retinex theory psych221 final project mike jahr march 16, 2000
TRANSCRIPT
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Retinex Theory
Psych221 Final Project
Mike Jahr
March 16, 2000
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Color Constancy
• Color depends on wavelength
• But, objects reflect different wavelengths
under different lighting conditions.
– Banana in daylight, fluorescent light, no light...
• To us, they seem to retain their color.
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How is this possible?
• There is more to color than wavelength.
• The visual system must somehow “discount the illuminant”
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A Juicy Burger
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A Closer Look...
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What’s going on?
• It’s not very saturated, but the red burger has browns, greens, tans…
• How can we see these colors in only red and white light?
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Enter Edwin H. Land
• Land was the founder of Polaroid; interested in color
• While running Maxwell’s experiments (3 color projectors), he noticed this
• It spawned decades of experiments
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The Mondrian Apparatus
• Land set up 3 filtered
light sources (LMS)
• Can calibrate each one;
precisely control light
• Telescopic photometerActually closer to a
Van Doesburg...
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Mondrian Experiments
• Measure reflectance from a green patch
• Calibrate lights so that a blue patch
reflects an identical spectrum
• It still looks blue!
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More Mondrian
• Calibrate lights for even reflectance
from the green patch
• Cover all other patches; looks gray
• Uncover all patches; looks green
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Land’s Conclusions
• Perceived color depends on reflected spectrum, but also on surroundings
• Relative reflectance is more important than absolute reflectance
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Discount the Illuminant: Retinex
• “A framework for computing perceived colors on the basis of the relative intensities of three wavelengths and their spectral interactions.”
• Processed in retina or cortex? Retinex!
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Principles of Retinex
• Process each receptor class independently
• Objective is to calculate illuminant-independent “lightness” values
• Lightness values represent perceived color
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The Algorithm
• Pick a starting pixel x1, then form a path by randomly selecting neighboring pixels
• Update an accumulator at each pixel:
• Threshold step: if difference is small, use previous sensor response
A xi( )← A xi( )+logρxi( ) −logρx1( )
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The Algorithm II
• Keep a counter N(x) for each pixel
• After a number of paths, normalize A(x) by N(x) for each pixel
• Result is L(x), the lightness value
• Algorithm has two parameters:– number of paths, length of each path
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What is Lightness?
• Should not depend on viewing conditions
• Should only depend on surface properties
• Results in a triplet that is tough to interpret– The retinex color space
• Issue: what to do with it?
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My Implementation
1 Convert image from RGB to LMS via
phosphor spectra and cone sensitivities
2 Run algorithm to get lightness values
3 Do something with lightness values??
• B&W implementation
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Retinex Variants
• McCann et al.– Retinex with reset
• Horn– Determining lightness from an image
• Marini– Retinex with Brownian motion
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Illusions under Retinex
Original image Processed image
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More Illusions
Original image Retinex image
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Biological Basis
• Some monkey neurons respond to colors, not wavelengths– Cortical area V4 in prestriate cortex
• Even goldfish can discount the illuminant
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Problems with Retinex
• Too dependent on composition of surfaces in image
• Higher-order processes influence color
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Conclusion
• Retinex is a long-lived theory, has sparked much debate and many imitators
• Although not a generally accurate model of human vision, it does perform well in some situations
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Appendix
• Source files, sample images, sample output, etc. can be found in src/ along with brief explanations of each.
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References
• E. H. Land, “Recent advances in retinex theory and some implications for cortical applications: Color vision and the natural image,” Proc. Nat. Acad. Sci. USA 80, 5163–5169 (1983).
• E. H. Land, “Recent advances in retinex theory,” Vision Res. 26, 7–22 (1986).
• B. K. P. Horn, “Determining lightness from an image,” Comp. Graphics Image Process. 3, 277–299 (1974).
• D. H. Brainard and B. A. Wandell, “Analysis of the retinex theory of color vision,” J. Opt. Soc. Am. 3, 1651–1661 (1986).
• J. J. McCann, “Lessons learned from Mondrians applied to real images and color gamuts,” IS&T Rep. 14, 6 (1999). http://www.imaging.org/pubs/reporter/articles/14_6_mccann/index.html
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References
• E. H. Adelsen, “Lightness perception and lightness illusions,” in M. Gazzaniga, M.S., Ed., The Cognitive Neurosciences, Cambridge, MA: MIT Press, pp. 339-351 (1999). http://www-bcs.mit.edu/people/adelson/publications/gazzan.dir/gazzan.htm
• F.W. Campbell, F.R.S., “Dr. Edwin H. Land,” Biographical Memoirs of Fellows of the Royal Society, 40, 195-219 (1994). http://www.rowland.org/land/land.html
• D. Marini and L. Marini, “Measuring the colours we receive,” Science Tribune, October (1997). http://www.tribunes.com/tribune/art97/mari.htm