Obtaining Bidirectional Obtaining Bidirectional Texture Reflectance of Texture Reflectance of

Human Skin by means of a Human Skin by means of a Kaleidoscope Kaleidoscope

Jude RadloffJude Radloff

Supervised by Shaun Bangay and Adele LobbSupervised by Shaun Bangay and Adele Lobb

Computer Science Honours - Rhodes University17 March 2004

ContextContext

Within field of Computer Graphics Within field of Computer Graphics and Virtual Realityand Virtual Reality

VRSIGVRSIG

REALISTIC IMAGE REALISTIC IMAGE RENDERINGRENDERING

Interaction of light with a Interaction of light with a surfacesurface

Interaction of Light with a SurfaceInteraction of Light with a Surface

PolarisationPolarisationSub-surface scatteringSub-surface scattering

FluorescenceFluorescencePhosphorescencePhosphorescence

ReflectionReflectionAbsorptionAbsorption

TransmittanceTransmittance

Bidirectional Texture Function- Sufficient physical accuracy

while being manageable

Bidirectional Texture FunctionBidirectional Texture Function

BTFλ ( θi, Φi, θo, Φo, u, v )

λ = wavelength of light (colour)θi, Φi = incoming light directionθo, Φo = outgoing light directionu, v = surface position co-ordinates

Describes how the incoming light is reflected

out, relative to its position

Measuring a BTFMeasuring a BTF

Standard technique:Standard technique:

Essentially an array of cameras all Essentially an array of cameras all pointing toward surface sample from pointing toward surface sample from different directionsdifferent directions

Difficult and cumbersomeDifficult and cumbersome Resulting sample for human skin: weakResulting sample for human skin: weak

Alternative TechniqueAlternative TechniqueHan and Perlin in ACM SIGGRAPH 2003 paperHan and Perlin in ACM SIGGRAPH 2003 paper

Use of a kaleidoscopeUse of a kaleidoscope Reflections off mirrors allow surface sample to Reflections off mirrors allow surface sample to

be viewed simultaneously from many directions, be viewed simultaneously from many directions, without the camera changing positionwithout the camera changing position Effect analogous to previous techniqueEffect analogous to previous technique

Richer BTF sampleRicher BTF sample

TechniqueTechnique

Table of BTFsTable of BTFs

x: view angle

y: illumination angle

My ProcessMy Process

1. Simulate kaleidoscope – ray-tracing1. Simulate kaleidoscope – ray-tracing 2. Build kaleidoscope, using a sample 2. Build kaleidoscope, using a sample

with known BTFwith known BTF 3. Optimise configuration to3. Optimise configuration to

sample human skinsample human skin 4. Write a surface shader using the BTF 4. Write a surface shader using the BTF

information obtainedinformation obtained

Implementation DetailsImplementation Details

Simulation PhaseSimulation Phase Radiance package – ray-tracingRadiance package – ray-tracing Unix operating systemUnix operating system

Implementation PhaseImplementation Phase Front Surface MirrorsFront Surface Mirrors High transmission glassHigh transmission glass Hi-resolution cameraHi-resolution camera Non-polarised projector, eg. DLPNon-polarised projector, eg. DLP

Experimental designExperimental design

Taper angleTaper angle DimensionsDimensions Choice of regular polygon for baseChoice of regular polygon for base

Details of PaperDetails of Paper

Han, Jefferson Y. and Perlin, Ken. Han, Jefferson Y. and Perlin, Ken. Measuring Bidirectional Texture Measuring Bidirectional Texture Reflectance with a KaleidoscopeReflectance with a Kaleidoscope. In . In Proceedings of ACM SIGGRAPH 2003, Proceedings of ACM SIGGRAPH 2003, ACM Press. New York. Computer ACM Press. New York. Computer Graphics Proceedings, Annual Conference Graphics Proceedings, Annual Conference Series, ACM.Series, ACM.

http://csshaun.ict.ru.ac.za/journals/acm/http://csshaun.ict.ru.ac.za/journals/acm/SIGGRAPH/2003/paper063.pdfSIGGRAPH/2003/paper063.pdf

Questions?Questions?