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Any questions about the current assignment?
(I’ll do my best to help!)
This Week: Color & Images
Next Week: Depth (Kevin Ponto) Image Compositing (Perry Kivolowitz)
This Week
Optics & Physics of Color
Color in Displays
How graphics people talk about color
Image Compression & Color Profiles
Misc (time permitting)
This Week
Optics & Physics of Color
Color in Displays
How graphics people talk about color
Image Compression & Color Profiles
Misc (time permitting)
Please ask questions if you have them!!!
What is Color?
“Color consists of the characteristics of light other than spatial and temporal inhomogeneities; light being that aspect of radiant energy of which a human observer is aware through the visual sensations which arise from the stimulation of the eye.”
--OSA Committee on Colorimetry
What is Color?
What is Color?
Both a physical quantity of light
A wavelength
and a visual sensation.
A perception
What is Color?
Both a physical quantity of light
A wavelength
and a visual sensation.
A perception
Color as a wavelength
Light Source
Color as a wavelength
Light Source Light
Color as a wavelength
Light Source
Light is composed of photons
Color as a wavelength
Light Source
Photons have specific wavelengths
Electromagnetic Spectrum:All wavelengths of light
Visible Light (390-700nm):Light we can see (colors)
Sun emits entire spectrum
Visible Light
Short WavelengthHigher Energy
Long WavelengthLower Energy
White = All Visible WavelengthsBlack = No Visible Wavelengths
What is Color?
Both a physical quantity of light
A wavelength
and a visual sensation.
A perception
Vision is measuring visible light
Short WavelengthHigher Energy
Long WavelengthLower Energy
Vision is measuring visible light
Retina is a bunch of sensors
RodsCones
Rods:
Shades of grey
Sensitive to low-light
Densest in the periphery
Cones:
Responsible for color vision
Long (L), Medium (M), Short (S)
Densest in the fovea (center)
No blue cones
Cone Distribution
From M. Fairchild
Measuring Visible Light
Short WavelengthHigher Energy
Long WavelengthLower Energy
S
MRod L
Trichromacy
Color perception is a function of the relative stimulation of the three cone types (L, M, & S)
In graphics, this isRed, Green, & Blue!
L Cones
S Cones
Normal
This Week
Optics & Physics of Color
Color in Displays
How graphics people talk about color
Color Spaces
Image Compression & Color Profiles
Misc (time permitting)
Blue
Red
Gre
en
“Cube” based on three primary colors
What (most) monitors use
Attempt to stimulate each cone in isolation
Computer Graphics: RGB
Monitors RGB
LCDCRT
From M. Stone
CRT Monitor
LCD Monitor
Two properties of monitors define what colors they produce
Gamma maps intensity of light emitted
Gamut maps the space of possible colors generated by a display
The amount of light emitted from a monitor is nonlinear
pixels
intensity
Gamma parameter makes light intensity linear
Original
Small Gamma
Large Gamma
Gamuts
Visible colors (grey) versus colors supported by the display (triangle)
Gamut: Colors that can be created using the three display primaries
Gamut Mapping
Moving an image from one devices’ gamut to another
How could you do gamut mapping when moving to a
smaller gamut (e.g. less available colors)?
Clip colors outside of the new gamut
Center the new colors with the old and then clip
Scale everything down towards the middle of the gamut
Scale just the primaries down towards the gamut
Original
This Week
Optics & Physics of Color
Color in Displays
How graphics people talk about color
Image Compression & Color Profiles
Misc (time permitting)
Who talks about color?
Graphics people
Physicists
Psychologists
Artists & Designers
Physicists
Short WavelengthHigher Energy
Long WavelengthLower Energy
Physicists: Spectral Distribution
From Stone’s A Field Guide to Digital Color
Blue
Red
Gre
en
“Cube” based on three primary colors
What (most) monitors use
Attempt to stimulate each cone in isolation
Computer Graphics: RGB
Why might choosing primaries that target cones (L, M, S) not
work?
Different cones respond to the same wavelengths
M L
Different cones respond to the same wavelengths
M L
Metamerism
Colors with different spectral distributions appear the same
From Stone’s A Field Guide to Digital Color
Different cones respond to the same wavelengths
M L
How could we fix this?
Negative light would need to shut down responses from a particular
cone
M L
Imaginary Primaries!
Negative light isn’t a thing…
Hence, imaginary
Instead, psychologists look at how the brain interprets color
An aside: aspects of color
Luminance: How light something is
Saturation: How colorful something is
Hue: What color something is
Psychologists: CIE XYZIntended to reflect perceptions based on cone primaries
Y: Lightness
XZ: Hue/saturation plane
From Stone’s A Field Guide to Digital Color
Opponent Color
From M. Stone
CIELAB
L: Lightness
AB: Hue/saturation plane based on opponent responses
Euclidean distance is meaningful!
CIELCh (Polar LAB)
Lightness
Hue
Colorfulness
Unique black and whiteUniform differences
Perception & design
From M. Stone
Psychologists’ way of talking about color is perceptually
accurate, but difficult to understand & implement.
Artists
Think in terms of lightness/hue/saturation
Munsell Look-up Tables
From Gretag-Macbeth
HSV/HSL
Color Blending
Physical Pigments
CMYK
Monitors & Light
Thursday
Optics & Physics of Color
Color in Displays
How graphics people talk about color
Image Compression & Color Profiles
Misc (time permitting)
