Computational Colour Vision Stephen Westl in addition to Centre as long as Colour Design Technology

Computational Colour Vision Stephen Westl in addition to Centre as long as Colour Design Technology www.phwiki.com

Computational Colour Vision Stephen Westl in addition to Centre as long as Colour Design Technology

Berra, Rich, Morning Show Host has reference to this Academic Journal, PHwiki organized this Journal Computational Colour Vision Stephen Westl in addition to Centre as long as Colour Design Technology University of Leeds s.westl in addition to @leeds.ac.uk June 2005 http://www.colourtech.org Ox as long as d Brookes University Computational Colour Vision Introduce some basic concepts – the physical basis of colour Computational approaches to how colour vision works Phenomenology of colour perception (the problem) Computational in addition to psychophysical studies of transparency perception The Physical Basis of Colour

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Cone spectral sensitivity L M S L = E(l)P(l)FL(l)dl M = E(l)P(l)FM(l)dl S = E(l)P(l)FS(l)dl Cone Responses L = E(l)P(l)FL(l)dl M = E(l)P(l)FM(l)dl S = E(l)P(l)FS(l)dl Each cone produces a univariant response L M S Colour perception stems from the comparative responses of the three cone responses Colour is a perception – ‘the rays are not coloured’ Colour Constancy Objects tend to retain their approximate daylight appearance when viewed under a wide range of different light sources The visual system is able to discount changes in the intensity or spectral composition of the illumination WHY / HOW

noon sunset X

X Computational Explanation L1 = E1(l)P(l)FL(l)dl M1 = E1(l)P(l)FM(l)dl S1 = E1(l)P(l)FS(l)dl L2 = E2(l)P(l)FL(l)dl M2 = E2(l)P(l)FM(l)dl S2 = E2(l)P(l)FS(l)dl Practical Use – Colour Correction Camera RGB values vary as long as a scene depending upon the light source colour correction In order to correct the images we need an estimate of the light source under which the original image was taken

Colour Constancy Adaptation is too slow to explain colour constancy “Any visual system that achieves colour constancy is making use of the constraints in the statistics of surfaces in addition to lights” – Maloney (1986) Is it possible as long as the visual system to recover the spectral reflectance factors of the surfaces in scenes from the cone responses P(l) S wiBi(l) Using a process such as SVD or PCA we can compute a set of basis functions Bi(l) such that each reflectance spectrum may be represented by a linear sum of basis functions – a linear model of low dimensionality. If we use n basis functions then each spectrum can be represented by just n scalars or weights. Basis Functions 1 Basis Function Original 1 BF P(l) = w1B1(l)

2 Basis Functions Original 1 BF 2 BF P(l) = w1B1(l) + w2B2(l) 3 Basis Functions Original 1 BF 2 BF 3 BF P(l) = w1B1(l) + w2B2(l) + w3B3(l) About 99% of the variance can be accounted as long as by a 3-D model (Maloney & W in addition to ell, 1986) But what proportion of the variance do we need to account as long as How many Basis Functions are Required 6-9 basis functions are required

Simultaneous Contrast

original original covered by filter original with small filter Colour Constancy – spatial comparisons “For the qualities of lights in addition to colours are perceived by the eye only by comparing them with one another” (Alhazen, 1025) “ object colour depends upon the ratios of light reflected from the various parts of the visual field rather than on the absolute amount of light reflected” (Marr) Spatial Comparison of Cone Excitations Retinex – L in addition to in addition to McCann (1971) Foster in addition to Nascimento (1994)

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Transparency Perception (Ripamonti in addition to Westl in addition to , 2001) e1/e2 = e’1/e’2 What is transparency An object is (physically) transparent if some proportion of the incident radiation that falls upon the object is able to pass through the object. What is perceptual transparency Perceptual transparency is the process ‘of seeing one object through another’ (Helmholz, 1867) Physical transparency is neither a necessary or sufficient condition as long as perceptual transparency (Metelli, 1974) Even in the complete absence of any physical transparency it is possible to experience perceptual transparency

Perceptual transparency Research Questions What mechanisms could drive perceptual transparency What are the chromatic conditions that cause transparency Could transparency in addition to colour constancy be linked Perceptual transparency

xB xP xQ xA xP = a xA + (1-a) g xQ = a xB +(1- a) g xP = a xA xQ = a xB Cone excitations are trans as long as med by a a diagonal matrix whose diagonal elements are all equal xB xP xQ xA xP = b xA xQ = b xB Cone excitations are trans as long as med by a a diagonal matrix whose diagonal elements are not necessarily all equal The two models can be made to be the same if the convergence model has no additive component in addition to if the invariance model has equal cone scaling

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