YIQ
From Free net encyclopedia
Template:Expert Image:CIExy1931.png YIQ is a color space, formerly used in the NTSC television standard. I stands for in-phase, while Q stands for quadrature, referring to the modulation schemes used in broadcast. NTSC now uses the YUV color space, which is also used by other systems such as PAL.
The Y component represents the luminance information, and is the only component used by black-and-white television receivers. I and Q represent the chrominance information. In YUV, the U and V components can be thought of as X and Y coordinates within the colorspace. I and Q can be thought of as a second pair of axes on the same graph, rotated 33°; therefore IQ and UV represent different coordinate systems on the same plane.
The YIQ system is intended to take advantage of human color-response characteristics. The eye is more sensitive to changes in the orange-blue (I) range than in the purple-green range (Q)--therefore less bandwidth is required for Q than for I. Broadcast NTSC limits I to 1.3 MHz and Q to 0.4 MHz. I and Q are frequency interleaved into the 4 MHz Y signal, which keeps the bandwidth of the overall signal down to 4.2 MHz. In YUV systems, since U and V both contain information in the orange-blue range, both components must be given the same amount of bandwidth as I to achieve similar color fidelity.
Very few television sets perform true I and Q decoding, due to the high costs of such an implementation.
Formulas
This formula approximates the conversion from the RGB color space to YIQ. R, G and B are defined on a scale from zero to one:
| <math>Y</math> | <math>= 0.299R + 0.587G + 0.114B</math> |
| <math>I</math> | <math>= 0.735514(R - Y) - 0.267962(B - Y)</math> |
| <math>= 0.595716R - 0.274453G - 0.321263B</math> | |
| <math>Q</math> | <math>= 0.477648(R - Y) + 0.412626(B - Y)</math> |
| <math>= 0.211456R - 0.522591G + 0.311135B</math> |
or using matrices
<math> \begin{bmatrix} Y \\ I \\ Q \end{bmatrix} = \begin{bmatrix} 0.299 & 0.587 & 0.114 \\ 0.595716 & -0.274453 & -0.321263 \\ 0.211456 & -0.522591 & 0.311135 \end{bmatrix} \begin{bmatrix} R \\ G \\ B \end{bmatrix} </math>
Two things to note:
- The top row is identical to that of the YUV color space
- If <math>\begin{bmatrix} R & G & B \end{bmatrix}^{T} = \begin{bmatrix} 1 & 1 & 1 \end{bmatrix}</math> then <math>\begin{bmatrix} Y & I & Q \end{bmatrix}^{T} = \begin{bmatrix} 1 & 0 & 0 \end{bmatrix}</math>. In other words, the top row coefficients sum to unity and the last two rows sum to zero.
References
- Buchsbaum, Walter H. Color TV Servicing, third edition. Englewood Cliffs, NJ: Prentice Hall, 1975. ISBN 0-13-152397-X
See also
- Color space:
- RGB color model commonly used for color monitors
- CMYK color model for color printing
- HSV color space
- HLS color space
- RYB color model the traditional color model used by artists.
- YUV for PAL televisionde:YIQ-Farbmodell