Making a copy of an image via a Loop not working - matlab

Question

Idk why the code won't work. All it does is give me a blank white image. And if you don't declare a matrix before by zeros(x, y) then it works fine. Wth is wrong here?
I tried not declaring the zeros matrix before and only that works. I even tried doing img2(i,j) = img2(i,j)+img1(i,j)
function [imgOut] = scaleLoopBased(img,s)
%UNTITLED4 Summary of this function goes here
% creating a zero matrix of the given scale
[rows,columns]=size(img);
imgTemp=zeros(rows, columns);
for i=1:rows
for j=1:columns
imgTemp(i, j) = img(i, j);
end
end
imshow(imgTemp);
imgOut = imgTemp;
end
Blank white image

Answer 1

This is a result of your new image (of type double, what zeros creates by default) not having the same type as your original image (typically type uint8). You can fix this by initializing your new image to have the same data type as the original using the class function and passing an additional argument to zeros:
imgTemp = zeros(rows, columns, class(img));
The reason it works correctly when you don't initialize imgTemp is because MATLAB initializes the variable for you using the data type of img by default when you perform the first indexed assignment.
The imshow utility expects one of the standard image types in MATLAB. An image of type double is expected to have values spanning the range [0 1], while images of type uint8 will have values in the range [0 255]. In your example you likely have a matrix imgTemp that is of type double but has values spanning [0 255]. Another way to fix your issue is to explicitly tell imshow what value range to use for the display (since the default [0 1] doesn't work):
imshow(imgTemp, [0 255]);
Always be aware of the data type when manipulating or processing images. You may need to scale or convert back and forth, using a double type for calculations (since integers saturate) and a uint8 type for display and reading/writing to files.