automatic calculation of length of object in a binary image

I have the following binary image:

http://www.4shared.com/download/BozvHQcHba/untitled2.jpg?lgfp=3000

I manually select the start and end points using the ruler in imtool to get the length. Is there a way to automatically get the length i.e the first white pixel to last white pixel (longest length) without doing it manually.

Answer 1

[Image Analyst]

The most robust way would be to call bwboundaries and run through the coordinates finding which two are farthest from each other. Using MajorAxisLength in regionprops() won't be as accurate as that since it fits the blob to an ellipse.

boundaries = bwboundaries(binaryImage);
x = boundaries(:, 1);
y = boundaries(:, 2);
maxDistance = -inf;
for index1 = 1 : length(x)
  for index2 = 1 : length(y)
    deltaX = x(index1) - x(index2);
    deltaY = y(index1) - y(index2);
    distance = sqrt(deltaX^2+deltaY^2);
    if distance > maxDistance

and so on. I trust you know how to finish it.

[Sonalika]

Thanks a lot! Yes I get the logic now.

[Yan Zhao]

thanks for your answer. I find a easy way accordingly:

D = pdist2(boundaries,boundaries);

maxDistance = max(max(D));

[Image Analyst]

Yes, if you have the Statistics and Machine Learning Toolbox, you can use that function.