Color Coordinates

* Two Attributes
which describe the color sensation of a human being: luminance and chrominance. Luminance refers to the perceived brightness of the light and it is proportional to the total energy in the visible band. Chrominance describes the perceived color tone of a light, which depends on the wavelength composition of the light.

* Trichromatic Theory of Color Mixture
Most colors can be produced by mixing three properly chosen primary colors.
- For illuminating light source, they are R, G and B
- For reflecting light source, they are C, M and Y (plus K for printing application)
- All present display systems use an RGB primary while printers are using CMYK primary.

* Color Specification by Luminance And Chrominance
In many applications, it is desirable to describe a color in terms of its luminance and chrominance content separately, to enable more efficient processing and tramission of color signals.

* Composite versus Component Video
A video in the format of RGB or YUV is called component video. Three signals could be multiplexed into a single signal to construct a composite video, which relies on the property that the chrominance signals have a significantly smaller bandwidth than the luminance component. Thus, it could be transmitted or stored more efficiently at a cost of image quality. As a compromise between data rate and image quality, S-video consists of two components: the luminance and a single chrominance component that is the multiplex of two original chrominance signals.

* Gamma Correction
In reality, the output signals of RGB from most cameras are not linearly related to the actual color values. Similarly, most of the display devices also suffer from such a nonlinear relation between input and displayed color intensity. In order to present true colors, one must do some compensation on the camera output and before sending real image values for display, compensation must be done for the gamma effect of display devices. These processes are known as gamma correction. In practice, one gamma correction is done in the side of sender for RGB signals and then they are converted to YUV or YIQ for transmission. Receivers need only to convert them back to RGB for display.

* YUV, YIQ, YPbPr and YCbCr
- As said before, for video capture and display, all the analog and digital systems are using RGB primary. However, the color coordinate systems used in transmission of analog TV systems including NTSC, PAL and SCEAM and digital system including BT601 and BT656 are different. In general, a luminance/chrominance coordinate is employed.
- YUV coordinate is the basic for all these coordinates. Y is the luminance component while U and V are proportional to color difference, B-Y and R-Y, respectively, scaled to have the desired range. YUV is used in PAL and SCEAM TV system.
- YIQ is used in NTSC, where I and Q components are the rotated(by 33 degree) version of the U and V components.
- YCbCr is the digital color coordinate introduced in BT601. The Y, Cb and Cr are scaled and shifted version of the Y, U and V so that the value range is in [0-255].
- YPbPr is the analog color coordinate used in BT709 for HDTV video interconnect signal. The offsets and scaling factors are specified to prevent saturation of the amplifiers of the analog signals.

* Common Video Input Formats
YUY2 4:2:2, interleaved Y,Cb,Y,Cr
YV12 4:2:0, Y,Cb and Cr are stored separately and in sequence