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Computer Graphics and Image ProcessingColor IPart 1 – Lecture 5

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Today’s OutlineToday s Outline Colors in the Real WorldCo o s e ea o d Interaction of Light with Materials Human Perception of Light

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COLORS IN THE REAL WORLD

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Electromagnetic Radiation: WavesElectromagnetic Radiation: WavesPhysics theories of light:1. waves (classical mechanics)2. particles (quantum mechanics) wavelength, λ

frequency =cycles/sec.

4© 2004 Lewis Hitchner, Richard Lobb & Kevin Novins

Some Neighbors of Visible LightSome Neighbors of Visible Light1000 nm

Infrared

700 nmTV

Radio

Red

Infrared Invisible

TV

h Mi

RedOrange

Yellow Visible light a k a

elen

gth Microwaves

Visible Light

Yellow

GreenInfrared

Visible light, a.k.a.the “visible spectrum”

400 nmX-RaysUV Rays

Wav

e gBlue

IndigoViolet 400 nm

10

GammaRays

X RaysUltra-violet(UV)

Invisible

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10 nmy (UV)

© 2004 Lewis Hitchner, Richard Lobb & Kevin Novins

Spectral Density Function (SDF): S(λ)Spectral Density Function (SDF): S(λ) S(λ) = power / unit wavelength = energy S(λ) power / unit wavelength energy

Ene

rgy

Ene

rgy

400 700Wavelength λ (nm)

E


E

spike or “single” wavelength= “spectral colour”

uniform white/gray light

Ene

rgy

Ene

rgy

400 700Wavelength λ (nm) 400 700Wavelength λ (nm)

white light plus a dominant wavelength arbitrary SDF: blue plus orange/yellow


SDFs for Different Light SourcesSDFs for Different Light SourcesSome other light spectra: sunlight, tungsten lamp, fluorescent lampg p g g p p

http://www.micro.magnet.fsu.edu/optics/lightandcolor/sources.html


Describing Colors using the SDFDescribing Colors using the SDF• Hue: dominant wavelengthgy Hue: dominant wavelength• Luminance (brightness): total power(integral of SDF)

• Saturation (also purity): % of luminance

Ene

rg

Saturation (also purity): % of luminanceresiding in dominant component400 700Wavelength λ (nm)

gy hi h S gy high S

Ene

rg high Smed L

Ene

rg high Slow L

400 700Wavelength λ (nm) 400 700Wavelength λ (nm)

rgy

low Srgy

Ene

low Slow L

Ene low S

high L

400 700Wavelength λ (nm)400 700Wavelength λ (nm)8

INTERACTION OF LIGHT WITH MATERIALS

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Interaction of Light with MaterialsInteraction of Light with MaterialsLight surface of some “body”: 3 possible results1. Absorption – energy of selected wavelengths retained within the body2. Transmission – energy of selected wavelengths travels through and exits

the body Refraction of light occurs at boundariesthe body. Refraction of light occurs at boundaries.3. Reflection – energy of selected wavelengths “bounces” off surface.

Angle of reflection = angle of incidence. Also combinations of these 3, such as “internal reflection” when light enters a

semi-translucent body, scatters, and some light reflects back out: human skin, Shrek’s skin

R = reflected energyR = reflected energy

A = absorbed

I = incident (incoming)energy

T = transmitted energy

energy


Interaction of Light with MaterialsInteraction of Light with Materials Incident (incoming) light energy

= absorbed energy + transmitted energy + reflected energy= retained + passed through + bounced off

Chemical properties of the body determine the % of each Chemical properties of the body determine the % of each.

opaque coloured surface: A > 0 , T = 0, R > 0 opaque black surface: A = I ,T = 0, R = 0(black felt cloth)(black felt cloth)

Comp ter graphics reflected (and refracted) transmitted lightperfect mirror surface: A = 0, T = 0, R = I transparent surface: A, R small, T ~= I

(clear glass)

Computer graphics: reflected (and refracted), transmitted light11

© 2004 Lewis Hitchner, Richard Lobb & Kevin Novins

Spectral Response Function (SRF)Spectral Response Function (SRF) Molecular structure of a body determines which wavelengths of light Molecular structure of a body determines which wavelengths of light

and what amount are absorbed, transmitted, or reflected Can be measured with a spectral response function (SRF) or filter ( )

function

ergy

100% ergy

100%

% E

ne

0%

% E

ne

0%

400 700Wavelength λ (nm)All pass filter (ideal , real )

