IMAGES IN MATLAB

Group Members:

Muhammad Raheel Ali (12-EE-131)

Rana Tassuvar (12-EE-123)

Muhib ullah (12-EE-214)

What is Image Data???

In Matlab, an image is a set of color or intensity

data.

Each image is formed by combination of pixels.

Each pixel is a matrix element.

Each pixel has its own intensity of red, blue and

green color.

A high level image is represented by a 3

dimensional array.

First plane in the third dimension represents the

red pixel intensities, the second plane represents

the green pixel intensities, and the third plane

represents the blue pixel intensities.

Popular Matlab functions associated

with Images:

In Matlab, there are a lot of useful functions. Some

of these are:

imread

imshow

rgb2gray

imhist

imadjust

imb2w

And many more….

Loading an image in Matlab:

For loading an image in Matlab, we use use

“imread” function.

Following is an example of loading an image:

a=imread('skardu.jpeg');

Showing an image in Matlab:

“imshow” function shows the loaded image in

Matlab window.

Following is an example:

a=imread('skardu.jpg');

imshow(a);

Output:

Getting image info using Matlab:

We mainly have two types of images that we use

to deal in Matlab. These are

1- RGB images

2- Indexed images

RGB images are stored in 3 matrices. 1st for red,

2nd for green and 3rd for blue.

Each element in matrix stores intensity value of

red, green or blue color for a particular pixel.

In indexed images, each pixel is given an index.

Each index is mapped to RGB value in colormap.

Pixel info of RGB image:

For obtaining pixels info of an rgb image, we have

a built in function “impixelinfo”.

Following syntax is used:

a=imread('rgb.jpg');

imshow(a);

impixelinfo;

On the output screen, we can see the particular

row and column of pixel with its corresponding

rgb value.

Output:

In the bottom left corner, we can see the pixel info.

We can also find the size of image by using given

command:

>> a=imread('rgb.jpg');

>> impixelinfo

>> size(a)

Output is:

ans =

768 1024 3

Output shows that it is a 768x1024 image with 3

planes.

Converting image from one format to

other:

Matlab can be used to change the format of image.

Following table gives information about image conversion:

As an example we will change format of image.

Converting image to grayscale

image:

“rgb2gray” function converts the image to

grayscale image.

Example is:

a=imread('skardu.jpg');

b=rgb2gray(a);

imshow(b);

Output

:

Histogram of Image:

“imhist” function is used to obtain histogram of

image.

This function plots a graph that shows how many

pixels are in a specific intensity in grayscale

image.

An example is:

a=imread('skardu.jpg');

b=rgb2gray(a);

imhist(b)

Output:

Adjusting the contrast ratio: “imadjust” function adjusts the contrast ratio in image.

Example is:

a=imread('skardu.jpg');

b=rgb2gray(a);

c=imadjust(b);

subplot(1,2,1);

imshow(b);

title('Not Adjusted');

subplot(1,2,2);

imshow(c);

title('Adjusted');

Output:

Using Matlab to obtain black and

white image:

“im2bw” is a function in Matlab that turns the

image into black and white image.

Example:

a=imread('skardu.jpg');

b=im2bw(a);

imshow(b);

Output:

Resizing the image: Matlab can also be used to resize the image.

By resizing, we mean that we can increase or decrease pixels of image.

Following code is an example:

a=imread('skardu.jpg');

subplot(1,2,1);

imshow(a);

title('original image');

c=imresize(a,1/5);

subplot(1,2,2);

imshow(c);

title('Resized');

Image is blurred showing decrease in number of pixels.

Output:

Performance issues in Matlab

images:

Matlab is very fast on vector and matrix

operations but

Correspondingly slow with loops.

So,

• Try to avoid loops

• Try to vectorize your code

Thank You!

Any

Question???