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Implementation of edge detection for a digital image

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par Innocent MBARUBUKEYE
KIST - AO Electronics and telecommunication engineering 2008
  

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3.1.6. TO WRITE MATLAB PROGRAMS

MATLAB programs can be written straight to command line (1), or in m-files. M-files are created by (2), or writing edit functionName on command line (1). Created files are stored as filename to current directory, which you can change in (3). You can, for example in case of a shared computer in laboratory exercises, create a temp directory under work for keeping your files organized.

FUNCTIONS AND SCRIPTS.

Script is simple M-file that executes all lines as they were written in the command line. It has no «headers» or anything, just plain program lines. Scripts see all variables created in Command line (i.e. workspace) and vice versa.

Functions are «advanced scripts», that take can take parameters and can produce outputs. Examples of both a script and a function are given bellow:

To use a script write scriptName on command line.

To use a function, write [output1,outpu2,...] = functionName(param1, param2) on command line.

In case of single output, drop the []'s. MATLAB has numerous functions to help your work. You can find out how to use a function by typing help functionName on command line.

3.1.7. TO END A PROGRAM.

After writing a program; you have to end it or to finish it in other to run the writing program. To do this, you type «end» at the final line of your program.

3.1.8. ACCESS TO IMPLEMENTATION DETAILS

MATLAB provides many functions for image processing and other tasks. Most of these functions are written in the MATLAB language and are publicly readable as plain text files. Thus the implementation details of these functions are accessible and open to scrutiny. The defense can examine the processing used in complete detail, and any challenges raised can be responded to in an informed way by the prosecution. This makes MATLAB very different from applications, such as Photoshop.

It should be noted that some MATLAB functions couldn't be viewed. These are generally lower level functions that are computationally expensive and are hence provided as 'builtin' functions running as native code. These functions are heavily used and tested and can be relied on with considerable confidence.

3.1.9. FUNDAMENTALS ON MATLAB.

A digital image is composed of pixels, which can be thought of as small dots on the screen. A digital image is an instruction of how to colour each pixel. We will see in detail later on how this is done in practice. A typical size of an image is 512-by-512 pixels. Later on in the course you will see that it is convenient to let the dimensions of the image to be a power of 2. For example, 29=512. In the general case we say that an image is of size m-by-n if it is composed of m pixels in the vertical direction and n pixels in the horizontal direction.

Let us say that we have an image on the format 512-by-1024 pixels. This means that the data for the image must contain information about 524288 pixels, which requires a lot of memory! Hence, compressing images is essential for efficient image processing.

APPLICATION OF MATLAB.

The matlab has more application:

Mathematical operation

v Vectors

v Matrices and Matrix operations

v Solving systems of linear equations

v Plotting

v Curve fitting, Interpolation

v MATLAB commands

v Preparing data files and M files

THE USE OF MATLAB.

MATLAB may not be as user friendly as an application like Photoshop, however, being a general purpose programming language it provides many important advantages for forensic image processing. It ensures the image processing steps used are completely documented, and hence can be replicated. In general, the source code for all image processing functions are accessible for scrutiny and test. It allows one to ensure numerical precision is maintained all the way through the enhancement process.

Image processing algorithms available under MATLAB are likely to be more advanced than those available from other image processing applications.

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