stretching or compressing part of a matrix

I have 400 by 400 pixels data set as in the attached txt file. When plotting it, the final figure is having a trapezoidal shape.

Is it possible in matlab to stretch the lower part of the figure or to compress the uppper part to have a square final shape.

Thanks for help.

[Stephen23]

https://www.mathworks.com/help/images/image-registration-and-geometric-transformation.html

M = readmatrix('Evsky.txt');
imshow(M,[])

Answer 1

[Sandeep Mishra]

Hi Mohammed,

I see that you are trying to modify a trapezoidal part of image into a square shape.

To achieve this, you can utilize the 'imresize' function to incrementally increase the width of the image. This process involves gradually scaling the top part with a smaller factor and the bottom part with a larger one.

Refer to the following code snippet:

% Reading the text file
M = readmatrix('Evsky.txt');

% Size of matrix
[rows, columns] = size(M);

% Scaling factors
topScale = 1.05;
bottomScale = 1.3;

% Initializing stretched image
stretchedImage = zeros(rows, columns * bottomScale);

% Scaling factors for each row
scalingFactors = linspace(topScale, bottomScale, rows);

for r = 1:rows
    % new width for each row
    newWidth = round(columns * scalingFactors(r));
    
    % Resizing row
    resizedRow = imresize(M(r, :), [1 newWidth]);
    
    % Streching row
    startIndex = round((columns * bottomScale - newWidth) / 2) + 1;
    % Place the resized row in the new matrix
    stretchedImage(r, startIndex:startIndex + newWidth - 1) = resizedRow;
end

% Stretched image
imshow(stretchedImage, []);

Please refer to the following MathWorks Documentation to learn more about ‘imresize’ function in MATLAB: https://www.mathworks.com/help/releases/R2024b/matlab/ref/imresize.html

I hope this helps you in resolving the query.