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Studies in Appearance of Skin

Simon Fraser University Computational Vision Lab Lilong Shi, Brian Funt and Tim Lee. Studies in Appearance of Skin. Overview. Studies of factors affecting skin colour Simple and linear model of skin Modelling Skin appearance under lights Applications:

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Studies in Appearance of Skin

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  1. Simon Fraser University Computational Vision Lab Lilong Shi, Brian Funt and Tim Lee Studies in Appearance of Skin

  2. Overview • Studies of factors affecting skin colour • Simple and linear model of skin • Modelling Skin appearance under lights • Applications: • Estimate melanin and hemoglobin concentrations • Correct imaged skin tones for lighting conditions

  3. Applications Skin tone correction Tone correction Preserve melanin Melanin/Hemoglobin separation

  4. Appearance of Human Skin • Appearance of human skin determined by • Biological factors • pigmentation, blood microcirculation, roughness, etc.. • Viewing conditions • Inducing lights • Acquisition devices • Cones in retina, RGB sensors of CCD digital cameras

  5. Schematic Model of Human Skin • Two-layered Skin Model [2] • Epidermis Layer: melanin absorbance • Dermis Layer: hemoglobin absorbance • A layer has properties of an optical filter

  6. Biological Factors • Various skin colour <= melanin + hemoglobin • Genetic: Race • Temporary: • Exposure to UV • Hot bath • Mixture varying by 2 independent factors • Analyse melanin and hemoglobin factors

  7. Blind Source Separation • Estimate melanin and hemoglobin concentration • Independent Component Analysis (ICA) • Statistical technique for revealing “hidden” factors • To “unmix” or “separate” signals composed of multiple sources • Independent and linear mixing • Related to Eigen-vector analysis

  8. Independent Component Analysis (ICA) Original Source Signals Mixing Observed Signals 70% v1 0% 20% 80% 30% s1 s2 100% v2 v3 s × A = v

  9. Independent Component Analysis (ICA) Melanin Melanin Hemoglobin Hemoglobin Skin samples

  10. A Example of Skin basis • Typical skin spectrum • Visible wavelength 400nm – 700nm • Extract skin bases from observed spectrum by ICA (left) 33 skin spectrum after normalization; (right) two independent basis spectrum – the melanin and hemoglobin, and the spectrum of chromophores other than melanin and hemoglobin pigments. ICA

  11. Linear Model of Skin • Arbitrary skin spectrum can be approximated are variables constru

  12. Signal from Digital Camera • Human vision • 3 types of Photoreceptors • L, M and S Cones • Digital Cameras • 3 sensors • Red, Green, and Blue • Reflectance spectrum recorded by 3 sensors => three values (R, G, B) for a skin colour

  13. Skin Model in Camera Space Given a pixel from skin, compute by projecting log(R,G,B) onto Possible skin colours lie within plane

  14. Real Image Result Melanin Image Hemoglobin Image Input Image [3]

  15. Results on 33 Skin Spectra - Inverse melanin concentration - Inverse hemoglobin concentration

  16. Modelling Skin + Illumination • Skin appearance greatly affected by lights • Reveal true skin colour by removing illum. • Common lights blackbody radiation • e.g. tungsten/halogen lamps, sunrise/sunset, etc • Varying colour temperature T • Redish -> white -> bluish

  17. Skin-Illumination Model • Colour: illumination times reflectance • In log space, multiplication => addition: Illum. basis

  18. Simplified Skin-Illum Model • In practice • Drop hemoglobin basis • Small angle between Illum and hemoglobin axes • Ignore brightness • Skin colour varying by T and

  19. Result based on Synthesized Data • 384 real skin reflectances times • 67 real light sources • => 25728 samples

  20. Result of Tone Correction • Skin tone correction example (UOPB DB [4]) 16 different illumination + camera settings Tone correction Preserve melanin

  21. Result of Tone Correction • Skin tone correction example (UOPB DB [4])

  22. Conclusion • Skin colour modelling: • Melanin and Hemoglobin concentration • Linear model in logarithm space • Estimation by Independent Component Analysis • Skin appearance + Light modelling: • Estimates light source • Preserves skin colour by melanin value • Applied to digital images from CCD cameras

  23. Reference • [1] Shi, L., and Funt, B., "Skin Colour Imaging That Is Insensitive to Lighting," Proc. AIC (Association Internationale de la Couleur) Conference on Colour Effects & Affects, Stockholm, June 2008 • [2] Angelopoulou, E., Molana, R., and Daniilidis, K. “Multispectral skin color modeling,” In IEEE Conf. on Computer Vision and Pattern Recognition, volume 2, pages 635-642, Kauai, Hawaii, Dec. 2001. • [3] Shimizu, H., Uetsuki, K., Tsumura, N., and Miyake, Y. Analyzing the effect of cosmetic essence by independent component analysis for skin color images. In 3rd Int. Conf. on Multispectral Color Science, pages 65-68, Joensuu, Finland, June 2001. • [4] Martinkauppi, B. “Face color under varying illumination-analysis and applications,” Ph.D. Dissertation, University of Oulu, 2002.

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