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This example shows how to classify objects based on their roundness using `bwboundaries`, a boundary tracing routine.

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Read in `pills_etc.png`.

```
RGB = imread('pillsetc.png');
imshow(RGB);
```

Convert the image to black and white in order to prepare for boundary tracing using `bwboundaries`.

I = rgb2gray(RGB); threshold = graythresh(I); bw = im2bw(I,threshold); imshow(bw)

Using morphology functions, remove pixels which do not belong to the objects of interest.

% remove all object containing fewer than 30 pixels bw = bwareaopen(bw,30); % fill a gap in the pen's cap se = strel('disk',2); bw = imclose(bw,se); % fill any holes, so that regionprops can be used to estimate % the area enclosed by each of the boundaries bw = imfill(bw,'holes'); imshow(bw)

Concentrate only on the exterior boundaries. Option 'noholes' will accelerate the processing by preventing `bwboundaries` from searching for inner contours.

[B,L] = bwboundaries(bw,'noholes'); % Display the label matrix and draw each boundary 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**

Estimate each object's area and perimeter. Use these results to form a simple metric indicating the roundness of an object:

metric = 4*pi*area/perimeter^2.

This metric is equal to one only for a circle and it is less than one for any other shape. The discrimination process can be controlled by setting an appropriate threshold. In this example use a threshold of 0.94 so that only the pills will be classified as round.

Use `regionprops` to obtain estimates of the area for all of the objects. Notice that the label matrix returned by `bwboundaries` can be reused by `regionprops`.

stats = regionprops(L,'Area','Centroid'); threshold = 0.94; % 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); % mark objects above the threshold with a black circle if metric > threshold centroid = stats(k).Centroid; plot(centroid(1),centroid(2),'ko'); end text(boundary(1,2)-35,boundary(1,1)+13,metric_string,'Color','y',... 'FontSize',14,'FontWeight','bold'); end title(['Metrics closer to 1 indicate that ',... 'the object is approximately round']);

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