Subject: Multimedia System

Topic: Color in Images & Video

Difficulty: Hard

RGB Color Model

Media that transmit light (such as television) use additive color mixing with primary colors of red, green, and blue, each of which stimulates one of the three types of the eye's color receptors with as little stimulation as possible of the other two. This is called "RGB" color space. Mixtures of light of these primary colors cover a large part of the human color space and thus produce a large part of human color experiences.

CMY Color Model

It is possible to achieve a large range of colors seen by humans by combining cyan, magenta, and yellow transparent dyes/inks on a white substrate. These are the subtractive primary colors. Often a fourth ink, black, is added to improve reproduction of some dark colors. This is called "CMY" or "CMYK" color space.

The cyan ink absorbs red light but transmits green and blue, the magenta ink absorbs green light but transmits red and blue, and the yellow ink absorbs blue light but transmits red and green

Used in electrostatic/ink-jet plotters that deposit pigment on paper

HSV and HSL Model

HSV models itself on paint mixture, with its saturation and value dimensions resembling mixtures of a brightly colored paint with, respectively, white and black.

HSL tries to resemble more perceptual color models such as NCS or Munsell. It places the fully saturated colors in a circle of lightness ½, so that lightness 1 always implies white, and lightness 0 always implies black.

HSV and HSL are both widely used in computer graphics, particularly as color pickers in image editing software.

YUV Color Model

YIQ and YUV are the two commonly used color models in video

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 differences.

U = B - Y

V = R – Y

If b/w image, then U = V = 0. --> No chrominance!

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 clockwise).

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