Post on 19-Mar-2018
Multimedia SystemsMultimedia Systems
Color SpaceColor Space
Course PresentationCourse Presentation
Mahdi Amiri
March 2014
Sharif University of Technology
Physics of ColorLightLight
Light or visible light is the portion of electromagnetic radiation that
is visible to the human eye.
The wavelength λ of a sinusoidal waveform
traveling at constant speed ν is given by
In the case of electromagnetic radiation, ν is
the speed of light, about 3×108 m/s.
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Color is Human SensationHuman EyeHuman Eye
Through cone and rod receptors in the retina
Three kinds of cones: Long (L, Red), Medium (M, Green), Short (S, BlueS, Blue)
Rod receptor is mostly for luminance perception(useful for night vision)
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Human EyeFoveaFovea
The fovea is responsible for sharp central vision
The human fovea has a diameter of about 1.0 mm with a high
concentration of cone photoreceptors.
Cone cells are densely packed in the fovea, but gradually
become sparser towards the periphery of the retina.
A rod cell is sensitive enough to respond to a single
photon of light and is about 100 times more sensitive to a
single photon than cones.
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Spatial DistributionSpatial Distribution
Multimedia Systems, Mahdi Amiri, Color Space
Human EyePhotoreceptor cellPhotoreceptor cell
Normalized typical human cone (and rod) absorbances
(not responses) to different wavelengths of light
IdeaIdea
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IdeaIdea
We can have different We can have different
colors by mixing colors by mixing
primary color primary color
componentscomponents
of “light” or “pigment”of “light” or “pigment”
Multimedia Systems, Mahdi Amiri, Color Space
Additive Color MixingMixing of LightMixing of Light
Usually red, green and blue
RGB color model
Application
LCD, LED, plasma and CRT
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LCD, LED, plasma and CRT
(picture tube) color video displays
Examine TV display with a Examine TV display with a
sufficiently strong magnifiersufficiently strong magnifier
Multimedia Systems, Mahdi Amiri, Color Space
RGBLCD TV CloseLCD TV Close--upup
RGB subRGB sub--pixels in an LCD TVpixels in an LCD TV
(on the right: an orange(on the right: an orange
and a blue color; and a blue color;
on the left: a closeon the left: a close--up)up)
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Subtractive Color MixingMixing of PigmentMixing of Pigment
Primary: Cyan, Magenta, Yellow
CMYK color model
Application
Printers
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Printers
Pigments Pigments
absorb lightabsorb light
Multimedia Systems, Mahdi Amiri, Color Space
CMYKWhy Including the Black?Why Including the Black?
Separated with Separated with
maximum black, maximum black,
to minimize ink to minimize ink
use.use.
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Separated for printing with Separated for printing with
process cyan, magenta, and process cyan, magenta, and
yellow inks.yellow inks.
A color photograph of A color photograph of
the Teton Range.the Teton Range.
Multimedia Systems, Mahdi Amiri, Color Space
CMYKCloseClose--up of a Printup of a Print
This closeThis close--up of printed halftone rasters up of printed halftone rasters
show that magenta on top of yellow show that magenta on top of yellow
appears as orange/red, and cyan on top appears as orange/red, and cyan on top
of yellow appears as greenof yellow appears as green
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of yellow appears as greenof yellow appears as green
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ModelsCIE 1931 XYZ color spaceCIE 1931 XYZ color space
One of the first mathematically defined color
spaces
Created by the International Commission on
Illumination (CIE)
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Illumination (CIE)
The CIE XYZ color space was derived from a
series of experiments done in the late 1920s by W.
David Wright and John Guild.
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ModelsCIE ExperimentsCIE Experiments
The observer would alter the brightness The observer would alter the brightness
of each of the three primary beams until of each of the three primary beams until
a match to the test color was observed.a match to the test color was observed.
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The Y primary is intentionally defined to have a The Y primary is intentionally defined to have a
colorcolor--matching function that exactly matches the matching function that exactly matches the
luminousluminous--efficiency function of the human eye.efficiency function of the human eye.
CIE XYZ primariesCIE XYZ primaries
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ModelsCIE 1931 XYZ color spaceCIE 1931 XYZ color space
The Result of the Experiments: The cone of visible colors
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The X + Y + Z = 1 plane is shown as a triangleThe X + Y + Z = 1 plane is shown as a triangle
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ModelsCIE xyY Chromaticity DiagramCIE xyY Chromaticity Diagram
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Y in Y in xyYxyY: measure of the brightness or : measure of the brightness or
luminance of a color.luminance of a color.
