How to detect rectangle in an image then crop it out?

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Hi All,
I wish to detect the rectangle as shown below in the image. After that, I want to crop out what's only inside the rectangle part.
Anyone can teach me how?
First, I changed this coloured image into grayscale image then to binary image. Then, I tried using 'regionprops' with 'BoundingBox' to detect the presence of rectangle. However, the cropped image is not exactly the rectangle size.
PS. I do refer from someone else code.
% Find boundary box
props = regionprops(binaryImage, 'BoundingBox');
boundingBox = props.BoundingBox;
width = boundingBox(3);
height = boundingBox(4);
% Crop off the top, shaded portion of column headers,
% which are known to be a fixed 29% of the height of the box.
newHeight = height * (1 - 0.29)
% Make a new bounding box.
xLeft = boundingBox(1);
yTop = boundingBox(2);
boundingBox2 = [xLeft, yTop + 0.29 * height, width, newHeight];
% Crop the image
newGrayScaleImage = imcrop(grayImage, boundingBox2);
subplot(2, 3, 6);
axis('on', 'image');
title('New Gray Scale Image', 'FontSize', fontSize);

Accepted Answer

Akira Agata
Akira Agata on 25 Feb 2020
How about the following?
% Load the image and convert it to gray scale
I = imread('image.jpeg');
Igray = rgb2gray(I);
% Detect black line
BW = Igray < 10;
% Detect rectangle region (inside the line)
BW = imclearborder(~BW);
BW = bwareafilt(BW,1);
% Calculate bounding box
s = regionprops(BW,'BoundingBox');
% Crop the image
Icropped = imcrop(I,s.BoundingBox);
% Show the result
Akira Agata
Akira Agata on 5 Mar 2020
This is because the array I is N x M x 3 (RGB) matrix. To apply immultiply to I, the N x M binary array BW should be expanded to N x M x 3. So I have applied repmat using [1,1,3].

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