How do i find the coordinates of a boundary

Hi, I am needing to find the length of only the line circled below. I am thinking if i can get the coorindates of the four corners of the boundary i would be able to subtract away the unwanted sides from the perimeter of the boundary, thus giving me the distance desired.

I = imread('ss3.png');
% imshow(I)
hold on
J=rgb2gray(I);
BW1=edge(J,'sobel',.009);

% imshow(imfill(BW1,'holes'))

mask = imclose(BW1, true(7));
mask = conv2(double(mask), ones(3), 'same') > 3.1; % Adjust number to control the size of the new mask.
mask = imfill(mask, 'holes'); % Remove any interior holes
% imshow(mask,'InitialMagnification', 'fit')


%Auto crop image using set points----------------------------------------
I2 = imcrop(mask,[407.51 0.51 83.98 1037.98]);
imshow(I2,'InitialMagnification', 'fit')
% 407.51 0.51 83.98 1037.98

 
I3 = bwareafilt(I2,[40 100000]);
% imshow(I3)
 

%boundary outline and trace -----------------------------------------
[B,L] = bwboundaries(I3,'noholes');
imshow(label2rgb(L, @gray, [0 0 0]))
hold on
for k = 1:length(B)
   boundary = B{k};
   plot(boundary(:,2), boundary(:,1), 'g', 'LineWidth', 0.5)
end

% numPixels(k) = size(boundary, 1)

[Matt J]

You need to attach ss3.png for us to run the code.

Answer 1

[Matt J]

How about simply,

[m,n]=size(Image);
I=boundary(:,1); J=boundary(:,2);


corner1 = [1, max(J(I==1))]; %row/column coordinates
corner2 = [m, max(J(I==m))];

corner3=[m,1];
corner4=[1,1];