Matlab: extrac i_th pixel of all images stored in a cell array

Question

I have an image_data cell array contains 400 images:

image_data = {img1, img2, ... img400}

each image, such as img1, is a 128 x 128 array of pixel values:

img1 = [0.2, 0.5, .....]

I want to get all the i_th pixel of all images stored in image_data, but not using a for-loop.

For Example:

image_data = {img1, img2, img3}
img1 = [0.1, 0.2, 0.3]
img2 = [0.4, 0.5, 0.6]
img3 = [0.7, 0.8, 0.9]

I want to calculate the variance the following quickly:

var([img1(1),img2(1),img3(1)]) = var([0.1,0.4,0.7]) 
var([img1(2),img2(2),img3(2)]) = var([0.2,0.5,0.8]) 
var([img1(3),img2(3),img3(3)]) = var([0.3,0.6,0.9])

Try to avoid using a loop, because it is slow.

Answer 1

The first step is to convert that unwieldy cell array into a 3-dimensional array by concatenating the images along the 3rd dimension:

image_array = cat(3, image_data{:});

Now, all we have to do is use a subscript to get the vector of the ith pixel from every image into a vector and pass that to var:

pixel_var = var(image_array(1,1,:));  % var for pixel 1

Or just do the whole image at once and index into the result:

image_var = var(image_array, 0, 3);   % take var along 3rd dimension