Color in Image and Video
Basics of Color
Light and Spectra
Visible light is an electromagnetic wave in the 400 nm - 700 nm range.
Most light we see is not one wavelength, it's a combination of many
The profile above is called a spectral power distribution or spectrum.
The Human Retina
Cones and Perception
The following figure shows the amounts of three primaries needed to match
all the wavelengths of the visible spectrum.
The negative value indicates that some colors cannot be exactly produced
by adding up the primaries.
CIE Chromaticity Diagram
Q: Does a set of primaries exist that span the space with only positive
A: Yes, but no pure colors.
In 1931, the CIE (Commission Internationale de L'Eclairage, or International
Commission on Illumination) defined three standard primaries
Z). The Y primary was intentionally chosen to be identical to
the luminous-efficiency function of human eyes.
The above figure shows the amounts of X, Y, Z needed to exactly reproduce
any visible color.
All visible colors are in a "horseshoe" shaped cone in the X-Y-Z space.
Consider the plane X+Y+Z=1 and project it onto the X-Y plane, we
get the CIE chromaticity diagram as below.
The edges represent the "pure" colors (sine waves at the appropriate frequency)
White (a blackbody radiating at 6447 kelvin) is at the "dot"
When added, any two colors (points on the CIE diagram) produce a point
on the line between them.
Q: how can we find a color's complement on the CIE diagram?
L*a*b (Lab) Color Model
A refined CIE model, named CIE L*a*b in 1976
Chrominance: a -- ranges from green to red, b -- ranges from blue to
Used by Photoshop
Color Models in Images
A color image is a 2-D array of (R,G,B) integer triplets. These triplets
encode how much the corresponding phosphor should be excited in devices
such as a monitor.
RGB Color Model for CRT Displays
CRT displays have three phosphors (RGB) which produce a
wavelengths when excited with electrons.
CMY Color Model
Cyan, Magenta, and Yellow (CMY) are complementary colors of RGB. They can
be used as Subtractive Primaries.
CMY model is mostly used in printing devices where the color pigments on
the paper absorb certain colors (e.g., no red light reflected from cyan
The RGB and CMY Cubes
Conversion between RGB and CMY:
-- e.g., convert White from (1, 1, 1) in RGB to (0, 0, 0) in CMY.
Sometimes, an alternative CMYK model (K stands for Black) is used
in color printing (e.g., to produce darker black than simply mixing CMY).
K := min (C, M, Y), C := C - K, M := M - K, Y := Y - K.
Comparison of Three Color Gamuts
The gamut of colors is all colors that can be reproduced using the
The Lab gamut covers all colors in visible spectrum
The RGB gamut is smaller, hence certain visible colors (e.g. pure yellow,
pure cyan) cannot be seen on monitors
The CMYK gamut is the smallest (but not a straight subset of the RGB gamut)
Color Models in Video
YIQ and YUV are the two commonly used color models in video
YUV Color Model
Initially, for PAL analog video, it is now also used in CCIR 601 standard
for digital video
Y (luminance) is the CIE Y primary.
Y = 0.299R + 0.587G + 0.114B
Chrominance is defined as the difference between a color and a reference
white at the same luminance. It can be represented by U and V -- the color
U = B - Y
V = R - Y
Sample YUV Decomposition:
Eye is most sensitive to Y. In PAL, 5 (or 5.5) MHz is allocated to Y, 1.3
MHz to U and V.
YCbCr Color Model
YIQ Color Model
YIQ is used in NTSC color TV broadcasting, it is downward compatible with
B/W TV where only Y is used.
Although U and V nicely define the color differences, they do not align
with the desired human perceptual color sensitivities. In NTSC, I and Q
are used instead.
I is the orange-blue axis, Q is the purple-green axis.
I and Q axes are scaled and rotated R - Y and B - Y (by 33 degrees
I = 0.877(R - Y) cos 33 - 0.492(B - Y) sin 33
Q = 0.877(R - Y) sin 33 + 0.492(B - Y) cos 33
I = 0.736(R - Y) - 0.268(B - Y) = 0.596R - 0.275G - 0.321B
Q = 0.478(R - Y) + 0.413(B - Y) = 0.212R - 0.523G + 0.311B
The YIQ transform:
Eye is most sensitive to Y, next to I, next to Q.
In NTSC broadcast TV, 4.2 MHz is allocated to Y, 1.5 MHz to I and 0.55
MHz to Q. For VCR, Y is cut down to 3.2 MHz and I to 0.63 MHz.
Color images are encoded as triplets of values.
RGB is an additive color model that is used for light-emitting devices,
e.g., CRT displays
CMY is a subtractive model that is used often for printers
Two common color models in imaging are RGB and CMY, two common color models
in video are YUV and YIQ.
YUV uses properties of the human eye to prioritize information. Y is the
black and white (luminance) image, U and V are the color difference (chrominance)
images. YIQ uses similar idea.
Besides the hardware-oriented color models (i.e., RGB, CMY, YUV, YIQ),
HSB (Hue, Saturation, and Brightness) and HLS (Hue, Lightness, and Saturation)
are also commonly used.
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