For and While loops

How do I use Embedded “For” or “While” loops to scan through an image in MATLAB?

[Chad Greene]

Can you be more specific about what you're trying to do? What have you tried so far?

Answer 1

[Simar]

Hi Zahraa Al-Waeli,

I understand that you are looking to use embedded ‘for’ or ‘while’ loops to scan through an image in MATLAB. Scanning through an image in MATLAB using embedded ‘for’ or ‘while’ loops is a common task in image processing. Here is a simple example of how you can do it:

1. Firstly, read the image file into MATLAB using ‘imread’ function and store the image data in a variable named ‘img’.

img = imread('your_image_file.jpg'); 

2. For this example, using the ‘if’ statement to check if image is a colored image (which has three layers - red, green, and blue). If it is, the ‘rgb2gray’ function converts the color image into a grayscale image (black and white) for simplicity.

if size(img, 3) == 3 
    img = rgb2gray(img); 
end 

3. Getting size of the image in terms of number of rows and columns. Think of the image as a grid, where each cell in the grid is a pixel (the smallest unit of an image). The number of rows and columns tells us the height and width of this grid.

[rows, cols] = size(img); 

4. In this example, using two ‘for’ loops (one inside the other) to scan through the image. The external loop iterates over the rows of the image and the internal loop iterates over the columns. Hence including every cell (or pixel) in the grid. For each pixel, retrieve its value, perform an operation (in this case, inversion), and then save the result back to the image.

for i = 1:rows 
    for j = 1:cols 
        % Getting value of the pixel at row i and column j. In a grayscale image, this value is a number between 0 (black) and 255 (white) 
        pixel = img(i, j); 
        % Perform operations on the pixel - Subtracting the pixel value from 255 (the maximum possible value), effectively inverting the image (making dark areas light, and light areas dark). 
        img(i, j) = 255 - pixel; 
    end 
end 

5. Finally, displaying the modified image using ‘imshow’ function.

imshow(img); 

Here is the complete code:

% Load an image 
img = imread('your_image_file.jpg '); 
% Convert to grayscale if the image is RGB 
if size(img, 3) == 3 
    img = rgb2gray(img); 
end 
% Get the size of the image 
[rows, cols] = size(img); 
% Scan through the image 
for i = 1:rows 
    for j = 1:cols 
        % Get the pixel value 
        pixel = img(i, j);  
        % Perform operations on the pixel here 
        % For example, let us invert the image 
        img(i, j) = 255 - pixel; 
    end 
end 
% Display the image 
imshow(img); 

The code loads an image, goes through each pixel one by one, inverts the color of the pixel, and then displays the modified image.

Remember that MATLAB is a high-level language, and it is more efficient to use matrix operations when possible, instead of loops. However, in some cases, loops are necessary and the above example shows how to use them.

Refer to the documentation links below to know more about ‘for’ and ‘while’ loops:

• https://www.mathworks.com/help/releases/R2023b/matlab/ref/for.html?searchHighlight=for&s_tid=doc_srchtitle
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/while.html?searchHighlight=while&s_tid=doc_srchtitle
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/imread.html?searchHighlight=imread&s_tid=doc_srchtitle
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/imshow.html?searchHighlight=imshow&s_tid=doc_srchtitle
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/size.html?searchHighlight=size&s_tid=doc_srchtitle
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/rgb2gray.html?searchHighlight=rgb2gray&s_tid=doc_srchtitle

Hope it helps!

Best Regards

Simar