Download - Lecture 21: Introduction to Color Science
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Computer Graphics and Imaging UC Berkeley CS184/284A
Lecture 21:
Introduction to Color Science
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Wassily Kandinsky, Color Study. Squares with Concentric Circles, 1913 Munich, The Städtische Galerie im Lenbachhaus
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Mark Rothko No. 61. Rust and Blue 1953, Museum of Contemporary Art, Los Angeles
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Ren NgCS184/284A
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Ren NgCS184/284A
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Kanazawa & NgCS184/284A
Watercolor Illusion
Pinna & Tanca, JOV, 2008
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Kanazawa & NgCS184/284A
Watercolor Illusion
Pinna & Tanca, JOV, 2008
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Kanazawa & NgCS184/284A
Watercolor Illusion
Pinna & Tanca, JOV, 2008
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Notice R, G, B sub-pixel geometry. Effectively three lights at each (x,y) location.
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Kanazawa & NgCS184/284A
Recall: Real LCD Screen Pixels (Closeup)
iPhone 6S
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Kanazawa & NgCS184/284A
• Mantis shrimp! A lot of photoreceptor classes. 16!
• Tetrachromacy — some women have 4 types of cone cells.
• Color is from different wavelengths
• Nothing inherently special to our visible spectrum, except H20 being transparent
• Synesthesia — hear music and see colors
• People couldn’t see or recognize blue until relatively modern times (?)
Discussion: Warm Ups (Breakout Rooms)What is a fact you know about color? What is something you want to learn about color?
• Your language shapes your perception of color. E.g. lack of blue/green distinction in some languages
• Our eyes are more sensitive to green — why our displays have more green pixels (?)
• Color does not always correspond to one wavelength
• Blue light disrupts sleep patterns — why we make our displays red at bedtime
• Red and yellow are most appetizing colors
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Kanazawa & NgCS184/284A
• 3D - 3 cone receptors in the human retina
• Some thought 3D, some thought wavelength (1D?)
• If function of frequency spectrum, then infinite dimensional
• Hidden dimensions of color — e.g. Ren’s research on Oz Vision project
Discussion: What is The Dimensionality of Color?
How do we know?
• Red green blue white black — 5D?
• CMYK printing — 4D?
• Position of the color may be important — e.g. the Lotto cube illusion
• HSV / RGB —
• Perception is 3D, but dimensionality of the ray of light itself perhaps infinite?
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Kanazawa & NgCS184/284A
What is Color?
• Color is a phenomenon of human perception; it is not a universal property of light
• Colors are the perceptual sensations that arise from seeing light of different spectral power distributions
• Technically speaking, different wavelengths of light are not “colors”
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Physical Basis of Color
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credit: Science Media Group.
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Ren NgCS294-164
Isaac Newton's Experimentum Crucis
Isaac Newton performing his crucial prism experiment – the 'experimentum crucis' – in his Woolsthorpe Manor bedroom. Acrylic painting by Sascha Grusche (17 Dec 2015)
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Kanazawa & NgCS184/284A
The Fundamental Components of Light
• Newton showed sunlight can be subdivided into a rainbow with a prism • Resulting light cannot be further subdivided with a second prism
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Kanazawa & NgCS184/284A
Electromagnetic radiation
• Oscillations of different frequencies (wavelengths)
The Visible Spectrum of Light
Image credit: Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:EM_spectrum.svg#/media/File:EM_spectrum.svg
Visible electromagnetic spectrum
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Kanazawa & NgCS184/284A
MonochromatorHegyi et al. "Introduction to Practical Biochem
istry" http://elte.prom
pt.hu/sites/default/files/tananyagok/IntroductionToPracticalBiochemistry/ch04s07.htm
l
A monochromator delivers light of a single wavelength from a light source with broad spectrum.
Control which wavelength by angle of prism.
