matrix to rgb conversion

When we write

i=imread('test.jpg');

if test.jpg is a colored image then it returns us i as a 3D matrix.

if we write

figure,imshow(i)

it shows the colored image test.jpg

but if we assign the values in i to another variable using the following syntax:

a=i(:,:,1); a=i(:,:,2); a=i(:,:,3);

then if i write

figure,imshow(a)

it does not return me the colored image as test.jpg

can any one say why this happen? How can i get the make a rgb image if i assign value to a 3D matrix manually how can i get the corresponding colored rgb image?

Answer 1

[Matt Fig]

The reason why you are having the problem is that when you call the ZEROS function, you are getting an array of doubles, whereas a jpg image is of type uint8.

I = imread('test.jpg');
figure
imshow(I)
a = zeros(size(I),'uint8'); % Or, a = zeros(size(I),class(I));
a(:,:,1) = I(:,:,1);
a(:,:,2) = I(:,:,2);
a(:,:,3) = I(:,:,3);

figure
imshow(a)

Note that the above is a horrible way to copy any data from one array to another. Simply use:

a = I;  %  Don't use variable name i!

[Mohammad Golam Kibria]

Yes. That works fine.

Thanks

Answer 2

[Jan]

i = imread('test.jpg');

a = i(:,:,1);
% Now [a] is a 2D-Matrix containing the first
% submatrix of i.

a = i(:,:,2);
% Now [a] is a 2D-Matrix containing the second
% Submatrix of i. The former values are overwritten.

a = i(:,:,3);
% Now [a] is a 2D-Matrix containing the third
% Submatrix of i. The former values are overwritten.

Therefore [a] is a matrix at the end. To copy the 3D-Array:

a = cat(3, i(:,:,1), i(:,:,2), i(:,:,3));
% Or simpler:
a = i;

[Mohammad Golam Kibria]

sorry, your answers to my code is right.But I have made a little mistake in the code. please see the following:

i = imread('test.jpg');

figure,imshow(i)

a=zeros(size(i));

a(:,:,1) = i(:,:,1);

a(:,:,2) = i(:,:,2);

a(:,:,3) = i(:,:,3);

figure,imshow(a)

c = cat(3, a(:,:,1), a(:,:,2), a(:,:,3));

figure,imshow(c)

The problem is that both a and c are not colored image as i found in i.

would you please explain me why this is happening.