Digital Images are the most common and convenient means of conveying or transmitting information. An image is worth a thousand words. Pictures concisely convey information about positions, sizes and inter-relationships between objects. They portray spatial information that we can recognize as objects. Human beings are good at deriving information from such images, because of our innate visual and mental abilities. About 75% of the information received by human is in pictorial form.
Digital Image Overview
Digital images are made of picture elements called pixels. Typically, pixels are organized in an ordered rectangular array. The size of an image is determined by the dimensions of this pixel array. The image width is the number of columns, and the image height is the number of rows in the array. Thus the pixel array is a matrix of M columns x N rows. To refer to a specific pixel within the image matrix, we define its coordinate at x and y. The coordinate system of image matrices defines x as increasing from left to right and y as increasing from top to bottom. Compared to normal mathematic convention, the origin is in the top left corner and the y coordinate is flipped. Why is the coordinate system flipped vertically? Originally, digital images were defined in terms of the electron beam scanning pattern of televisions. The beam scanned from left to right and top to bottom. Other than this historical reason, there is no purpose served by this inversion of the y coordinate.
Definition of Digital image
A digital image is an electronic file that forms into square picture elements (pixels) when displayed on a viewing device (e.g., a computer monitor). The displayed image is a two-dimensional matrix of thousands or millions of pixels each of which has its own address, size, and colour representation. You might think of pixels as serving a role similar to the grains in a photograph.3 digitizing a photograph means converting or capturing its image electronically through a scanner or digital camera. Digital image processing software allows you to magnify an image to see the pixels, and to sometimes measure the numeric color values for each pixel – like a sophisticated, computer generated, and paint-by-number matrix
Significance of Digital Image
Digital imaging has become more than just a popular pastime in contemporary culture. Personal computers, in addition to a plethora of hand-held electronic devices, have become the preferred mode of communication for increasingly large portions of the population. Digital cameras and camera phones have made taking, processing, and sharing photos almost instantaneous, making digital images a common component of hi-tech communications. With all this fast-paced instant gratification going around, one might not notice the extent to which professional industries also rely on digital imaging. Publishers of both print and web materials depend heavily on the quality and accessibility of their images. Attention to the details of how an image is created, how it is stored, who will be using or accessing it, and how, is crucial to successful digital imaging.
Digital images are composed of pixels (short for picture elements). Each pixel represents the color (or gray level for black and white photos) at a single point in the image, so a pixel is like a tiny dot of a particular color. By measuring the color of an image at a large number of points, we can create a digital approximation of the image from which a copy of the original can be reconstructed. Pixels are a little like grain particles in a conventional photographic image, but arranged in a regular pattern of rows and columns and store information somewhat differently. A digital image is a rectangular array of pixels sometimes called a bitmap.
Category of Digital Images
There are four category of digital images are as following.
- Binary: Binary images are useful for fingerprints, architectural plan, and text (printed or handwriting. This images use only two color black and white. It require only 1 bit per pixel for storage. This images also known as two-level or bi-level and store as bitmap in memory.
- Gray-Scale: Gray-Scale images suitable for recognition of natural objects, printed work images, Diagnostic photography using X-ray. In this type of images pixels range is between black and white. In 8 bit representation 256 states are available. These images are different from black-and-white images which have just two color black and white. Gray-Scale images have shades of gray between black and white colors.
- True colour or RGB: An RGB colour image is M*N*3 array of colour pixels where each colour pixel is equivalent to third component of RGB image. The three components of RGB image is referred as red, green, blue module images. The data class of RGB image determines the range of values and this image is of class double range between [0, 1]. The range values of this image are [0,255] or [0, 65535] for RGB images of class unit16 respectively. The pixel value of the component image verifies the bit depth of the RGB image for instance, if each component image is 8-bit image then analogous RGB image is said to be 24 bit deep. In this case, the number of possible colours in an image is (2^b) ^3, where b is the number of bits in each component image for 8-bit case, the number is 16,777,216 colour.
Bitmap File Formats
There are many bitmap file formats presently available for storing images in computers. Some of these are as follows .
- GIF(Graphics Interchange Format): This file format stores 1 bit to 8 bit images. It is the most commonly used file format on the internet. It was developed by CompuServe in 1987.
- TIFF ( Tagged Image File Format): This file format is quite flexible for storing 1 bit to 24 bit images. It was developed by Adobe System in 1993. It is flexible and adaptable file format.
- SGI ( Silicon Graphics Images): This file format stores 24 bit RGB color images. It was developed by Paul Haeberli. It is produced by many program that run on SGI workstations.
- Sun Raster: This file format was developed by Sun Microsystems. It is native bitmap format of the Sun UNIX platform. It stores black- and–white, Color Bitmapped and Gray-Scale Images.
- BMP: This File format store 2D digital images of arbitrary height, resolution and width. It stores both monochrome and color images. This file format also known as, Device Independent File Format (DIFF) or Bitmap File.
- JPEG (Joint Photographic Expert Group): This is the most commonly used file format for digital camera and other capturing device. It is used for storing and transmitting images on the internet. It is able to store up to 24 bits of color images.
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 Kit A. Peterson, “Introduction to Basic Measures of a Digital Image for Pictorial Collections”, Digital Conversion Specialist, June 2005