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Kanazawa & NgCS184/284A
Spectral Power Distribution (SPD)
Salient property in measuring light
• The amount of light present at each wavelength
• Units:
• radiometric units / nanometer (e.g. watts / nm)
• Can also be unit-less
• Often use “relative units” scaled to maximum wavelength for comparison across wavelengths when absolute units are not important
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Kanazawa & NgCS184/284A
Daylight Spectral Power Distributions Vary
Blue sky Solar disk
[Brian Wandell]
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Kanazawa & NgCS184/284A
Spectral Power Distribution of Light Sources
Describes distribution of energy by wavelength
Figure credit:
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Kanazawa & NgCS184/284A
SpectrometerHegyi et al. "Introduction to Practical Biochem
istry" http://elte.prom
pt.hu/sites/default/files/tananyagok/IntroductionToPracticalBiochemistry/ch04s07.htm
l
For unknown light source, use a monochromator to isolate each wavelength of light for measurement
Detector
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Kanazawa & NgCS184/284A
Superposition (Linearity) of Spectral Power Distributions[Brian W
andell]
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Biological Basis of Color
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Kanazawa & NgCS184/284A
Anatomy of The Human Eye
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Kanazawa & NgCS184/284A
Retinal Photoreceptor Cells: Rods and Cones
Rod cells Cone cellsRods are primary receptors in very low light (“scotopic” conditions), e.g. dim moonlight
• ~120 million rods in eye
• Perceive only shades of gray, no color
Cones are primary receptors in typical light levels (“photopic”)
• ~6-7 million cones in eye
• Three types of cones, each with different spectral sensitivity
• Provide sensation of colorhttp://ebooks.bfwpub.com/life.php Figure 45.18
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Kanazawa & NgCS184/284A
Human Retinal Cone Cell Response Functions (L, M, S Types)
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Brainard, Color and the Cone Mosaic, 2015.
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Three types of cone cells: S, M, and L (corresponding to peak response at short, medium, and long wavelengths)
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Kanazawa & NgCS184/284A
An Aside: Spatial Resolution of Rods and Cones in the Retina
• Highest density of cones is in fovea (and no rods there)
• “Blind spot” at the optic disc, where optic nerve exits eye
[Roorda 1999]
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Kanazawa & NgCS184/284A
Fraction of Three Cone Cell Types Varies Widely
Hof
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Distribution of cone cells at edge of fovea in 12 different humans with normal color vision. Note high variability of percentage of different cone cell types. (false color image)
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Measuring Light
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Kanazawa & NgCS184/284A
A Simple Model of a Light Detector
Produces a scalar value (a number) when photons land on it
• Value depends only on the number of photons detected
• Each photon has a probability of being detected that depends on the wavelength
• No way to distinguish between signals caused by light of different wavelengths: there is just a number
This model works for many detectors:
• based on semiconductors (such as in a digital camera)
• based on visual photopigments (such as in human eyes)
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Kanazawa & NgCS184/284A
A Simple Model of a Light DetectorCredit: M
arschner
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Kanazawa & NgCS184/284A
Mathematics of Light Detection
Same math carries over to spectral power distributions
• Light entering the detector has its spectral power distribution, s(λ)
• Detector has its spectral sensitivity or spectral response, r(λ)
measured signal
input spectrum
detector’s sensitivity
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Kanazawa & NgCS184/284A
If we think of s and r as discrete, sampled representations (vectors) rather than continuous functions, this integral operation is a dot product: We can also write this in matrix form:
Mathematics of Light Detection
X =⇥
s⇤2
64r
3
75
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Kanazawa & NgCS184/284A
Dimensionality Reduction From ∞ to 1
At the detector: • SPD is a function of wavelength (∞ - dimensional signal)
• Detector result is a scalar value (1 - dimensional signal)
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Tristimulus Theory of Color
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Kanazawa & NgCS184/284A
Maxwell's Crucial Color Matching Experiment
http://designblog.rietveldacademie.nl/?p=68422
Portrait: http://rsta.royalsocietypublishing.org/content/366/1871/1685
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To Be Continued