Transcript Chapter 1
Outline
• Covered from Chapter 4 of Fundamentals of
Multimedia
– Color Science (Sections 4.1, 4.1.1 … 4.1.10)
– Covered in previous set of slides
• Covered from Chapter 3 of Multimedia
Signals and Systems
– Color Models (Sections 3.3, 3.3.1...3.3.4)
– More Color Models and Transformation of
Primaries (Section 3.3.5)
Color Representation
• Although we can differentiate a hundred
different grey-levels, we can easily
differentiate thousands of colors
Perceptual Attributes of Color
• Brightness
– Perceived luminance
• Hue
– Attribute we commonly describe as “blue”,
“red”, “yellow”, etc.
• Saturation
– Human’s impression of how different the color
is from an achromatic (white or gray) color.
Hue
• In an RGB color space, hue can be thought
of as an angle φ in standard position.
• To calculate φ, let R, G, B be the color
coordinates in RGB space, defined on a
scale from zero to one. Then, after obtaining
the brightness μ and the saturation σ, the
hue could be obtained from
Saturation
• Pastel colors are of low saturation, whereas
spectral colors are of high saturation
– From Encyclopedia Britannica Online
Saturation
• Spectral colors are of high saturation
Perceptual Representation of Color
Three-Receptor Model
• Designing a system that can individually display
thousands of colors is very difficult
• Instead, colors can be reproduced by mixing an
appropriate set of three primary colors
– It has been discovered that there are three different
types of cone cells in the human retina. When light falls
on the retina, it excites the cone cells. The excitation of
different types of cone cells determines the color seen
by the observer
– See http://colorvisiontesting.com/ for more information
on color-blindness
Three-Receptor Model
Three-Receptor Model
• The excitation of the three types of cone cells can
be calculated as:
where C() is the spectral distribution of the
incoming color light
– Note that two colors will be perceived identical if the
two different spectral distributions C1() and C2()
produce identical {R, B, G}
Color Matching
• The science of color measurement is known as
colorimetry.
• Some laws for color matching
– Any color can be matched by mixing at most three
colored lights
– The luminance of a color mixture = sum of the
luminance of its components
– Color Addition: If colors A & B match with colors C &
D, respectively, then color (A+B) matches color (C+D).
– Color Subtraction: If color (A+B) matches color (C+D),
and color A matches color C, then color B matches
color D.
red
Additive Color Mixing
500
600
700 nm
400
500
600
700 nm
yellow
green
400
When colors combine by
adding the color spectra (w.r.t.
cones excited). Example color
displays that follow this mixing
rule: CRT phosphors, multiple
projectors aimed at a screen,
Polachrome slide film.
Red and green make…
Yellow!
400
500
600
700 nm
Additive Color Mixing
Red
Ligh
t
Blue
Light
Green
L i gh t
cyan
Subtractive color mixing
500
600
700 nm
yellow
400
500
600
700 nm
green
400
When colors combine by
multiplying the color spectra
(occurs when “mixing paints”).
Examples that follow this
mixing rule: most photographic
films, paint, cascaded optical
filters, crayons.
Cyan and yellow (in crayons,
called “blue” and yellow)
make…
Green!
400
500
600
700 nm
Subtractive Color Mixing
Yellow
Paint
Blue
Paint
Absorbed by
blue pigments
Reflectance
Absorbed by
yellow pigments
400
425
450
475 500 525 550
Wavelength (in nm)
575
600
Tristimulus Value
• Recommended by the international
commission on color standards
(Commission Internationale de l’Eclairage)
aka CIE, are three monochromatic colors at
wavelength 700nm (red), 546.1 nm (green),
and 435.8 nm (blue).
• Let k be the amount of k-th primary and
k the reference white color needed to
produce a color C. Then, k / k is called
the tristimulus values of color C.
Tristimulus Value
• Note that some colors have to be added in
negative amounts to produce certain colors
– Which means that these primary sources cannot
produce all possible colors!!!
– Since no set of three primaries can produce all colors,
many color coordinate systems exist, each with their
own advantages and disadvantages.
Tristimulus Value
• Example Color Coordinate Systems
– CIE {X,Y,Z} system with hypothetical primary
sources such that all the spectral tristimulum
values are positive
• X: Supersaturated red
• Y: Green
• Z: Blue
Chromaticity Diagram
• Colors can be expressed in terms of
chromaticity coordinates
where X,Y, and Z represent the CIE system
Color Models and Transformation of
Primaries
• A single application may use two different
color representations of the same visual
signal
– E.g. NTCS Receiver System vs. NTCS
Transmission System
RGB XYZ Transformation
• MXYZRGB = (MRGBXYZ)–1
• Note: XYZ or RGB need not be orthogonal but
need to specify three unique directions in space
RGB XYZ Transformation
NTSC Receiver Primary
• NTSC: National Television Systems Committee.
• The TV Standard in North America
• The true color of an object is revealed under ideal
white light only.
–
–
–
–
Ideal white light is difficult to produce
Illuminant A: Tungsten filament lamp
Illuminant B: Midday sunlight
Illuminant C: Typical daylight
• NTSC reference white where RN=GN=BN=1
XYZ to NTCS Receiver Transformation
NTSC Transmission System
• Employs YIQ system to facilitate the use of
monochrome television channels without
increasing the bandwidth.
– Y: Luminance of the color and simultaneously
acts as the monochrome channel in a
monochrome receiver
– I and Q: Jointly represent the hue and saturation
of the color.
– The bandwidth required for I and Q is much
less than that for Y.
NTSC Receiver Primaries Trans.
Chromaticity of PAL and NTSC
Chromaticity of PAL and NTSC
• Camera output signal of the PAL system is normalized
with respect to the reference white D6500 with
chromaticity (0.31,0.33)
– Chromaticity of NTSC’s reference white is (0.31,0.32)
• The chromaticity diagram is useful for color mixing
experiments
– Colors corresponding to various points on the straight line
between two primaries represent various shades of the two
primaries.
– Colors corresponding to various points on the straight line
between C and any monochrome color X will represent the
hue X with different saturation values.
• How many colors does PAL or NTSC count for
according to the chromaticity diagram? Does that affect
TV signal perceived quality?
Example
• Determine the tristimulus and chromaticity
values of cyan (GN=BN=1, RN=0) in CIE
primary and XYZ systems.
• Solution:
CIE-UCS Color Coordinates
• One disadvantage of the previous color
system coordinates is that the Euclidean
distance between two colors may not
necessarily represent the perceptual
“distance”.
CIE-UCS Color Coordinates
• Perceptual distance experiment using two
concentric circles in RGB color space.
– a) Inner circle: r=0.2, g=0.6, b=0.2;
outer circle: r=0.2, g=0.62, b=0.2
– b) Inner circle: r=0.2, g=0.2, b=0.6;
outer circle: r=0.2, g=0.2, b=0.62.
• The values of r, g, and b are normalized where 1
corresponds to a pixel value of 255.
CIE-UCS Color Coordinates
• The CIE Uniform Chromaticity Scale
diagram was derived from the CIE
chromaticity diagram by stretching it
unevenly such that the chromaticity distance
corresponded more closely to the
perceptible difference.
• This color space is denoted by UVW and its
transformation matrix has been shown
earlier.
CMY Model
• Since printing applications employ the
subtractive color model, the RGB model is
not suitable.
• Instead, the three primaries used for color
printing are
– Cyan = White – Red
– Magenta = White – Green
– Yellow = White – Blue
CMY Model
• Assuming that 1 represents white, the CMY
model can be expressed as:
• Note that an additional color channel K has
been added to reproduce the black color.