# Can I use PCA effectively on a greyscale image?

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Neo on 18 Dec 2015
Answered: Aya Ahmed on 5 Apr 2020
Hello!
I found this code online:
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,[]);
provided from another commentary form I was reading and I was wondering if there was any code that performed PCA that did not give the pc as a color channel. I don't want to do PCA on the colors of the image composite I want something else (not sure what else is but something not color), so applying PCA to a gray scale image.
Thanks for you any and all suggestions.
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Neo on 21 Dec 2015
Edited: Neo on 21 Dec 2015
Oh I see, but just because an image is not RGB, it doesn't necessarily mean that it has only one axis, it might mean that the image has axes that are not related like in a RGB image. And let me get a picture for you to give an idea of what I am talking about actually, great suggestion. I read your response to quickly. Here ya go:

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### Accepted Answer

Walter Roberson on 18 Dec 2015
Sure. For example,
I = double(imread('cameraman.tif'));
X = reshape(I,[],4);
coeff = pca(X);
This would correlate vertical quarters of the image.
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Neo on 29 Dec 2015
Haha, thanks Analyst. But I am more concerned with how I can feed multiple images into the PCA code so that I can get more than one PC from the image. Do you have an average face answer to that? Cheers, Neo Cornel

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### More Answers (4)

mugahid albadawy on 9 Feb 2017
i ve already used the same function for dicom image but it's not working properly
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Stefan Karlsson on 27 Dec 2015
I think the answer by Walter, while technically correct, confounds a VERY simple topic. Being technically correct here amounts to what exactly?
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Image Analyst on 28 Dec 2015
Nah, you were fine. But is a little frustrating waiting for Neo to verbalize what features he wants to characterize in his image. It's almost like he heard about PCA and thought that it sounded cool and wanted to apply it to his image without considering if it was appropriate or not, or whether there might be better methods. I still don't know what kind of result he would want.

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Stefan Karlsson on 28 Dec 2015
... I guess one can also give another piece of advice to anyone who stumbles onto this looking for info on PCA. Read the posts by Image Analyst. They are as high in quality as they usually are.
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Neo on 28 Dec 2015
I'd say Walter Roberson was also very high quality as well.

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Aya Ahmed on 5 Apr 2020
Neo can you tell me please ..
i want to make feature extraction using PCA ,using matlab code on galaxy grayscale image ,
I was wondering if you could help me with a few steps or even code to make feature extraction from images .
I would like to extract the features of galaxy images and then classify them in the classification learner app.
The data I have is a set of galaxy imagse.
The aim is to extract the features and then compare them in the classification app with each other.
Any help is appreciated!
i want to know how pca work ? does it work in gray images only ??
Thanks ..
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