A color model in Image Processing is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted (viewing conditions, etc.), the resulting set of colors is called Color Space.

Color images/video are captured and displayed in the RGB format. However, they are often converted to an intermediate representation for efficient compression and processing.

RGB Color Model

RGB color model is an additive color model in which red, green and blue colors are mixed together in various proportions to form a different array of colors. White color having all colors in it and black without the presence of any color. RGB color model is used in various digital displays like TV, Compute, digital cameras, and other light-based display devices.

Color Wheel showing RGB Color Model
Color Wheel showing RGB Color Model

CMYK Color model

The CMYK color model is a subtractive color model, based on the CMY color model, used in color printing. CMYK refers to the four ink plates used in some color printing: cyan, magenta, yellow, and key (black). CMY color model is based on the fact that initially we have a white sheet of paper, and it reflects (virtually) the entire spectrum of RGB, and all inks applied to it, act as filters, each of which “steals” own color (red, or green, or blue). Thus, these color inks are determined by subtracting one from the white colors RGB.

Luminance-Chrominance Color Model

The luminance-chrominance color model was used to develop an analog color TV transmission system that is backwards compatible with the legacy analog black and white TV systems. The luminance component, denoted by Y, corresponds to the gray-level representation of video. Chrominance components are denoted by U and V for analog video or Cr and Cb for digital video. Chrominance components U and V represent the deviation of color from the gray level on blue–yellow and red–cyan axes respectively.

Most of the parameters defined for the digital YCbCr color space remains the same for the YPbPr color space. YCbCr and YPbPr color spaces are closely related. Individual color components of YCbCr color space are luma Y, chroma Cb and chroma Cr.

Human visual system is less sensitive to variations (higher frequencies) in chrominance components. This has resulted in the subsampled chrominance formats, such as 4:2:2 and 4:2:0. In the 4:2.2 format, the chrominance components are subsampled only in the horizontal direction. While in 4:2:0 format, chrominance component are subsampled in both directions. The luminance-chrominance representation offers higher compression efficiency, compared to the RGB representation due to this subsampling.

ITU-R BT.709 defines the conversion between RGB and YCrCb representations as:

RGB to YCrCb conversion
RGB to YCrCb conversion

Above relation states that the human visual system perceives the contribution of R-G-B to image intensity approximately with a 3-6-1 ratio, i.e., red is weighted by 0.3, green by 0.6 and blue by 0.1. Y-Cr-Cb is not a color space. It is a way of encoding the RGB information, and actual colors displayed depends on the specific RGB space used.

Hue-Saturation-Intensity Color Model

Color features that best correlate with human perception of color are hue, saturation, and intensity. Hue relates to the dominant wavelength, saturation relates to the spread of power about this wavelength (purity of the color), and intensity relates to the perceived luminance (similar to the Y channel). amily of color spaces that specify colors in terms of hue, saturation, and intensity, known as HSI spaces. The HSI space specifies color in cylindrical coordinates.