Difference Between RGB, CMYK, HSV, and YIQ Color Models

The colour spaces in image processing aim to facilitate the specifications of colours in some standard way.

Different types of colour models are used in multiple fields like in hardware, in multiple applications of creating animation, etc.  

Let’s see each colour model and its application.

• RGB
• CMYK
• HSV 
• YIQ 

RGB: The RGB colour model is the most common colour model used in Digital image processing and openCV. The colour image consists of 3 channels. One channel each for one colour. Red, Green and Blue are the main colour components of this model. All other colours are produced by the proportional ratio of these three colours only. 0 represents the black and as the value increases the colour intensity increases.

Properties:

• This is an additive colour model. The colours are added to the black.
• 3 main channels: Red, Green and Blue.
• Used in DIP, openCV and online logos.

Colour combination: 
Green(255) + Red(255) = Yellow 
Green(255) + Blue(255) = Cyan
Red(255) + Blue(255) = Magenta
Red(255) + Green(255) + Blue(255) = White

CMYK: CMYK colour model is widely used in printers. It stands for Cyan, Magenta, Yellow and Black (key). It is a subtractive colour model. 0 represents the primary colour and 1 represents the lightest colour. In this model, point (1, 1, 1) represents black, and (0,0,0) represents white. It is a subtractive model thus the value is subtracted from 1 to vary from least intense to a most intense colour value.

1-RGB = CMY
Cyan is negative of Red.
Magenta is negative of Green.
Yellow is negative of Blue.

HSV: The image consists of three channels. Hue, Saturation and Value are three channels. This colour model does not use primary colours directly. It uses colour in the way humans perceive them. HSV colour when is represented by a cone. 

Hue is a colour component. Since the cone represents the HSV model, the hue represents different colours in different angle ranges. 

Red colour falls between 0 and 60 degrees in the HSV cone.
Yellow colour falls between 61 and 120 degrees in the HSV cone.
Green colour falls between 121 and 180 degrees in the HSV cone.
Cyan colour falls between 181 and 240 degrees in the HSV cone.
Blue colour falls between 241 and 300 degrees in the HSV cone.
Magenta colour falls between 301 and 360 degrees in the HSV cone.

Saturation as the name suggest describes the percentage of the colour. Sometimes this value lies in the 0 to 1 range. 0 being the grey and 1 being the primary colour. Saturation describes the grey colour.

The value represents the intensity of the colour chosen. Its value lies in percentage from 0 to 100. 0 is black and 1 is the brightest and reveals the colour. 

HSV model is used in histogram equalization and converting grayscale images to RGB colour images.

YIQ: YIQ is the most widely colour model used in Television broadcasting. Y stands for luminance part and IQ stands for chrominance part. In the black and white television, only the luminance part (Y) was broadcast. The y value is similar to the grayscale part. The colour information is represented by the IQ part.

There exist a formula to convert RGB into YIQ and vice-versa.

YIQ model is used in the conversion of grayscale images to RGB colour images.