400 700Wavelength λ (nm)low pass and high pass filters

ergy

100% ergy

100%

% E

ne

100%

0%

% E

ne100%

0%

400 700Wavelength λ (nm)Narrow band filter

400 700Wavelength λ (nm)Notch filter

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Light Source SDF x SRF = Result SDFLight Source SDF x SRF = Result SDFSDF of result = product of SRF and light source SDF

i.e. at each wavelength, multiply SRF % times source energy

gygy

% E

nerg 100%

0%

ΧEne

rg

400 700Wavelength λ (nm)0%

Narrow band filter400 700Wavelength λ (nm)

Sunlight SDF

Ene

rgy

== Ene

rgy

400 700Wavelength λ (nm)E


E

Filtered sunlight SDF


SDF x SRF = Result SDFSDF x SRF = Result SDFWhy is this relevant for computer graphics?1. All light sources can be defined by their SDF

Natural light source: sun, fire Artificial light source: light bulb, laser, LED, computer display

2. All light absorbers, transmitters, or reflectors can be defined by g ytheir SRF Sensing devices: absorbed light SRF

Camera (digital photocell, film), human eye (retina) Definition of “color” =

integral of (light source SDF sensor’s SRF) Glass, still water, cellophane: transmitted light SRF Surface material of an object: reflected light SRF


HUMAN PERCEPTION OF LIGHT

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The EyeThe Eye Four types of receptors (sensors): R/G/B cones + rods, each has unique SRF

http://webvision.med.utah.edu/imageswv/fovmoswv.jpeghttp://webvision.med.utah.edu/imageswv/Sagschem.jpeg

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Colour “Blindness”Colour Blindness

http://members.aol.com/protanope/card2.htmlIf you didn’t see both a yellow circle and a faint brown square, you are somewhat “colour

blind” (in USA 5 0% of males 0 5% of females) To find out more visit:blind (in USA 5.0% of males, 0.5% of females). To find out more, visit: http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/cblind.htm


Colors and the SDFColors and the SDF Many different SDFs are perceived by us as the same color! When describing a color (as seen by the eye) exactly, we

do not need to know full SDF Three parameters are enough For example: just use hue, luminance and saturation For example: just use hue, luminance and saturation

ergyrgy

Ene

Ene

400 700Wavelength λ (nm)400 700Wavelength λ (nm)


SRFs for the Eye and a CameraSRFs for the Eye and a Camera

http://www.stanford.edu/class/ee392b/handouts/color.pdfp phttp://www.ecs.csun.edu/~dsalomon/DC2advertis/AppendH.pdf


Seeing Red Green and BlueSeeing Red, Green and Blue A cone cell in the retina measures amount of red, green, or blue wavelength

(3 SRF’ ) R d l i b i ht li htenergy (3 SRF’s). Responds only in bright light. SRF of a rod cell covers all wavelengths (measures “gray level” or intensity)

Responds in low light, but not in bright light.p g , g g Integral of R, G, or B cone response produces a single value

Note: SRF’s really L, M, S wave responses (long, medium, short), not R, G, B.Note: low response of short (blue) is scaled up by vision system (after retina)Note: low response of short (blue) is scaled up by vision system (after retina).

= =

Χ

Short (blue) (scaled)

==

SDF of source

ΧMedium (green)

= =SRF’s of eye (L, M, S)

=Long (red)

=


Seeing Red Green Blue (cont’d)Seeing Red, Green, Blue (cont d) Example L, M, S responses for various SDF’s

= =

Sunlight SDF Blue reflecting object SDFg g j

==

Yellow reflecting object SDFGreen reflecting object SDF

Resulting L, M, and S SRF responses are independent values The 3 SRF response values are interpreted as hues by our brain,

e g red + green = yellow red + green + blue = whitee.g. red + green = yellow, red + green + blue = white

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SUMMARY

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SummarySummary1. Spectral Density Function (SDF): describes the wave

composition of light with power for each wave length segment2. Spectral Response Function (SRF): can be used to specify

h h % f h l th b b d fl t dhow much % of each wave length are absorbed or reflected or transmitted

3 Light ith different SDFs can ha e the same color for o r e e3. Light with different SDFs can have the same color for our eye

References: Li ht d C l Hill Ch t 11 1 Light and Colors: Hill, Chapter 11.1 Dominant Wave Length: Hill, Chapter 11.2.1

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QuizQuiz1. What is a spectral density function (SDF)?2. In what ways can light interact with a material?3. How can we describe this interaction?3. How can we describe this interaction?4. What are hue, luminance and saturation?

How manydifferent colors?

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computer graphics and image processing color i › compsci373s1c › christ... · microsoft...

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