bpc: art and computation – fall 2006 digital media ii: light, vision & digital images glenn...

BPC: Art and Computation – Fall 2006BPC: Art and Computation – Fall 2006

Digital Media II:Digital Media II:Light, Vision & Digital ImagesLight, Vision & Digital Images

Glenn Bresnahan

[email protected]

BPC: Art and Computation – Fall 2006 2

OutlineOutline

What is light?Properties of lightHow do we see?Digital representation of imagesComputer displayDigital image formats


How Do We See?How Do We See?


How Do We Hear?How Do We Hear?

Sound waves move through the air Waves interact (e.g. reflect) w/ environment Sounds wave reach our ear


How Do We See?How Do We See?

Light is emitted from a source Waves interact (e.g. reflect) w/ environment Light reaches our eyes


What is LightWhat is Light

Light is a

wave

Packets of light energy are called photons


Waves RevisitedWaves Revisited


Waves – PropertiesWaves – Properties

Sine Wave

-1.5

-1

-0.5

0

0.5

1

1.5

Amplitude

Wavelength (distance)


Waves in Motion – PropertiesWaves in Motion – Properties

Sine Wave

-1.5

-1

-0.5

0

0.5

1

1.5

Period (time for one cycle)

TimeTime 11 22Frequency cycles per time interval


Cycles and CirclesCycles and Circles

Sine waves and circles are closely related

Y axis

X axisangle

(x,y)


Cycles and CirclesCycles and CirclesY Value vs Angle

-1.5

-1

-0.5

0

0.5

1

1.5

0 30 60 90 120 150 180 210 240 270 300 330 360

Angle

Y v

alu

e

Y axis

X axisangle

(x,y)

X Value vs Angle

-1.5

-1

-0.5

0

0.5

1

1.5

0 30 60 90 120 150 180 210 240 270 300 330 360

Angle

X v

alu

e


Properties of SoundProperties of Sound

Pitch is the perception of frequencyHuman perception: 20 Hz – 20 KHzSound travels at approx. 1100

feet/second in air– Approx. 750 miles/hour or 1 mile every

4.8 sec.Loudness perception of amplitude


Properties of LightProperties of Light

Color is the perception of frequencyHuman perception: 430 – 750 THz

(red – violet)– 1 THz = 1,000,000,000,000 Hz

Light travels at approx. 186,000 miles/second in air– Approx 1 foot every nanosecond

Brightness is perception of energy level (number of photons)


How Fast is Light?How Fast is Light?

186,00 miles/sec or 300,000 meters/sec

8 minutes to reach earth from sun


WavelengthWavelength

Wavelength = Speed / Freq– E.g. 1 ft/sec at 1 Hz = 1 ft wavelength– Higher frequencies == shorter wavelengths

Red = 300KM/Sec / 430 THz = 698 nm (nano (billionth) meters)

Violet = 300KM/Sec / 750 THz = 400 nm


Visible SpectrumVisible Spectrum

Where is the white light?

What happens at higher/lower frequencies?


Electromagnetic SpectrumElectromagnetic Spectrum

Visible light is electromagnetic force in a particular frequency range


Light Interaction with MaterialsLight Interaction with Materials

When light hits a surface, several things can happen. The light can be:– Absorbed by the surface• Converted to another form of energy

– Reflected (bounced) off the surface– Transmitted (refracted) through the

surface


Absorption and ReflectionAbsorption and Reflection

Different materials will absorb different frequencies

The absorption vs. reflection determines the color of the material– Black materials absorbs all wavelengths– White material reflects all wavelengths– Blue material reflects blue and absorbs all

other wavelengths Combining pigments causes more

wavelengths to be absorbed, fewer wavelengths to be reflected– Subtractive color


Reflection and RefractionReflection and Refraction


ReflectionReflection

Light reflects at an opposite and equal angle– Specular (mirror) reflection

Some light will be scattered in all directions


RefractionRefraction

Speed of a wave varies by material Index of refraction is relative speed in the

medium– Vacuum 1.0000– Air 1.0003– Ice 1.31– Water 1.33– Quartz 1.46– Flint glass 1.57-1.75– Diamond 2.417



When a wave chances speed it changes direction, i.e. bends

The angle

depends of the

change in

refractive

index



Objects appear to bend in water



Lens change size of objects


Combination of LightCombination of Light

White light?– Combination of multiple colors (freq) of

lightWhat happens when we combine

different frequencies of light, say red and green?

What happens when we combine different frequencies of sound, say an C and an E note?


Color ExperimentColor Experiment

If we combine red, green and blue light we get new colors in the region of overlap

Colors seem to “add”


How We SeeHow We See

Light is emitted from a sourceLight interacts with surfaces in the

environmentLight is reflected into our eyes


Human VisionHuman Vision

Light passes into the cornea, though a liquid filled chamber and out through the lens. These focus the light

The pupil acts as diaphragm, controlling the amount of light

The light is projected onto the retina at the back of the eye



The retina is covered with photosensitive receptor cells

Photoreceptor cells are attached to the optical nerve which feeds signals to the brain

Light (photons) enter the cell cause a chemical reaction which causes the cell to fire


Eat Your Carrots!Eat Your Carrots!

