Mumbai University > Electronics > Sem 7 > Digital image processing

Marks: 10M

Year: May 2012

Image types

Images can be classified as follows:

(1) Monochrome image (Binary images).

(2) Grey scale images.

(3) Colour (24-bit) images.

(4) Half-toned images.

(1) Monochrome Image: In this, each pixel is stored as a single bit (0 or 1.) Here, 0 represents black while 1 represents while. It is a black and white image in the strictest sense. These images are also called bit mapped images, we have only black and white pixels and no other shades of grey.

(2) Grey scale Image: Here each pixel is usually stored as a byte (8-bits). Due to this, each pixel can have values ranging from 0(black) to 255(white). Grey scale images, as the name suggests have black, white and various shades of grey present in the image.

(3) Colour Image (24-bit): Color images are based on the fact that a variety of colors can be generated by mixing the three primary colors viz, Red, Green and Blue, in proper proportions. In color images, each pixel is composed of RGB values each of these colors require 8-bits (one byte) for it’s represented by 24-bits [R (8-bits,), G (8-bits), B (8-bits)]. A 24-bit color image supports 16, 777,216 different combination of colors. Color images can be easily converted to grey scale images using the equation

X = 0.30R + 0.59 G +0.11 B

An easier formula that could achieve similar results is

$$X = \frac{R +G+B}{3}$$

(4) Half Toning: It is obvious that a grey scale image definitely looks than the monochrome image as it utilizes more grey levels. But there is a problem in hand. Most of the printers, that we use (inkjet, lasers, dot matrix) are all bi-level devices. i.e., they have only a black cartridge and can only produce two levels (black on a white back-ground). In fact, most of the printing jobs are done using bi-level devices.