What are algorithms for computing contours from given edges? Let's say I have a set of edges which makes the contour, however there are too many of them and some are too long. I have to remove edges, and shorten them to make contour. I cannot add new edges!

I am looking at least for the name of the algorithm to further look how this problem is solved.

Here is the example (please don't mind the grid) -- input:



and output in pink (the pink is the real output, the rest is left for comparison -- it should be removed by algorithm I am looking for):



As you can see I have a lot of redundant edges, too long, internal ones.
Note also that this is not the convex hull problem; my shape is not convex.

The question: what are known algorithms for solving this problem?
This problem is not surprisingly called Concave Hull
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CONCAVE HULL: A K-NEAREST NEIGHBOURS APPROACH FOR THE COMPUTATION OF THE REGION OCCUPIED BY A SET OF POINTS by Adriano Moreira and Maribel Yasmina Santos