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

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G Prasanth Reddy on 24 Dec 2014
Commented: Image Analyst on 14 Sep 2021
This question was flagged by Walter Roberson
Dimensionality reduction
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Image Analyst on 14 Sep 2021
@SHEETAL AGRAWAL, perhaps. You obviously need at least two features. What would be your two features? Maybe gray level is one, but what is the other? Or do you just have two different features, like blob area and blob texture or brightness?

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### 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.
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Aya Ahmed on 4 Mar 2020

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### 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
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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];

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Shaveta Arora on 30 Jan 2016
Can I have the pca code used in this color image example
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Image Analyst on 31 Jan 2016
I can't. It would not be legal. You either have to buy the toolbox from the Mathworks, or implement it yourself from low level code.

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Anitha Anbazhagan on 17 Sep 2016
I have 200 ROIs from each of the 50 images. For each ROI, I have 96 feature vectors for four different frequency bands. It seems very high dimensional. How to apply PCA for this? PCA should be applied to data matrix. Do I have to apply for each image or each ROI?
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Image Analyst on 17 Sep 2016
It depends on if you want PCA components on each image individually, or the PCA components of the group as a whole.

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Mina Kh on 11 Dec 2016
Hi. I have multispectral( multi channel) data and I want to apply PCA to reduce the number of channel. Can u give me some hint?Which code i have to use?
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Arathy Das on 20 Dec 2016
How can i extract three texture features among the 22 using PCA?
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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.

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joynjo on 24 Mar 2018
How to visualize the result of PCA image in pseudocolor?
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Image Analyst on 24 Mar 2018
imshow(PC1); % Display the first principal component image.
colormap(jet(256));

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F M Anim Hossain on 6 Apr 2018
I'm new to the concept of PCA. I'm trying to develop something that can recognize color features from different images. Is it possible to do it with the help of PCA?
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