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
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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
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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
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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
=
=
=
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( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )
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
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Additive Subtractive
R G B C M Y (K)
Mixing light and mixing pigment Mixing light and mixing pigment
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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Different blending versions(how to combine color values)