Subsurface Scattering Rendering

Subsurface scatteringModel of light transport in translucent

materials Marble, jade, milk, skin

Light penetrates material and exits at different point Not simple reflection

Light absorbed the further it travels into material

BRDFBRDF - Bidirectional

Reflectance Distribution Function

Defines how light is reflected on an opaque surface

Assumes light enters and leaves material at same position

BSSRDFBSSRDF -

Bidirectional Sub-surface Scattering Reflectance Distibution Function

Can describe light transport between any two rays that hit a surface

Comparison – BRDF vs BSSRDF

Comparison – BRDF vs BSSRDF

BSSRDF – How to calculate?Outgoing radiance equation:

Integrating over incoming directions and area gives:

BSSRDF functionBSSRDF function:

Comprised of diffuse and single scattering components

BSSRDF – Diffuse scattering term

Fresnel terms – light refraction from entering material and then exiting

Diffuse sub-surface reflectance function

BSSRDF – Diffuse sub-surface reflectance function

BSSRDF – Single scatteringOccurs when

refracted incoming and outgoing rays intersect

BSSRDF – Single scattering termTotal outgoing radiance due to single

scattering:

BSSRDF – ImplementationCan be implemented in a ray tracerSimilar to sampling area light source in distributed ray

tracingFor each ray-object intersection integrate light over area of

surfaceSample single scattering term along refracted outgoing raySample diffuse scattering term around outgoing position

Fast BSSRDFCan speed up calculation by using optimising

BSSRDFContribution of single scattering term is

small for materials with high albedo, so can be ignored

Therefore only need to calculate diffuse term

Fast BSSRDF - ImplementationTwo passes

Sampling irradianceEvaluating diffusion approximation

Fast BSSRDF – Sampling IrradianceCreate sample points on mesh

Distance between sample points equal to average distance at which light is scattered within material

For each point store location, area, and irradiance

Fast BSSRDF – Evaluating Diffusion ApproximationSum the contribution of all irradiance

samples for each outgoing pointContribution from irradiance samples

decreases according to distance from outputWe can cluster distant samples

Implement using a hierarchical structure – eg. octree, indexed by point and area

Fast BSSRDF – Output

ReferencesJensen, Marschner, Levoy and Hanrahan: “A

Practical Model for Subsurface Light Transport”

http://graphics.ucsd.edu/~henrik/papers/bssrdf/

Jensen and Buhler: “A Rapid Hierarchical Rendering Technique for Translucent Materials”http://graphics.ucsd.edu/~henrik/papers/fast_b

ssrdf