Any questions about the current assignment? (I’ll do my best to help!)
Depth 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?
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 ( nm): Light we can see (colors) Sun emits entire spectrum
Visible Light Short Wavelength Higher Energy Long Wavelength Lower Energy White= All Visible Wavelengths Black= 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 Wavelength Higher Energy Long Wavelength Lower Energy
Vision is measuring visible light
Retina is a bunch of sensors Rods Cones
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 Wavelength Higher Energy Long Wavelength Lower Energy S M Rod L
Trichromacy Color perception is a function of the relative stimulation of the three cone types (L, M, & S) In graphics, this is Red, 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 Green “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 Wavelength Higher Energy Long Wavelength Lower Energy
Physicists: Spectral Distribution From Stone’s A Field Guide to Digital Color
Blue Red Green “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
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 XYZ Intended 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 white Uniform 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)