Obtaining Bidirectional Texture Reflectance of Human Skin by means of a Kaleidoscope Jude Radloff Supervised by Shaun Bangay and Adele Lobb Computer Science Honours - Rhodes University 17 March 2004

Context  Within field of Computer Graphics and Virtual Reality  VRSIG REALISTIC IMAGE RENDERING

Interaction of light with a surface

Interaction of Light with a Surface Polarisation Sub-surface scattering Fluorescence Phosphorescence Reflection Absorption Transmittance Bidirectional Texture Function - Sufficient physical accuracy while being manageable

Bidirectional Texture Function BTF λ ( θ i, Φ i, θ o, Φ o, u, v )   λ = wavelength of light (colour)   θ i, Φ i = incoming light direction   θ o, Φ o = outgoing light direction   u, v = surface position co-ordinates Describes how the incoming light is reflected out, relative to its position

Measuring a BTF Standard technique:  Essentially an array of cameras all pointing toward surface sample from different directions  Difficult and cumbersome  Resulting sample for human skin: weak

Alternative Technique Han and Perlin in ACM SIGGRAPH 2003 paper  Use of a kaleidoscope  Reflections off mirrors allow surface sample to be viewed simultaneously from many directions, without the camera changing position  Effect analogous to previous technique  Richer BTF sample

Technique

Table of BTFs x: view angle y: illumination angle

My Process  1. Simulate kaleidoscope – ray-tracing  2. Build kaleidoscope, using a sample with known BTF  3. Optimise configuration to sample human skin  4. Write a surface shader using the BTF information obtained

Implementation Details Simulation Phase  Radiance package – ray-tracing  Unix operating system Implementation Phase  Front Surface Mirrors  High transmission glass  Hi-resolution camera  Non-polarised projector, eg. DLP

Experimental design  Taper angle  Dimensions  Choice of regular polygon for base

Details of Paper  Han, Jefferson Y. and Perlin, Ken. Measuring Bidirectional Texture Reflectance with a Kaleidoscope. In Proceedings of ACM SIGGRAPH 2003, ACM Press. New York. Computer Graphics Proceedings, Annual Conference Series, ACM.  SIGGRAPH/2003/paper063.pdf

Questions?