Length of a thin curved object in an image

I’d like to use image analysis to calculate the length of a thread.

Because the thread is not entirely straight, I can’t just use regionprops, so I think the best option is to use the function bwdistgeodesic

In the bwdistgeodesic function, what is it expecting for the variable “mask”? How could I use image analysis to generate said mask? I’ve looked into using regionprops ‘Extrema’ property, but since I’m not sure what the “mask” variable is in bwdistgeodesic, I’m not sure how I would use the x,y dimensions from ‘Extrema’ in a mask.

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Because the thread is not entirely straight, I can’t just use regionprops,

Why not? Couldn’t you just do

S=regionprops(  bwareafilt(BW,1,'smallest') ,'Perimeter');
    length=S.Perimeter/2

Length of a thin curved object in an image