Image Representation in MATLAB – How to Represent Binary,Grayscale & Color Images

Avaneet Ranjan October 23, 2013 1 Comment

In this article, we are going to talk about images, their types and some basic functions for Images. This article is part of our MATLAB tutorial series, which we are developing for Image processing through MATLAB. We expect you to give us feedback, so that we can keep the information flow more efficient, informative and up to the mark. In the first chapter, you have learned an introduction to MATLAB and its commands. In this chapter you will learn how to represent different types of images in MATLAB. In addition you will learn how to read & display images in MATLAB and how to create an M file.

Taking Up Images

Technically whatever we categorize as a picture, is an Image. We already talked about the basics of Image processing in our first article, we would focus today on technical representation of images.

Different Types of Images 

Binary Image: An image that consists of only black and white pixels. Technically these types of images are called as Black and White Image. (Although it makes me sad to break my reader’s heart but till now what you called black and white images have some other technical name). 

Grayscale Image: In  daily  language  what  we  refer  to  as  black-and-white  (as  in  old  photos)  are  actually grayscale. It contains intensity values ranging from a minimum (depicting absolute black) to a maximum (depicting absolute white) and in between varying shades of gray. Typically, this range is between 0 and 255.

Color Image: We all have seen this! Such an image is composed of the three primary colors, Red, Green and Blue, hence also called an RGB image.

How to represent binary image in MATLAB

Binary Image

 

How to represent grayscale image in MATLAB

Grayscale Image

How to represent color image in MATLAB

Color Image

RGB  value:  All  colors  which  we  see  around  us  can  be  made  by  adding  red,  blue  and  green components in varying proportions. Hence, any color of the world can uniquely be described by its RGB value, which stands for Red, Blue and Green values. This triplet has each value ranging from 0 to 255,  with  0  obviously  meaning  no component  of  that particular  color and  255  meaning  full component. For example, pure red color has RGB value [255 0 0], pure white has [255 255 255], pure black has [0 0 0] and   has RGB value [55 162 170].

Representation of an Image in MATLAB

An image in MATLAB is stored as a 2D matrix (of size mxn) where  each element  of the matrix represents the intensity of light/color of that particular pixel. Hence, for a binary image, the value of each element of the matrix is either 0 or 1 and for a grayscale image each value lies between 0 and 255. A color image is stored as an mxnx3 matrix where each element is the RGB value of that particular pixel (hence it’s a 3D matrix). You can consider it as three 2D matrices for red, green and blue intensities.

Reading and Displaying Images

imread(): To read an image and store in a matrix.

Syntax: IM=imread(‘filename’)

where IM is a matrix. If the file is in the current directory (as described above), then you only need to write the filename, else you need to write the complete path. Filename should be with extension (.jpg, .bmp,..). There are some default images of MATLAB like ‘peppers.png’, ‘cameraman.tif’, etc. You can try reading them as

>>im=imread(‘peppers.png’);

It is always  advised  to use a semi-colon  (;)  at  the end  of the  statement  of reading  an  image, otherwise… you can try yourself what happens!

imshow(): Displays the image. Syntax:

imshow(‘filename’)

or imshow(im)

 

Example:-

>>imshow(‘cameraman.tif’);

 

OK, now let’s make our own image, try this:

>>a(1,1)=0;

>>for i=1:200; for j=1:200

a(i+1,j+1)=1-a(i,j);

end end

>>imshow(a);

You try out making many different types of images like this just to make yourself comfortable with the commands learnt till now.

Data cursor: To see the values of the colors in the figure window, go to Tools>Data Cursor (or select from the toolbar), and click over any point in the image. You can see the RGB values of the pixel at location (X,Y).

Image representation in MATLAB

A better option of data cursor is the function imtool(). Type the following

 >>imtool(‘peppers.png’);

And see the pixel info on lower left corner as you move mouse pointer over different pixels.

Now, before we move on to our next article, we take a little detour in this article and talk about some MATLAB resources which will be extensively be used in coming article.

Making M Files & Functions in MATLAB

It is a provision in MATLAB where you can execute multiple commands using a single statement. Here the group of commands is stored as a MATLAB file (extension .m).

M Files in MATLAB - How to create & save an M File in Matlab

Here we have saved the m-file by the name “test.m”. Now as you type

>>test

And in MATLAB command window, all the above commands will execute.

Comments: As we have comments in C/C++/ Java using double slash (//), in MATLAB we use symbol  to  write  comments,  i.e.,  statements  that  are  not  considered  for  execution.  You can see comments in green in the snapshot above.

 Functions in MATLAB

Functions,  as  some  of  you might  know,  are  written  to organize  the  code  efficiently  and  make debugging easier. The set of statements within a function can be executed as and when required by just calling it, thereby avoiding repetitions. The data which is needed within the function can be passed as arguments and then the required values can be returned.  You can return any no. of values and they can be matrices also.

 We value your suggestions and feedback very much. Please drop by your comments. This encourages us to move forward!

Until next article is up this all folks! Keep reading and keep learning.

Comments
  • amol varwade
    February 11, 2014

    Audio reading files in matlab
    Image conversion in matlab

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