MATLAB: How to measure a crack length (using skeleton function)

binary imagebwmorphcrack measurementcracksimage analysismeasurementskeleton

I am trying to calculate the length of the crack (attached) however I can't figure out a way to remove the extra sections of the skeleton that branch off from the main crack path – crackoverlay.png.
Does anyone have any advice as to how I could do this?
clc
clear variables
RGB = imread('crack2.jpg');                                                % Aquires original image
I = rgb2gray(RGB);                                                         % Creates greyscale image
BW = imbinarize(I,'global');                                               % Binarises it
BW2 = ~(bwmorph(BW,'majority',Inf));                                       % Fills in holes
BW3 = bwmorph(BW2,'skel',Inf);                                             % Reduces image to center areas of the white sections
BW4 = bwmorph(BW3,'spur',Inf);                                             % Removes spurs
Ov = imfuse(RGB,BW4);                                                      % Overlay of original image and skeleton
figure('Name','All_Steps')
subplot(3,2,1), imshow(RGB), title('Original Image')
subplot(3,2,2), imshow(BW) , title('Binarised Image')
subplot(3,2,3), imshow(BW2), title('Binarised Inverted')
subplot(3,2,4), imshow(BW3), title('Skeleton')
subplot(3,2,5), imshow(BW4), title('Simplified Skeleton')
subplot(3,2,6), imshow(Ov),  title('Overlay')

Best Answer

Here is my solution code.
Thanks to Adam Danz for the pointers.
Blog with useful code
clc
clear variables
RGB = imread('crack2.jpg');                                                % Aquires original image
I = rgb2gray(RGB);                                                         % Creates greyscale image
BW = imbinarize(I,'global');                                               % Binarises it
BW2 = ~(bwmorph(BW,'majority',Inf));                                       % Fills in holes
BW3 = bwmorph(BW2,'skel',Inf);                                             % Reduces image to center areas of the white sections
BW4 = bwmorph(BW3,'spur',Inf);                                             % Removes spurs
Ov = imfuse(RGB,BW4);                                                      % Overlay of original image and skeleton
imshow(BW4)                                                                % Displays the skeleton de spurred
y1 = 151;                                                                  % Start of crack co-ordinates
x1 = 23;    
y2 = 169;                                                                  % End of crack co-ords
x2 = 307; 
hold on
plot(x1, y1, 'g*', 'MarkerSize', 15)                                       % Puts * at location of start and end of crack
plot(x2, y2, 'g*', 'MarkerSize', 15)
hold off
D1 = bwdistgeodesic(BW4, x1, y1, 'quasi-euclidean');
D2 = bwdistgeodesic(BW4, x2, y2, 'quasi-euclidean');
D = D1 + D2;
D = round(D * 8) / 8;
D(isnan(D)) = inf;
skeleton_path = imregionalmin(D);                                          % Returns regional minima 
P = imoverlay(BW4, imdilate(skeleton_path, ones(3,3)), [1 0 0]);
imshow(P, 'InitialMagnification', 200)
hold on
plot(x1, y1, 'g*', 'MarkerSize', 15)                                       % Start position marker
plot(x2, y2, 'g*', 'MarkerSize', 15)                                       % End poisition marker
hold off
path_length = D(skeleton_path);                                            % Path length calculation
path_length = path_length(1)
figure('Name','Montaged')                                                  % Montage of stages 
montage({RGB, BW, BW2, BW3,BW4, Ov})
figure('Name','All_Steps')
subplot(3,2,1), imshow(RGB), title('Original Image')
subplot(3,2,2), imshow(BW) , title('Binarised Image')
subplot(3,2,3), imshow(BW2), title('Binarised Inverted')
subplot(3,2,4), imshow(BW3), title('Skeleton')
subplot(3,2,5), imshow(BW4), title('Simplified Skeleton')
subplot(3,2,6), imshow(Ov), title('Overlay')
figure('Name','Reduced')
subplot(2,3,1), imshow(RGB), title('Original Image')
subplot(2,3,2), imshow(BW2), title('Binarised Inverted')
subplot(2,3,3), imshow(BW4), title('De-Spurred Skeleton')
subplot(2,3,4), imshow(Ov),  title('Overlay')
subplot(2,3,6), imshow(P),   title('Path on Skeleton')
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