COLOR - Uppsala University€¦ · (CMYK common in printing, where K is black pigment) RGB within...

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Lecture 3 October 30, 2003 Ingela Nyström 1 COLOR Angel 1.4, 2.4, 7.12 J. Lindblad 2001-11-01 Elements of color Color = The eye’s and the brain’s impression of electromagnetic radiation in the visual spectra. How is color perceived? light source reflecting object detector s () λ r() λ r g b () () () λ λ λ rods rods & cones red-sensitive green-sensitive blue-sensitive Visible spectrum The Fovea There are three types of cones, S, M and L

Transcript of COLOR - Uppsala University€¦ · (CMYK common in printing, where K is black pigment) RGB within...

Page 1: COLOR - Uppsala University€¦ · (CMYK common in printing, where K is black pigment) RGB within CIE XYZ-space HLS color space Hue Lightness Saturation Hue=dominant wavelength, tone

Lecture 3 October 30, 2003

Ingela Nyström 1

COLORAngel 1.4, 2.4, 7.12

J. Lindblad 2001-11-01

Elements of color

Color = The eye’s and the brain’s impression of electromagnetic radiation in the visual spectra.

How is color perceived?

light source

reflecting object

detector

s( )λr( )λ

r

g

b

( )

( )

( )

λλλ

rodsrods & cones

red-sensitive

green-sensitive

blue-sensitive

Visible spectrum

The Fovea

Thereare three typesof cones, S, M and L

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Lecture 3 October 30, 2003

Ingela Nyström 2

RodsSense luminance, or ”brightness”, but not color.Are spread out across the whole retina, and dominate when the pupil is large, i.e. night vision. Less color is seen at night. The response is not linear, but logarithmic.The appearance of an object’s intensity dependson the surroundings; the sensation is relative and not absolute.

Three kinds of conesr

g

b

( )

( )

( )

λλλ

red-sensitive

green-sensitive

blue-sensitive

wavelength

r=700nmg=546.1nm

b=435.8nm

In order to standardize the description of color, a large number of people were instructed to say what combination of basic colorsa certain color sample consisted of in standard lighting. This resulted in the color matching curves, i.e. transform

r g b( ), ( ), ( )λ λ λ x y z( ), ( ), ( )λ λ λ

CIE standard(Commission Internationale de L’Eclairage, 1931)

Color perception•Different spectra can result in identical sensations, called metamers•Color perception results from the simultaneousstimulation of 3 cone types (trichromat)•Our perception of color is affected by surrounding effectsand adaptation

standard lightsource

object reflectance

CIE 1931 standard observer

CIE XYZ values

400nm 700nm 400nm 700nm 400nm 700nm

xy

z

xx =X=14.27Y=14.31Z=71.52

s( )λ r( )λ

Each color is represented by a point (X,Y,Z) in the 3D CIE color space. The point is called the tristimulus value.

X s r x d

Y s r y d

Z s r z d

=

=

=

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

λ λ λ λ

λ λ λ λ

λ λ λ λ

Projection of the CIE XYZ-space

Perceptual equal distances

RGB/CMY color space

RGB - for additive color mixing, e.g. computer screen.CMY - for subtractive color mixing, e.g. printing or painting.

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Lecture 3 October 30, 2003

Ingela Nyström 3

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Additive Subtractive

R G B C M Y (K)

Mixing light and mixing pigment Mixing light and mixing pigment

green

bluered

yellow cyan

magenta

green

blue

red

yellow

cyanmagenta

CMY

RGB

= 1-[][] R+B+G=white (additive) R+G=Y C+M+Y=black (subtractive) C+M=B etc...

(CMYK common in printing, where K is black pigment)

RGB within CIE XYZ-space

HLS color spaceHue Lightness Saturation

Hue=dominant wavelength, toneLightness=intensity, brightnessSaturation=purity, dilution by white

Important aspects:•Intensity decoupledfrom color•Related to howhumans perceive color

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Lecture 3 October 30, 2003

Ingela Nyström 4

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L

S

H

red

magentablue

green yellow

cyan

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YIQ color space

Y= LightnessI= Inphase = amount red-greenQ= Quadrature = amount blue-yellow

• Optimised for transmission (TV broadcast).

• Compatible with BW monitors (use only Y component)

• Human eye is more sensitive to variations in lightnessthan variations in hue and saturation and more bandwith(bits) is used for Y.

NCS color description

NCS=Natural Color SystemA psychological more than a physiological description of color.Common among artists, designers etc.

w

b

c

2060-R50B=20% white60% blackred with 50% blue20

60

40

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Ingela Nyström 5

Color is relative

grgröönn lilalila gulgul blblåå svartsvart

Hjärnan är lättlurad

greengreen purplepurple yellowyellow blueblue blackblack

It is easy to trick the brain

Non-existing colors(without useof psychadelic drugs)

Blind spot; look at left cross with your right eye

Colour context

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Shape contextChromaticadaption

Mach bands Gamma correctionMost displays have non-linear intensity scales. The most common correction method is called gamma correction (usually implementedwith a lookup table)

Sometimes in computer graphics this effect is exaggerated to compensatefor the adaptation of the eye.

True-color framebuffer

Store R,G,B valuesdirectly in the framebuffer.

Each pixel requiresat least 3 bytes => 2^24 colors.

Indexed-color framebuffer

Store index into a color map in the frame buffer.

Each pixel requiresat least 1 bytes => 2^8 simultaneouscolors.

Enablescolor-mapanimations.

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Ingela Nyström 7

Different blending versions(how to combine color values)

Additive blending

C=A+B

e.g. combining light

Subtractive blending

C=A-(1-B)

e.g. filter effect

Averageblending

C=(A+B)/2 or C=uA+vB

e.g. for anti-aliasing

Multiplicative blending

C=A*B

e.g. combining light and matter