Any questions about the current assignment? (I’ll do my best to help!)

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)