MATLAB: How to visually isolate and measure a round object within the MATLAB image

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How can I visually isolate and measure a round object within my MATLAB image?
Do you have an example that shows how to do the following in MATLAB:
1. Select a round object in the image
2. Fill the rest of image with a background color
3. Draw an outline around the object
4. Find the diameter and center of the object

Best Answer

Here is an example on how to visually isolate and measure a round object within an image in MATLAB:
 
I = imread('eight.tif');
imshow(I)
BW = roipoly(I);
% manually select region here
BW1 = not(BW);
J = roifill(I,BW1); imshow(J)
% using ipexsegcell demo
% type 'ipexsegcell' from the MATLAB command prompt
% to view documentation for this demo
BWs = edge(J, 'sobel', (graythresh(I) * .1));
figure, imshow(BWs), title('binary gradient mask');
se90 = strel('line', 3, 90);
se0 = strel('line', 3, 0);
BWsdil = imdilate(BWs, [se90 se0]);
figure, imshow(BWsdil), title('dilated gradient mask');
BWdfill = imfill(BWsdil, 'holes');
figure, imshow(BWdfill);
title('binary image with filled holes');
BWnobord = imclearborder(BWdfill, 4);
figure, imshow(BWnobord), title('cleared border image');
seD = strel('diamond',1);
BWfinal = imerode(BWnobord,seD);
BWfinal = imerode(BWfinal,seD);
figure, imshow(BWfinal), title('segmented image');
BWoutline = bwperim(BWfinal);
Segout = I; 
Segout(BWoutline) = 255; 
figure, imshow(Segout), title('outlined original image');
% find diameter and center of the selected coin
[i,j] = find(BWfinal);
i1 = unique(i);
j1 = unique(j);
diameter = max(i)-min(i)
center_i = floor(mean(i1))
center_j = floor(mean(j1))
Additionally, another demonstration you can look into is the following MathWorks FileExchange submission:
http://www.mathworks.com/matlabcentral/fileexchange/25157-image-segmentation-tutorial
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