How to crop out a detected object from image and store it ?

The code bellow classifies objects based on their roundness using bwboundarie. It estimates each object's area and perimeter and uses these results to form a simple metric indicating the roundness of an object:( metric = 4*pi*area/perimeter^2).

This metric is equal to 1 only for a circle and it is less than one for any other shape. But with this code, I'm using a threshold of 0.80 so that only object with a metric value greater than 0.80 will be classified as round.

My question is: when a given object classified as a round, how can I crop it out from the original image 'img' (not 'I' nor 'bw') and save it as a new image?

I think using label matrix and boundary matrix would be enough to do so, but still don't know how to manipulate them.

Thank you for your help.

img=imread('cap.png');

I = rgb2gray(img);


% Step 2: Threshold the Image

bw1 = imbinarize(I);

bw = imcomplement(bw1);


% Step 3: Remove the Noise

bw = bwareaopen(bw,30);      % remove small objects

bw = imfill(bw,'holes');    



% Step 4: Find the Boundaries

[B,L] = bwboundaries(bw,'noholes');

imshow(label2rgb(L,@jet,[.5 .5 .5]))

hold on

for k = 1:length(B)
  
boundary = B{k};
  
plot(boundary(:,2),boundary(:,1),'w','LineWidth',2)

end




% Step 5: Determine which Objects are Round

stats = regionprops(L,'Area','Centroid');

threshold = 0.80;

% loop over the boundaries

for k = 1:length(B)

  
% obtain (X,Y) boundary coordinates corresponding to label 'k'
  
boundary = B{k};

  
% compute a simple estimate of the object's perimeter
  
delta_sq = diff(boundary).^2;    
  
perimeter = sum(sqrt(sum(delta_sq,2)));

  
% obtain the area calculation corresponding to label 'k'
  
area = stats(k).Area;

  
% compute the roundness metric
  
metric = 4*pi*area/perimeter^2;

  
% display the results
  
metric_string = sprintf('%2.2f',metric);

  
% Test if the current object classified as a round
  
if metric > threshold
    
% HERE, I want to crop the current object from the 'img' 
    
% and save it as a new image 
  
end
end

title(['Metrics closer to 1 indicate that ',...

       'the object is approximately round'])

Answer 1

[Mohammed Sadiq]

The rectangle boundary values for cropping an object from image can be extracted from the list of boundary points using min() and max() functions. boundary(:,2) represents the X coordinates and boundary(:,1) represents the Y coordinates for the contour of object. For more information, refer to bwboundaries documentation.

Using the following code will save the image after cropping on the round object.

if metric > threshold
    % Calculate x limits
    left = min(boundary(:, 2));
    right = max(boundary(:,2));
    % Calculate y limits
    up = min(boundary(:, 1));
    down = max(boundary(:,1));
    % Saving cropped image
    imwrite(img(up:down, left:right, :),['img_round', int2str(k), '.png']);
end