How to separate RGB values of an image.

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We know that each pixel i in the input image I has rgb colors. My question is how to store them in a 3-vector Ii. Each color is being represented using a 64-bit floating-point number scaled to between 0 and 1.

Accepted Answer

Jim Dowd
Jim Dowd on 1 Feb 2011
Sean de has told you how to get each plane, but it sounds like you want some kind of vector. I am not sure what you mean by a 3 vector, but here are a couple of guesses.
If you want 3 vectors:
R = reshape(I(:,:,1),[],1);
G = reshape(I(:,:,2),[],1);
B = reshape(I(:,:,3),[],1);
Now if you want them all together in a 3 column vector:
V3 = reshape(A(:),[],3) % R is first col, G is second col, B is third.
  3 Comments
Walter Roberson
Walter Roberson on 6 Mar 2016
MATLAB goes "down columns" in constructing the above. So if there were, for example, 5 rows of pixels in the image, the order in the file would be
(1,1)
(2,1)
(3,1)
(4,1)
(5,1)
(1,2)
(2,2)
(3,2)
(4,2)
(5,2)
(1,3)
(2,3)
...

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More Answers (3)

Sailesh Sidhwani
Sailesh Sidhwani on 25 Oct 2018
Edited: Sailesh Sidhwani on 3 Sep 2020
Starting R2018b, Image Processing Toolbox has a new function "imsplit" which does exactly this: https://www.mathworks.com/help/images/ref/imsplit.html

Sean de Wolski
Sean de Wolski on 1 Feb 2011
Rchannel = I(:,:,1);
Gchannel = I(:,:,2);
Bchannel = I(:,:,3);
  2 Comments
Walter Roberson
Walter Roberson on 25 Oct 2018
It is by definition: MATLAB interprets the first pane of the third dimension as being red values, the second pane of the third dimension as being green values, the third pane of the third dimension as being blue values.
That particular ordering is arbitrary: there is no reason why it could not have been Green, Red, Blue for example, which would have made some sense in terms of the sensitivity of the human eye to brightness in daylight. Or it could have been Blue that was first, reflecting the fact that the human eye is most sensitive to contrast in the blues. But for whatever reason, RGB was the order standardized on.

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Sean de Wolski
Sean de Wolski on 1 Feb 2011
You could also do it so you have a cell array with each index being a 3x1 vector, perhaps:
I2 = cellfun(@squeeze,num2cell(I,3),'uni',false);

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