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I want to create a picture like this:

enter image description here

This picture was found in Regular Polygon Divisionby Diagonals.

But these codes are too old to run.

I think I can use Graphics`Mesh`FindIntersections to find all the intersection points.

lines=Line/@Tuples[CirclePoints[7],2];
pts=Graphics`Mesh`FindIntersections[lines]
Graphics[{{Thickness[0.005],lines},{Directive[Red,PointSize[0.02]],Point[pts]}}]

enter image description here

Looks it meets a bug. I need another way or try to fix it.

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    $\begingroup$ Indeed, there seems to be a problem with Graphics`Mesh`FindIntersections[]. In the meantime: use Subsets[] instead of Tuples[]: Line /@ Subsets[CirclePoints[7], {2}] $\endgroup$ Commented Nov 13, 2017 at 5:33
  • $\begingroup$ Still have bug when $n$ is bigger, just try $n=8$ . $\endgroup$
    – Aster
    Commented Nov 13, 2017 at 5:49

1 Answer 1

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This is a brute-force solution that works in version 11.1, but not in the latest version:

lines = Line /@ Subsets[CirclePoints[7], {2}];
pts = DeleteCases[RegionIntersection @@@ Subsets[lines, {2}], _EmptyRegion];

Graphics[{lines, Directive[Red, PointSize[Large]], pts}]

intersection points on the complete graph

In 11.2, the corner points are missed:

missing points

because RegionIntersection[] apparently lost the ability to find intersections of two line segments sharing an endpoint in 11.2:

RegionIntersection[Line[{{0, 0}, {1, 0}}], Line[{{0, 0}, {0, 1}}]]
   EmptyRegion[2]

where 11.1 would give the result Point[{{0, 0}}].

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    $\begingroup$ Please report.. $\endgroup$
    – yode
    Commented Nov 13, 2017 at 6:18
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    $\begingroup$ Just did.$\phantom{}$ $\endgroup$ Commented Nov 13, 2017 at 6:20
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    $\begingroup$ For workaround, you can convert coords to numerical values. lines = N[lines]. $\endgroup$
    – halmir
    Commented Nov 13, 2017 at 14:04

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