Hope this answers your question.
img = imread("pic1.jpg"); % loading your picture
img = rgb2gray(img); % convert to black-and-white pic
row = 450; % choosing random row
idx = find(img(row,:)==0); % position of all white pixels in row 450
diff = idx(1) - idx(end) % difference of first and last pixel in row 450
This should work fine. I noticed your white == 0 and black == 255.
Also, this is for chosen row. If you change this a bit, you can get it for any column.
