Matlab: Crop objects from binary image - matlab

Question

I'm new to matlab, I have a image and i want to crop all the three circles and store them. My code works for single circle in an image. But fails to work when i have more circles in image.
My code:
im=imread('D:\capture.png');
im_gray = rgb2gray(im);
BW = im2bw(im_gray, graythresh(im));
se = strel('disk',3);
bw2=imopen(BW,se);
bw2=~bw2;
s = regionprops(bw2, 'BoundingBox');
rectangle('Position', s.BoundingBox);
imCrop = imcrop(bw2, s.BoundingBox);
figure, imshow(imCrop);
Any ideas about it?

Answer 1

You almost have it working. Bear in mind that when you do s.BoundingBox by itself, you are only extracting the first circle. As such, I would recommend you make a cell array that stores the individual circles for each bounding box, then run a for loop through all of your bounding boxes. As such, each element in your cell array would be a cropped circle. As such, try doing this:
%// Your code
im=imread('D:\capture.png');
im_gray = rgb2gray(im);
BW = im2bw(im_gray, graythresh(im));
se = strel('disk',3);
bw2=imopen(BW,se);
bw2=~bw2;
s = regionprops(bw2, 'BoundingBox');
%// New code here
circles = cell(1,numel(s));
for idx = 1 : numel(s)
rect = s(idx).BoundingBox;
circles{idx} = imcrop(bw2, rect);
end
circles will now be a cell array of cropped circles. To access the ith circle, simply do:
imCrop = circles{i};
Edit
From your comments, you want to detect the largest and smallest circles. This can easily be done by checking the Area attribute from regionprops. You would find the bounding box that generates the minimum and maximum areas. You would need to modify your regionprops call to include the Area flag. As such:
s = regionprops(bw2, 'BoundingBox', 'Area');
[~,indMin] = min([s.Area]);
[~,indMax] = max([s.Area]);
circleSmall = circles{indMin};
circleLarge = circles{indMax};
The above code will find the circles with the minimum and maximum area, then extract those corresponding circles, assuming you've run the code to extract all of those circles in that for loop I wrote earlier. Bear in mind that I had to enclose s.Area in square braces. The reason why is because when you do this, you'll be able to extract all of the areas as a single array instead of a matrix with singleton dimensions, and min/max can't work on something like that.