Photoreceptor cells contain opsin (a protein) + retinal = rhodopsin

Photo excitation causes the rhodopsin to twist and release the retinal

The released retinal causes a reaction which cause the attached nerve to fire

Retinal is destroyed in the processRetinal is synthesized from vitamin AVitamin A is derived from beta-

carotene

BPC: Art and Computation – Fall 2006BPC: Art and Computation – Fall 2006

Digital Media II:Digital Media II:Light, Vision & Digital ImagesLight, Vision & Digital ImagesPart 2Part 2

Glenn Bresnahan

[email protected]


Question?Question?

When we combine light of two different frequencies we seem to get light of a different color. Why does this happen? Sound waves don’t combine this way.


Combining WavesCombining Waves

Sound waves do not combine to make new frequencies (pitch)

C + E not equal DC = 523.25 Hz / 65.9 cm (2.162 ft)

D = 587.33 Hz / 58.7 cm (1.925 ft)

E = 659.29 Hz / 52.3 cm (1.716 ft)


Length of Light WavesLength of Light Waves

Human hair ~ 1/500”– 0.005 cm– 50,000 nm

Cyan light = 500 nm 100 wavelengths

across a human hair



Light passes into the cornea, though a liquid filled chamber and out through the lens. These focus the image

The pupil acts as diaphragm, controlling the amount of light

The light is projected onto the retina at the back of the eye where a chemical reaction causes neurons to fire


PhotoreceptorsPhotoreceptors

The retina contains two types of receptor cells: rods and cones

Approx. 90 million rods; 4.5 million cones


Photoreceptors - RodsPhotoreceptors - Rods

Rods react to very low light levels– As few as several photons

Rods react to a broad spectrum of frequencies (max at 498 nm)

Rods react slowly (~100 milliseconds)


Photoreceptors - ConesPhotoreceptors - Cones

Cones require much more light to fireCones react much more quickly (10-

15 ms)Cones are much denser in the center

(fovea) of the eye


Photoreceptors – DistributionPhotoreceptors – Distribution


Photoreceptors - ConesPhotoreceptors - Cones

Three types of cones: S, M, L which react to different wavelengths of light– L Cones: peak at 564 nm–M Cones: peak at 533 nm– S Cones: peak at 437 nm


Photoreceptors – Response SpectrumPhotoreceptors – Response Spectrum

S = blue, M = green, L = red


Photoreceptors – Seeing ColorsPhotoreceptors – Seeing Colors

Any response can be synthesized by combining red, green and blue light


Color MixingColor Mixing

Adding red, green and blue light in various proportions can generate the perception of all colors


Cell FiringsCell Firings

Light reaching photoreceptors causes some number of cells to fire (after an interval)

Cells can not continually fireReceptors can become saturatedCell firings are discrete


Saturation – After ImagesSaturation – After Images


Flicker FusionFlicker Fusion

If light is flashed fast enough, it becomes indistinguishable from a steady light

The rate is called the flicker fusion or critical flicker frequency

Dependent on intensity, but about 45 Hz


Flicker Fusion & AnimationFlicker Fusion & Animation

Flicker fusion makes animation possible

Each frame is

displayed a

fraction of a

second


Flicker Fusion in Film and VideoFlicker Fusion in Film and Video

Film uses 24 frames per secondVideo uses 30 frames per secondFlicker fusion is >45 FPSHow does this work??


Flicker Fusion in FilmFlicker Fusion in Film

Film projector has a shutterEach frame is displayed 3 times


Flicker Fusion in VideoFlicker Fusion in Video

Frame is broken up into strips (scan lines)

Frame is divided into two fields: odd lines and even lines

Fields are displayed at 60 Hz


SeeingSeeing

The rods and cones cause nerves to fire and electrical signals to be send to the brain.

There are ~100 million receptor cells generating impulse streams

Impulses are combined by other nerve cells

1.2 million nerve fibers in optic nerve bundle


Vision Is ComplicatedVision Is Complicated


Digital ImagesDigital Images



Red = 100%Green = 80%Blue = 60%



Image is constructed from a grid (array) of individual color dots

Individual dots are called pixels (picture elements) Resolution is the number of elements in each

direction (e.g. 1280 in x, 1024 in y = 1.3 Mpixel) Each pixel is composed of three color

components representing levels of R,G,B light Each level (R,G,B) is represented by a number

– One byte (0-255) per component, e.g. 255,204,153 The array of pixel values is stored in a graphics

frame buffer (memory) The pixel values are read out (at approx. 60 fps)

and used to display the image on a monitor


Graphics DisplayGraphics Display

RGB RGB RGB …

RGB RGB RGB …

RGB RGB RGB …… … …

Frame Buffer Computer Monitor


Cathode Ray Tube DisplayCathode Ray Tube Display

Same technology as a TV screen Electron beam is aimed at the screen When the beam hits a phosphor on the surface it

glows Three different colored (RGB) beams phosphors


Other DisplaysOther Displays

Liquid Crystal (LCD) Plasma panel Digital Light Projection


Digital Storage of ImagesDigital Storage of Images

Need to store RGB value for each pixel 1024x1280 pixels = 3.9 million numbers Different images files use different ways of

storing the numbers– Most file formats store additional information– Formats vary in how much information per pixel is

stored Some formats compress the information

– Compression can lead to loss of detail– Uncompressing the compressed image is NOT the same

as the original image.– Repeatedly storing (compressing) and retrieving

(uncompressing) can cause an image to degrade

bpc: art and computation – fall 2006 digital media ii: light, vision & digital images glenn...

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