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

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Taha KAMIL
Taha KAMIL 2019-5-24
编辑： Mohammed Sadiq 2021-6-15
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'])

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回答（1 个）

Mohammed Sadiq
Mohammed Sadiq 2021-6-15
编辑：Mohammed Sadiq 2021-6-15
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

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