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COLORAngel 1.4 and 2.4
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( )r( )
r
g
b
( )
( )
( )
rodsrods & cones
red-sensitive
green-sensitive
blue-sensitive
Visible spectrum
The Fovea
There are three types of cones, S, M and L
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 respons is not linear, but logarithmic.The appearance of an object’s intensity depends on the surroundings; the sensation is relativ and not absolute.
Three kinds of conesr
g
b
( )
( )
( )
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 colors a certain color sample consisted of in standard lighting. This resulted in the color matching curves, i.e. transform
r g b( ), ( ), ( ) x y z( ), ( ), ( )
CIE standard(Comission Internationale de L’Eclairage, 1931)
Color perception•Different spectra can result in identical sensations, called metamers•Color perception results from the simultaneous stimulation of 3 cone types (trichromat)•Our perception of color is affected by surrounding effects and 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( ) r( )
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
( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )
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.
Additive Subtractive
R G B C M Y (K)
Mixing light and mixing pigment
Mixing light and mixing pigment
green
blue
red
yellow
cyan
magenta
green
blue
red
yellow
cyan
magenta
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 spaceHueLightnesSaturation
Hue=dominant wavelength, toneLightness=intensity, brightnessSaturation=purity, dilution by white
Important aspects:•Intensity decoupled from color•Related to how humans perceive color
YIQ color space
Y= Lightness I= Inphase = ammount red-greenQ= Quadrature = ammount blue-yellow
• Optimised for transmission (TV broadcast).
• Compatible with BW monitors (use only Y component)
• Human eye is more sensitive to variations in lightness than 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
Color is relative
gröngrön lilalila gulgul blåblå svartsvart
hjärnan är lättlurad
Non-existing colors(without use of psychadelic drugs)
Blind spot; look at left cross with your right eye
Colour context
Shape context
Chromaticadaption
Mach bands
Gamma correctionMost displays have non-linear intensity scales. The most common correction method is called gamma correction (usually implemeted with a lookup table)
Sometimes in computer graphics this effect is exaggerated to compensate for the adaptation of the eye.
True-color frame buffer
Store R,G,B values directly in the frame-buffer.
Each pixel requires at least 3 bytes => 2^24 colors.
Indexed-color frame buffer
Store index into a color map in the frame-buffer.
Each pixel requires at least 1 bytes => 2^8 simultaneous colors.
Enables color-map animations.
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
Average blending
C=(A+B)/2 or C=uA+vB
e.g. for anti-aliasing
Multiplicative blending
C=A*B
e.g. combining light and matter