MATLAB: How to convert grey image to RGB

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hi All,
i have 60 slice image SPECT in grey scale. How to convert all to RGB?
Below is my coding. 
P = zeros(103, 103, 60);
for K = 1 : 60
    K_file=30+10*K;
    petname = sprintf('I%d.dcm', K_file);
    P(:,:,K) = dicomread(petname);
         scale = 130/103 ;
 Pi(:,:,K) = imresize(P(:,:,K),scale) ; % where P is your 103*103 3D matrix
 
end
rgbImage = gray2rgb(Pi);
imshow3D(rgbImage)
this is my function
function [Image]=gray2rgb(Image)
%Gives a grayscale image an extra dimension
%in order to use color within it
[m n]=size(Image);
rgb=zeros(m,n,3);
rgb(:,:,1)=Image;
rgb(:,:,2)=rgb(:,:,1);
rgb(:,:,3)=rgb(:,:,1);
Image=rgb/255;
end
i got error
Unable to perform assignment because the size of the left side is 130-by-7800 and the size of
the right side is 130-by-130-by-60.
Error in gray2rgb (line 6)
rgb(:,:,1)=Image;
Error in sliderspect1 (line 12)
rgbImage = gray2rgb(Pi);
I DONT KNOW HOW COME LEFT SIDE-BY-7800…

Best Answer

This is wrong:
[m n]=size(Image);
Why? See Steve's blog:
Too much information about the size function | Steve on Image Processing
It should be
[rows, columns, numberOfSlices] = size(Image);
but don't use Image as the name of your variable since that's a built-in function (almost). Also, the pet/spect image is already grayscale. You shouldn't need to convert to gray scale.
Also, you did not make a color image, you made a volumetric image with 60 gray scale slices in it.
I'm not sure what you're trying to do, but what you did doesn't make sense to me.
If you're trying to make a volumetric image with 60 RGB slices in it instead of 60 gray scale images, you'll need a 4-D image where the 4th dimension is the slice number and the third dimension is the color channel (1, 2, or 3).
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