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I would like to identify if a polygon is open then close it

Sean G
Posted 3 years ago

Hi All,

I'm still very much a beginner in Mathematica and have struggled with this one.
I would like to load an image, identify polygons, then if any polygons are open, close them with a line or a curve, then save the new image.
I have no idea even how to approach this.
Any advice would be appreciated.
POSTED BY: Sean G
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That is a nice problem! Here is one way to separate single candidates for polygons and close them:

img = Import[
   "https://community.wolfram.com//c/portal/getImageAttachment?filename=shapes.jpg&userId=1703851"];
pts = PixelValuePositions[DeleteSmallComponents[Thinning@ColorNegate@Binarize[img], 10], 1];
clusters = FindClusters[pts, Method -> "SpanningTree"];
orders = Last@*FindShortestTour /@ clusters;
polygs = MapThread[Polygon[#1[[#2]]] &, {clusters, orders}];
grph=Graphics[{Orange, EdgeForm[{Thick, Black}], polygs}]

enter image description here

This can be exported as image like so:

Export["ClusterPolygs.png", grph]

Regards -- Henrik

POSTED BY: Henrik Schachner

Well, thank you, Rohit! But it is rather Mathematica which makes it really easy and elegant!

Best Regards --Henrik
POSTED BY: Henrik Schachner

Ahmed,

I was wondering why there are 8 clusters ...

well, that is strange! This effect seems to be depending on the version of MMA:

  • on 13.1 (where the code was originally written) you get 7 clusters;
  • on 13.2 you actually get 8 clusters - as you did;

One not really elegant solution is to explicitly input the number of clusters:

clusters = FindClusters[pts, 7, Method -> "SpanningTree"]

Maybe anyone has a better method.
POSTED BY: Henrik Schachner

Same here. Thank you, Henrik, for the tip.
POSTED BY: Ahmed Elbanna
Sean G
Sean G
Posted 3 years ago

Thanks again Henrik. Lots of ideas to play around with!
POSTED BY: Sean G

Ahmed,

here is annother idea that might help separating the clusters in a clean way, just replace this line:

pts = PixelValuePositions[
  Thinning@Dilation[DeleteSmallComponents[ColorNegate@Binarize[img], 10], 3], 
  1]

The dilation of the contours (before Thinning again) makes sure that a single contour is really connected. With this I always get:

clusters = FindClusters[pts, Method -> "SpanningTree"];
Length[clusters]
(*   Out:   7   *)
POSTED BY: Henrik Schachner
POSTED BY: Henrik Schachner

I wrote the exact same code line to get 7 clusters. Anyway, this doesn't affect the final result. Thank you again.
POSTED BY: Ahmed Elbanna
POSTED BY: Ahmed Elbanna
Sean G
Sean G
Posted 3 years ago

I wonder if there are any other approaches to this problem? If anyone has any alternative suggestions, it would be great to hear from you?
POSTED BY: Sean G
Rohit Namjoshi
Rohit Namjoshi
Posted 3 years ago

Hi Henrik

That is a nice problem!

And your solution is very nice too!
POSTED BY: Rohit Namjoshi
Sean G
Sean G
Posted 3 years ago

Wow, that's really elegant. Thanks Henrik!
POSTED BY: Sean G
Sean G
Sean G
Posted 3 years ago

Attached is what I was thinking of.

Image showing open shapes or polygons
POSTED BY: Sean G
Rohit Namjoshi
Rohit Namjoshi
Posted 3 years ago

Can you provide some sample images?
POSTED BY: Rohit Namjoshi
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