency.

Obtain the distance travelled by the light inside an object.




Fourth method: Real-Time realistic skin translucency

Computing the distance traveled through the object.

light

eye





Use the path length with the transmittance function T (s):

T (s) =k∑

i=1wie−s2/vi (8)

Using d’Eon and Lubeke weights.



Outline

1 Motivation




5 Results




Extensions

Extensions

Problems with current methods:Halos.Incorrect diffusion.

Based in non-physically-based previous work:Modulate the subsurface scattering effect with mesh localcurvature.



Halos

Halos

Close points in screen space may be far away in the geometry.Problem partially tackled by correction factors (but notenough):

//correctionfloat depth = texture(depthTex, offset).r;float s = min(correction * abs(depthM - depth), 1.0);colorS.rgb = mix(colorS.rgb, colorM.rgb, s);



Halos

Halos

Mikkelsen uses a Cross Bilateral Filter to weight the diffusionprofile.

CBF [I,E ]p =∑

q∈S Gσs e−||p−q||Gσr e−(Ep−Eq)Iq∑q∈S Gσs e−||p−q||Gσr e−(Ep−Eq) (9)

It works like a bilateral filter but uses an auxiliary image forweighting.

I(p) = I(x(p)) ∗ cos3(φi )||x(p)||2cos(φj)

(10)

φi is the angle between the z-axis and the view.φj is the angle between the point normal and the -view.



Halos

Halos

The original implementation depends on the distance betweenthe eye and the object.

Assume x(p) lies on the zNear plane.This technique produces strange artifacts when used directlywith our subsurface scattering techniques.

Slightly modify the subsurface scattering algorithms to takethis artifacts into account.



Halos

Halos

Results:The Halos artifacts are eliminated using this extension.



Incorrect diffusion

Incorrect diffusion

Limitations:Our meshes do not differentiate between different scannedelements (skin, hair, cloths).In video games artists provide this information (texture ormeshes).

We do not have this information.Blurring between different zones.



Incorrect diffusion

Incorrect diffusion

Proposed Solution:Weight the filter using the lab-color distance to differentiatebetween skin and non-skin zones.Bilateral filter:

BF [I]p =∑

q∈S Gσs e−||p−q||Gσs e−(Ip−Iq)Ip∑q∈S Gσs e−||p−q||Gσs e−(cp−cq) (11)



Incorrect diffusion

Incorrect diffusion

Results:The diffusion between skin and non-skin zones is eliminatedusing this startegy:



Scattering modulation


Non-physically based subsurface scattering: the subsurfacescattering effect is more noticeable in high curvature zones.

Modulate the scattering strength with the screen spacecurvature.





Various strategies to modulate the subsurface scattering.Increase the effect according to its local curvature.Decrease the effect in zones with lower curvature and increaseit otherwise.Reduce the effect up to a minimum at zones with lowcurvature and increase it at zones with high curvature.



Outline

1 Motivation




5 Results




PBR

Scene Lighting

Features:Subsurface scattering.Specular Reflections.Environment lighting.



Speculars

BRDF Specular

BRDF specular:

Cspec(l , v) = F (l , h)G(l , v , h)D(h)4(n · l)(n · v) Lc(n · l) (12)



Speculars

BRDF Specular

Fresnel: defines the fraction of light reflected from anoptically flat surface.

Fslick(F0, l , h) = F0 + (1− F0)(1− (l · h))5 (13)

Shadow-masking function: defines the percentage ofmicrofacets with h as their normal vector that are notshadowed or masked.

Gimplicit(l , v , h) = (n · l)(n · v) (14)

Distribution of normals function: concentration of microfacetsthat are oriented such that they could reflect light from l intov .

DPhong (h) = παr + 2

2π (n · h)αr (15)



Speculars

BRDF Specular

Finally, our physically-based specular BRDF is defined asfollows:

Cspec(l , v) = αr + 28 (n · h)αr Fslick(F0, l , h)Lc(n · l) (16)



Speculars

BRDF Specular



Global Ilumination

Irradiance maps

Ambient occlusion.Environment lighting using an irradiance map.

EMdiff (n) =∑

k∈Ω max(0, lk · n)L(lk)∑k∈Ω max(0, lk · n) (17)



Global Ilumination

Irradiance maps



Extras

Color linearity

Important to take into account that the color captured by asensor is not stored in a linear way.



Extras

Color linearity

When a non-linear color is used, it produces an incorrectrendering.



Application

Snapshot



Application

Pipeline



Outline

1 Motivation




5 Results




Performance

Results

Application performance:

ApplicationShadow map MainRender AddSpecular Tonemap

0.447 ms 1.262 ms 0.148ms 0.977 ms



Performance

Results

Subsurface scattering methods:

View Gausian sum Artistic Pre-int KernelClose 9.428 ms 1.731 ms 1.799 msMid 2.01 ms 0.492 ms 0.487 msFar 0.676 ms 0.312ms 0.36 ms



Performance

Results

Performance vs #samples:



Performance

Results

Without subsurface scattering:



Performance

Results

With subsurface scattering:



Outline

1 Motivation




5 Results




Conclusions

Conclusions

Explored different algorithms to render the skin.Proposed and explained some extensions to improve therendering quality.Implemented a testbed application integrating the subsurfacescattering methods plus other techniques in order to producehigh quality renders.Methods performance analysis.



Future work

Future work

Explore non-physically based methods to render the skin.Improve the re-usability of the forward scattering method.Explore segmentation methods to distinguish between skinand non-skin zones.



Questions

Questions

Questions?


Physically based skin rendering

Technology

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