Issue while plotting image (wrong scale)

Hello, I am getting inverted image, and when I am trying to invert it using (reverse) I got the the corrected image but the y axis scale also got inverted. Is this correct or is there a way to correct it. Please suggest possible solution, I am confused whether it is right or not for single valued decomposition.

I have attached the MATLAB script and the output image for your reference after inversion but the scale is also inverted and is going from high to low.

for i = 1:52;
    data(:,:,i) = rgb2gray(imread(sprintf('~/Downloads/4_68_image/scene_%0.4d.bmp',i)));
end
r = 1024;
c = 1024;
 
data = double(reshape(data,r*c,52));
 
mode = 1
freq = 1

[Phi ,~, C]=svd(data-repmat(mean(data,2),[1 size(data,2)]),'econ');
% Plot the figures
close all
figure('name','POD')
subplot(1,1,1)
imagesc(reshape(Phi(1:r*c,mode),r,c));axis image;set(gca,'Ydir','reverse')

Answer 1

[Raag]

Hi Priyank,

This is a common issue when displaying reshaped image data in MATLAB. The confusion arises because MATLAB’s ‘imagesc’ function, by default, places the (1,1) pixel at the top-left, with the y-axis increasing downwards ('YDir','normal').

To keep the image upright and the y-axis increasing from top to bottom (the MATLAB default), you can flip the image data before displaying, and use 'YDir','normal'. Here’s how you can modify your code:

imagesc(flipud(reshape(Phi(1:r*c,mode), r, c))); 
axis image; 
colormap(jet);      % Optional: for better visualization 
colorbar;           % Optional: show color scale 
set(gca, 'YDir', 'normal');   % Standard y-axis direction (top-to-bottom)

Here ‘flipud()’ flips the image data vertically, correcting any inversion caused during reshaping.

'YDir','normal' ensures the y-axis increases from top to bottom, which is standard in MATLAB.

This is how the output may look like:

For more information, refer the following documentation:

imagesc: https://www.mathworks.com/help/matlab/ref/imagesc.html