Downsizing and rescaling an image using for loops - matlab

Question

I'm relatively new to Matlab, and trying to understand why a piece of code isn't working.
I have a 512x512 image that needs to be downsized to 256, and then resized back up to 512.
How I understand the mathematics, is that I would need to mean the pixels in the image to get the 256, and then sum them back to get the 512. Is that correct ? Following is the code that I'm looking at, and if someone can explain me whats wrong(its giving a blank white image), I would appreciate it:
w = double(imread('walkbridge.tif'));
%read the image
w = w(:,:,1);
for x = 1:256
for y = 1:256
s256(x,y) = (w(2*x,2*y)+ w(2*x,(2*y)-1) + w((2*x)-1,2*y)+ w((2*x)-1,(2*y)-1))/4;
end
end
for x = 1 : 256
for y = 1 : 256
for x1 = 0:1
for y1 = 0:1
R1((2*x)-x1,((2*y)-y1)) = s256(x,y);
end
end
end
end
imshow(R1)

Answer 1

I got your code to work, so you might have some bad values in your image data. Namely, if your image has values in range 0..127 or something similar, it will most likely show as all white. By default, imshow expects color channels to be in range 0..1.
You might also want to simplify your code a bit by indexing the original array instead of accessing individual elements. That way the code is even easy to change:
half_size = 256;
w = magic(2*half_size);
w = w / max(w(:));
figure()
imshow(w)
s = zeros(half_size);
for x = 1:half_size
for y = 1:half_size
ix = w(2*x-1:2*x, 2*y-1:2*y);
s(x,y) = sum(ix(:))/4;
end
end
for x = 1:half_size
for y = 1:half_size
R1(2*x-1:2*x, 2*y-1:2*y) = s(x,y);
end
end
figure()
imshow(R1)
I imagine the calculations could even be vectorised in some way instead of looping, but I didn't bother.