Select a random patch from an image using matlab

Question

I have a gray image of size 400 x 600 . And i want to select randomly from this image, a patch of size 50 x 50.

By the way, i tried to write a code for this, and it worked fine. But according to my code below, there is another solution ? In other words, is there another code which can be more sexy than my own code ?

clear all;
close all;
clc;

image=imread('my_image.jpg');
image_gray=rgb2gray(image);
[n m]= size(image_gray); % 400 x600

L=50;

x=round(rand(1)*n); % generate a random integer between 1 and 400
y=round(rand(1)*m); % generate a random integer between 1 and 600

%verify if x is > than 400-50 , because if x is equal to 380 for example, so x+50 become %equal to 430, it exceeds the matrix dimension of image...
if(x<=n-L)
a=x:x+(L-1);
else
a=x-(L-1):x;
end

if(y<=m-L)
b=y:y+(L-1);
else
b=y-(L-1):y;
end

crop=image_gray(a,b);
figure(1);
imshow(crop);

Answer 1

This is as "sexy" as it gets. Satisfaction guaranteed ;-)

% Data
img=imread('my_image.jpg');
image_gray=rgb2gray(img);
[n m]= size(image_gray);
L = 50;

% Crop
crop = image_gray(randi(n-L+1)+(0:L-1),randi(m-L+1)+(0:L-1));

If using a Matlab version that doesn't support randi, substitute last line by

crop = image_gray(ceil(rand*(n-L+1))+(0:L-1),ceil(rand*(m-L+1))+(0:L-1));

Comments to your code:

• You shouldn't use image as a variable name. It overrides a function.
• You should change round(rand(1)*n) to ceil(rand(1)*n). Or use randi(n).
• Instead of randi(n), use randi(n-L+1). That way you avoid the ifs.

Answer 2

For anyone struggling to get this code to work for RGB images, when you compute the size you'll want to add an extra variable for the third dimension. i.e.

% Data
img=imread('my_image.jpg');
% Not making anything gray anymore 
[n, m, ~]= size(img);
L = 50;

% Crop - add a : in order to get 3 or more dimensions at the end
crop = img(randi(n-L+1)+(0:L-1),randi(m-L+1)+(0:L-1), :);

Super simple but not necessarily obvious at first.