Opencv : convert curves to shapely Linestrings

I have a pre-processed image containing strands of white pixels representing curves. Some closed, some not closed. The thickness is 1 pixel

I would like to be able to convert each one of these to a shapely Linestring.

I already tried to use the "findContours" function. This works very well on the closed curves, however on the open ones, opencv returns a closed contour.

This breaks my code later when, for instance, i need to get the length of these curves. (For instance, it roughly returns twice the length of each curve)

Note that i absolutely need linestrings. I can not simply count the number of white pixels in the image.

Is there a simple way to extract these lines using opencv or i will have to write the algorithm myself (i'd really like to avoid this solution as i dont have much time to do it and performance is kind of important for this app)

example image:

I'm not familiar with shapely but you seem to want to get some kind of vector representation, so the following may help. It is a bit long for a comment.

You can use potrace to convert bitmap data to vector format. It accepts input in PNM/PGM/PBM/PPM or BMP format, so I am using ImageMagick to convert your PNG into PGM and then pump it into potrace. I am chopping out the shape at the top-left of your image so you can see the results for a single contour:

magick 6JVH1.png -crop 176x200+0+0 pgm: | potrace - --svg > a.svg

That is cropping out a 176px wide by 200px tall rectangle starting at 0,0 (top-left) and making it into a PGM, then piping into potrace and selecting the SVG backend which I assume is something you can make into a shapely line-string.

The result is this:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20010904//EN"
 "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
<svg version="1.0" xmlns="http://www.w3.org/2000/svg"
 width="176.000000pt" height="200.000000pt" viewBox="0 0 176.000000 200.000000"
 preserveAspectRatio="xMidYMid meet">
<metadata>
Created by potrace 1.16, written by Peter Selinger 2001-2019
</metadata>
<g transform="translate(0.000000,200.000000) scale(0.100000,-0.100000)"
fill="#000000" stroke="none">
<path d="M0 1000 l0 -1000 880 0 880 0 0 1000 0 1000 -880 0 -880 0 0 -1000z
m1150 -165 c6 -71 18 -192 25 -269 31 -288 15 -338 -82 -267 -23 16 -101 86
-174 155 -158 148 -357 341 -285 275 28 -25 114 -105 191 -179 251 -239 324
-294 346 -259 14 22 11 119 -11 329 -28 283 -40 499 -38 715 1 186 1 184 9
-90 4 -154 12 -338 19 -410z"/>
</g>
</svg>