# how PCA can be applied to an image to reduce its dimensionality with example?

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This question was flagged by Walter Roberson

Dimensionality reduction

##### 3 Comments

Image Analyst
on 14 Sep 2021

### Accepted Answer

Image Analyst
on 24 Dec 2014

Edited: Image Analyst
on 14 Apr 2020

Here's code I got from Spandan, one of the developers of the Image Processing Toolbox at the Mathworks:

Here some quick code for getting principal components of a color image. This code uses the pca() function from the Statistics Toolbox which makes the code simpler.

I = double(imread('peppers.png'));

X = reshape(I,size(I,1)*size(I,2),3);

coeff = pca(X);

Itransformed = X*coeff;

Ipc1 = reshape(Itransformed(:,1),size(I,1),size(I,2));

Ipc2 = reshape(Itransformed(:,2),size(I,1),size(I,2));

Ipc3 = reshape(Itransformed(:,3),size(I,1),size(I,2));

figure, imshow(Ipc1,[]);

figure, imshow(Ipc2,[]);

figure, imshow(Ipc3,[]);

In case you don’t want to use pca(), the same computation can be done without the use of pca() with a few more steps using base MATLAB functions.

Hope this helps.

-Spandan

Also attached are some full demos.

### More Answers (7)

Devan Marçal
on 13 Aug 2015

Hi,

in your example you used PCA in just one image. I have an image bank a total of ~ 800 images. If I make a loop (if, while, etc ..) using the PCA function for each image individually, will be using this command wrong or inefficiently?

Thanks a lot.

Devan

##### 8 Comments

Image Analyst
on 25 Jul 2019

Etworld, I just ran the colored chips image and it ran fine. Did you change my code at all?

Darshan: where did your colors come from? I don't understand what your "approximations" are supposed to be. But anyway, you can stitch images side by side if they are all RGB images to begin with:

wideImage = [rgbImage1, rgbImage2, rgbImage3];

Shaveta Arora
on 30 Jan 2016

Can I have the pca code used in this color image example

##### 6 Comments

Image Analyst
on 31 Jan 2016

Anitha Anbazhagan
on 17 Sep 2016

##### 1 Comment

Image Analyst
on 17 Sep 2016

Mina Kh
on 11 Dec 2016

##### 0 Comments

Arathy Das
on 20 Dec 2016

How can i extract three texture features among the 22 using PCA?

##### 1 Comment

Image Analyst
on 20 Dec 2016

I think you should start your own discussion with your own data or images. If you have 22 PCA columns, then just extract the 3 you want as usual.

pca3 = pca22(:, 1:3); % or whatever.

joynjo
on 24 Mar 2018

How to visualize the result of PCA image in pseudocolor?

##### 1 Comment

Image Analyst
on 24 Mar 2018

imshow(PC1); % Display the first principal component image.

colormap(jet(256));

F M Anim Hossain
on 6 Apr 2018

##### 0 Comments

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