How can I rearrange the RGB planes of an image in MATLAB 7.9 (R2009b)?

I would like to reorder the color planes of an RGB image. Note that this has no effect on the image dimensions. This only changes the image coloring.

Answer 1

[MathWorks Support Team]

Images in MATLAB are commonly represented by arrays. RGB images in particular have a third dimension of length 3. The first dimension is red, the second is green, and the third is blue.

The simplest way to change the color panes of an image is to permute them. For example, to switch red and blue of image 'im', the code is

im = im(:,:,[3 2 1])

Alternatively, here is a function to change color panes via a character array. For example, 'rgb' can become 'gbr':

function out=switchColors(im,order)

indexs = [0 0 0];

for i=1:3
    switch (order(i))
        case 'r'
            indexs(i)=1;
        case 'g'
            indexs(i)=2;
        case 'b'
            indexs(i)=3;
    end
end

out = im(:,:,indexs);

end

Here is an example of how to use this function to swap the Red and Blue planes of "football.jpg":

im=imread('football.jpg');
figure, imshow(im), title('Original');
bgr=switchColors(im,'bgr');
figure, imshow(bgr), title('Red/Blue Swapped');

 

Answer 2

[DGM]

Why does this question exist for this specific version? Why does that answer exist? Did I miss something?

So you want to permute the channels of an image? You don't need to write a huge clumsy thing to do it -- not in R2009b or any version. Just permute the channels.

% an RGB image
A = imread('peppers.png');

% a specified channel order
chanorder = [3 2 1];

% permute the channels
B = A(:,:,chanorder);

% concatenate the results for compact display
imshow([A B])

While imshow() creates an IPT dependency in older versions, the method of permuting channels should not depend on anything. This works from R2022b to R2009b, and while I don't have anything older than R2009b, I don't know why it wouldn't work in prior versions as well. It's basic array indexing.

If you want to replicate the behavior of the function in the accepted answer:

% an RGB image
A = imread('peppers.png');

% a specified channel order
chanorder = 'bgr';

% permute the channels
B = permutechans(A,chanorder);

% concatenate the results for compact display
imshow([A B])

function outpict = permutechans(inpict,orderstr)
    % strip spaces and force lowercase
    orderstr = lower(orderstr(orderstr~=' '));
    
    % if you want to require the output to be 3 channels
    if numel(orderstr) < 3
        error('not enough output channels specified')
    end
    orderstr = orderstr(1:3); % just discard any extras
    
    % get the order from the string
    [valid order] = ismember(orderstr,'rgb');
    if ~all(valid)
        error('expected channel designations to be one of the following: r, g, b')
    end
    
    % permute the array
    outpict = inpict(:,:,order);
end

This will do the same without all the repeated code, and since it's class-agnostic, it will actually work with non-uint8 images without destroying them. Again, all of the tools used here predate R2006a.