Obtaining Bidirectional Texture Reflectance of Human Skin by means of a Kaleidoscope

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Obtaining Bidirectional Texture Reflectance of
Human Skin by means of a Kaleidoscope
Computer Science Honours - Rhodes University 17
March 2004

• Jude Radloff
• Supervised by Shaun Bangay and Adele Lobb

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Context

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

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Interaction of light with a surface
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Interaction of Light with a Surface

• Polarisation
• Sub-surface scattering
• Fluorescence
• Phosphorescence
• Reflection
• Absorption
• Transmittance

• Bidirectional Texture Function
• Sufficient physical accuracy
• while being manageable

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Bidirectional Texture Function

• BTF? ( ?i, Fi, ?o, Fo, u, v )
• ? wavelength of light (colour)
• ?i, Fi incoming light direction
• ?o, Fo outgoing light direction
• u, v surface position co-ordinates
• Describes how the incoming light is reflected
  out, relative to its position

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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

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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

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Technique
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Table of BTFs
x view angle y illumination angle
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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

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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

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Experimental design

• Taper angle
• Dimensions
• Choice of regular polygon for base

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

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Questions?