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Graph visualizers, such as Graphviz, can take a formal representation of the nodes and edges of a graph and produce an image from them. What's a program or programming library that can do the reverse; that is, recognize an image of a graph and produce a formal representation? I would be interested in software for recognizing a bitmap as well as the easier case of recognizing a graph from an SVG that follows a standard structure. Either way, restrictions on the graph's appearance are okay. Also, I require only the extraction of node-and-edge relationships, not other things like text labels of nodes.

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This can be done with OpenCV and some custom code in C or python, (it is not a very common requirement).

The answers to this question do a great job of discussing how to go about this in C.

For a ready rolled solution you might wish to take a look at NEFI: enter image description here

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Mathematica has a built-in function called MorphologicalGraph.

The open source image analysis software Fiji can do this too. The rough steps are:

  • Smoothen the image if needed
  • Threshold the image to make it binary
  • Process -> Binary -> Skeletonize
  • Analyze -> Skeleton -> Analyze Skeleton, tick Show Detailed Info. When a table comes up, save it as CSV.
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  • Mathematica is closed-source and hence doesn't match the open-source tag, but Fiji looks like a good suggestion.
    – Kodiologist
    Commented Nov 22, 2018 at 16:01
  • @Kodiologist It is better to spell out the requirements to avoid confusion. Tags are for categorization first, not for communicating requirements.
    – Szabolcs
    Commented Nov 22, 2018 at 16:09

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