image splitting: what is the logic behind the following code? I need explanation.

v = imread('embedded.jpg');
[s1 s2 s3] = size(v);
i = 1; 
j = 1;
in = 0;
cond = true;
while (cond)
    img = v(i:i+(s1/4)-1, j:j+(s2/4)-1,:);
    j = j+(s2/4);
    if j > s2
        i = i+(s1/4);
        j = 1;
        if i > s1
            cond = false;
        end
    end
    imshow(img);
    imwrite(img, strcat('img', num2str(in), '.jpg'));
    in = in + 1;
end

Answer 1

[yanqi liu]

yes，sir，it use to cut image top-left in to 4*4=16 block area，such as

clc;
clear all;
close all;

v = imread('football.jpg');
[s1 s2 s3] = size(v);
i = 1; 
j = 1;
in = 0;
cond = true;
figure(1); imshow(v, []);
while (cond)
    img = v(i:i+(s1/4)-1, j:j+(s2/4)-1,:);
    % display now rect
    bbox=[i j s1/4 s2/4];
    figure(1); hold on; rectangle('position', bbox, 'EdgeColor', 'g', 'LineWidth', 2)
    j = j+(s2/4);
    if j > s2
        i = i+(s1/4);
        j = 1;
        if i > s1
            cond = false;
        end
    end    
    %imshow(img); 
    imwrite(img, strcat('img', num2str(in), '.jpg'));
    in = in + 1;
end

[Image Analyst]

You said

bbox=[i j s1/4 s2/4];

However the form is [xLeft, yTop, width, height].

But s1 is the number of rows, not columns, and s2 is the number of columns, not rows:

[s1 s2 s3] = size(v); % [rows, columns, numberOfColorChannels]

so you should have

bbox=[i, j, s2/4, s1/4];

That's why your green rectangles don't fit an integer number of times in the image.

Also it's not good to use i and j for things. Not only because they're often used for the imaginary variable, but also because they're deceptive and not clearly named. Since you're using i for the first index, it should be called row, and j should be called column. Good names can avoid mistakes. (I know that some of the poor name choices were the original poster's.)

The whole while loop and if block to check if i and j went outside the image could be done vectorized in a few lines of code using linspace(). Like

startingRows = round(linspace(1, s1+1, 4));
startingCols = round(linspace(1, s2+1, 4));

etc. Now the indexes will never be outside the image and you don't have to check for that.

[yanqi liu]

yes，sir，if use grid，can get the total split，if the row number equal to column number，may be split equal.

but not the origin code，in fact the origin code is use on top left

of course，if we modify it

clc;
clear all;
close all;
I = imread('football.jpg');
sz = size(I);
th = 5;
[x, y] = meshgrid(linspace(1,sz(2),th), linspace(1,sz(1),th));
z = ones(size(x));
figure;
imshow(I, []);
hold on; axis off;
mesh(x, y, z, 'FaceColor', 'none', ...
    'EdgeColor', 'g', 'LineWidth', 2, ...
    'Marker', 'o', 'MarkerFaceColor', 'k', ...
    'MarkerSize', 3);

% use for imwrite
for i = 1 : length(x)-1
    for j = 1 : length(y)-1
        bbox = round([x(i,j) y(i,j) diff(x(1,1:2)) diff(y(1:2,1))]);
        imi = imcrop(I, bbox);
        imwrite(imi, sprintf('%02d-%02d.png', i, j));
    end
end