Fundamentals

Fundamentals 

What is an image?

Image can be represented using a two dimensional function f(x,y) where x and y are the spatial coordinates of each point in the image and the amplitude of f at any coordinate is known as the intensity or gray value of that point. 

What is a digital image?

If the image has discrete and finite quantities of coordinates and intensity, we call it as a digital image. Each image contains a finite number of elements each of which has a location and value. These elements are referred to as pixels or picture elements.  

What is digital image processing?

Processing a digital image by means of computer is known as digital image processing.

Basically we can identify two stems of image processing,

1.                   Processing pictorial information for the human interpretation and

2.                   Processing image data to store transmit and represent for autonomous machine perception.

Image sources?

There are various image sources available which can produce images. The well known image source is Electro Magnetic (EM) energy spectrum. Figure 1.1 depicts the EM spectrum and human can see only the visible band. In addition to the EM spectrum acoustic, ultra sonic and electronics can be considered as other sources which can generate images. Electromagnetic waves are conceptually sinusoidal and formation of different wave lengths. It can be thought of as a stream of massless particles, each travelling in a wave like pattern and moving at the speed of light. Each massless particle contains a certain amount of energy and a bundle of energy is known as photon. 

 Figure 1.1 Electro Magnetic Spectrum

Utilizing the EM Spectrum for generating Images 

Other bands of the EM spectrum can be used to generate images apart from the visible band.

Imaging in Gamma Ray Band

Gamma rays are used in nuclear medicine and astronomical observations. X-ray Tomography is one example for the Gamma Ray Imaging. In this, patient is injected radioactive isotope that emits Gamma rays as it decays. Images are produced from the emission collected by Gamma Ray detector. Positron Emmition Tomography (PET) is another example for Gamma Ray Imaging. In PET, patient is given radioactive isotope and it emits Positrons as it decays. When a positron meets electron, both are annihilated and two Gamma Rays are given off. These are detected and topographic image is created using the basic principles of tomography. 

Imaging in X-Ray Band

X-ray is used in medical diagnostics, industry and astronomy. X Rays are generated from X Ray tube which is a vacuum tube with anode and cathode. When cathode is heated, free electrons are moving towards the positively charged anode in high speed. When these free electrons strike nucleus energy is released as X Rays. The energy of X Rays is controlled by the voltage applied across the anode and the number of X Rays controlled by the current applied to the filament in the cathode. Bone structure visualization, angiogram and Computed Axial Tomography (CAT) are common examples of the X-rays in medical domain.

Imaging in Ultra Violet Band

Imaging in Ultra Violet band includes lithography, industrial inspections, microscopy, lacers, biological imaging and astronomical observations. One good example of Ultra Violet imaging is Fluorescence Microscopy.  

Imaging in the Visible and Infrared Bands

Infrared band is often use in conjunction with visible band. Basically it is used in numerous applications like low enforcement (figner print reading, reading the number plate of the vehicles), industrial verifications, remote sensing. Astronomy, light microscopy and many more. Remote sensing, weather observation and predication are utilizing some energy bands in both visible and infrared regions.  

Imaging in the Microwave Band

The common utilization of Microwave is radar. Radar can form images virtually any region at any time. It does not care about the lighting conditions and the weather conditions. 

Imaging in the Radio Band

Radio signals are used in medicine for diagnosis and astronomy. Magnetic Resonance Imaging (MRI) is one good example for the Radio band imaging. In this technique, patient is places in a powerful magnet and passes radio waves through his body in short pulses. Each pulse causes a responding pulse of radio waves to be emitted by the patients’ tissue. The location from which these signals originate and their strength are determined by a computer, which produces a two dimensional picture of a section of the patient. MRI can produce pictures in any plane. 

Other Image formation Sources  

Acoustic imaging, electron microscopy, computer generated images (Synthetic images) can be considered as other image formation sources. Acoustic imaging is used in geological explorations (Mineral and Oil exploration), medicine and industry. Ultrasound rays are inherited from sound source and it is used in medical diagnosis. Electron Microscope is one of the examples for utilizing electrons for generating images. 

Main Steps in Digital Image Processing 

Some image processing application we do input images and we get the results as images but in some applications we get some attributes as the output. Various steps are available to apply during the image processing and depending on the complexity of the application we choose those steps to achieve the expected results. Following section explains some of the steps used in the image processing applications.  

Image Acquisition

This will give some hints regarding the origin of digital image. Preprocessing is also one part of the image acquisition. 

Image Enhancement  

Image enhancement means bring out details that is obscured, or simply to highlight certain features of interest in an image. Image enhancement is subjective.  

Image Restoration

It is an area which deals with improving the appearance of an image. 

Color Image Processing

It discuss about the various color models and basic color processing in digital domain. 

Wavelet and Multi Resolution Processing

Wavelets are the foundation for representing images in various degrees of resolution.

Compression

It deals with techniques for reducing the storage required to save an image or the bandwidth required to transmit it.

Morphological Processing

It deals with tools for extracting image components that are useful in the representation and description of shape. 

Segmentation

Partitioning image into its constituent parts or objects is known as image segmentation. 

Representation of the segment follows output of the segmentation stage, which usually is raw pixel data constituting either boundary or region. Description also called feature selection, deals with extracting attributes that results in some quantitative information of interest or are basic for differentiating one class of objects from another. 

Recognition

It is the process that assigns a label to an object based on its descriptors. 

Components of an Image Processing System  

Figure 1.2 depicts the components of general purpose image processing system. With reference to sensing, two devices are required to acquire digital images. The first is a physical device that is sensitive to the energy radiated by the object we wish to image. The second, called the digitizer, is the device for converting the output of the physical sensing device into digital form. For instance, in a digital video camera the sensors produce electrical output proportional to the light intensity and the digitizer converts these outputs to digital data. Digitizer and Arithmetic and Logical Unit (ALU) can be considered as specialized image processing hardware. In the image processing applications, the computers can be scaling from general purpose computer to super computer depending on the nature of the application.

     

Figure 1.2 – Components of the general purpose image processing system

Software for image processing applications indicates the specific software modules that help to the programmer to write efficient programs easily. Storage is required to store the image temporarily, online storage for fast recall and archival storage for rarely access.  Displays are the units which visualize the output of the image processing application. It may be color monitor or graphic card. Hardcopy devices are important to recording images includes laser printers, film cameras, heat sensitive devices, inkjet units or digital units such as optical disk or CD-ROM. Network provides the media to interchange the image processing outputs. 

Reference

Gonzalez, R.C., Woods, R.E., 1992. Digital Image Processing, 3rd ed. Addison-Wesley Pub (Sd).