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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
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.
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
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
Lecture 3 October 30, 2003
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
Lecture 3 October 30, 2003
Ingela Nyström 6
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.
Lecture 3 October 30, 2003
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