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ModelsColor GamutColor Gamut
The range of color representation of a display device
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A typical CRT gamutA typical CRT gamut
Multimedia Systems, Mahdi Amiri, Color Space
Color SpaceHSVHSV
Hue: Quantity that distinguishes color family,
say red from yellow, green from blue.
Saturation (Chroma): Color intensity (strong to
weak). Intensity of distinctive hue, or degree of
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weak). Intensity of distinctive hue, or degree of
color sensation from that of white or grey.
Value (Luminance): Light color or dark color.
Multimedia Systems, Mahdi Amiri, Color Space
Color SpaceHSVHSV
Photoshop uses this model to get more control over color
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Color SpaceHSV vs. RGBHSV vs. RGB
Example color control
From a relatively colorful orange ���� a less saturated orange
RGB: we would need to readjust all three R, G and B sliders
HSV: we can just readjust the ‘Saturation’ related slider
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HSV: we can just readjust the ‘Saturation’ related slider
Sample Application of HSV color space: Object detection based on color filtering.
Multimedia Systems, Mahdi Amiri, Color Space
Color SpaceYUVYUV
Y: Luminance
U and V: Chrominance
Taking human perception into account, Taking human perception into account,
allowing reduced bandwidth for allowing reduced bandwidth for
YUV was invented when engineers
wanted color television in a black-
and-white infrastructure.
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Example of UExample of U--V color planeV color plane
allowing reduced bandwidth for allowing reduced bandwidth for
chrominance componentschrominance components
(Since the human eye is less sensitive
to chrominance than luminance)
Multimedia Systems, Mahdi Amiri, Color Space
No need for color space conversion No need for color space conversion
when our image processing technique when our image processing technique
only uses luminance component.only uses luminance component.
YUV's corresponding color space in
ITU standards is YCbCr
U corresponds to the Cb = B – Y
V corresponds to the Cr = R – Y
Color SpaceSubsampling in YUV or YCbCrSubsampling in YUV or YCbCr
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Color SpaceDifference Between Four SchemesDifference Between Four Schemes
This image shows the difference between four This image shows the difference between four subsamplingsubsampling
schemes. Note how similar the color images appear. The schemes. Note how similar the color images appear. The
lower row shows the resolution of the color information.lower row shows the resolution of the color information.
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Color Space ConversionRGB and YUVRGB and YUV
RGB to YUVRGB to YUV
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YUV to RGBYUV to RGB
Multimedia Systems, Mahdi Amiri, Color Space
Color Space ConversionRGB and CMYKRGB and CMYK
CMY to CMYKCMY to CMYKCMY to RGBCMY to RGB
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CMYK to CMYCMYK to CMYRGB to CMYRGB to CMY
Multimedia Systems, Mahdi Amiri, Color Space
Color SpaceGamma CorrectionGamma Correction
Ou
tpu
t li
gh
t
(bri
gh
tnes
s)
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Input signal level
Ou
tpu
t li
gh
t
(bri
gh
tnes
s)
Low gamma (e.g. 1.5)
Low
gamma
Looks “milky”, flat
and lacks punch
With visible details in
the black areas and
Source: Piers Clerk, ISF Calibrator,
www.homecinemaengineering.comGamma control in HDTVs
(May be found in advanced
picture settings menu)
Page 29Multimedia Systems, Mahdi Amiri, Color Space
Normal gamma (e.g. 2.2)
High gamma (e.g. 3.0)
High
gammaHas loads of punch;
but no detail in the
black areas
the black areas and
white areas.
Color SpaceGamma CorrectionGamma Correction
Difference between a scale with linearly-increasing encoded luminance signal (linear input)
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Difference between a scale with linearly-increasing encoded luminance signal (linear input)
and a scale with linearly-increasing intensity (i.e., gamma-corrected) scale (linear output).
Multimedia Systems, Mahdi Amiri, Color Space
Visual perceptionVisit Mind Lab WebsiteVisit Mind Lab Website
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http://jvsc.jst.go.jp/find/mindlab/english/index.htmlhttp://jvsc.jst.go.jp/find/mindlab/english/index.html
How is our consciousness connected to the world?How is our consciousness connected to the world?
Explore the unconscious functions of the brain with visual Explore the unconscious functions of the brain with visual
illusions and mysterious perceptual phenomenaillusions and mysterious perceptual phenomena
Multimedia Systems, Mahdi Amiri, Color Space