How to detect the boundary (binary image)

How do I write a program to delete the line L1 (binary image) and make the red line around the boundary area (B2) with the matlab language. From [e-mail]

Answer 1

[Image Analyst]

One way is to call imclose(), then cast the image to double and then blur it with conv2() and threshold it. Then remove any holes that remain.

mask = imclose(mask, true(7));
mask = conv2(double(mask), ones(3), 'same') > 3; % Adjust number to control the size of the new mask.
mask = imfill(mask, 'holes'); % Remove any interior holes.

Another way might be to use activecontour(). See attached demo. Attach your image if you can't figure it out.