what is the difference between these two types of rgb matlab code ?

i have a 3D color image , when i use a matlab code as below

x=imread('....');

R=x(:,:,1); % red

G=I(:,:,2); % green

B=I(:,:,3); %blue

figure, imshow (R);

it display grayscale image. and if i use the code given below

% Red channel only

im=imread('....');

im_red = im;

im_green = im;

im_blue = im;

im_red(:,:,2) = 0;

im_red(:,:,3) = 0;

figure, imshow(im_red);

it display red color image.

i need help in undestanding difference between these codes ?

Answer 1

[Ken Atwell]

In the first case, R is not a 3-D image, but a 2-D image, which MATLAB will display as grayscale. You created R as 2-D when you indexed in the red-only portion of the images in your statement:

 >> R=x(:,:,1); % red

In the second case, im_red is a 3-D image, It begins as identical to 'im', but then you set both the green and blue planes to zeros with:

 >> im_red(:,:,2) = 0;
 >> im_red(:,:,3) = 0;

Look at the dimensions of your matrices in the Variable window as you execute these command one-by-one, and I believe you will be able to follow along with what is happening.

[namita chandrakar]

Quite nice explanation, now it's clear for me. Thanks for your time and answer.