How I can normalize a range of columns of an image

I am taking the mean of few coloumns of an image to replce the whole row of the image with mean value. The code is given below

lines = mean(img(:, 400:700), 2);

new_img = img- repmat(lines, 1, size(img, 2));

But the value from coloumns 400:700 varies, so I want to normalize these coloumns. How I can do this?

Answer 1

[Image Analyst]

It doesn't matter that the values in those columns vary. But if img is of type uint8, you need to cast it to double otherwise when subtracting the mean if any values are below 0 it will clip them to 0.