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.


enter image description here

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

  • 1
    $\begingroup$ Indeed, there seems to be a problem with Graphics`Mesh`FindIntersections[]. In the meantime: use Subsets[] instead of Tuples[]: Line /@ Subsets[CirclePoints[7], {2}] $\endgroup$ – J. M. will be back soon Nov 13 '17 at 5:33
  • $\begingroup$ Still have bug when $n$ is bigger, just try $n=8$ . $\endgroup$ – GalAster Nov 13 '17 at 5:49

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}}]]

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

  • 1
    $\begingroup$ Please report.. $\endgroup$ – yode Nov 13 '17 at 6:18
  • 2
    $\begingroup$ Just did.$\phantom{}$ $\endgroup$ – J. M. will be back soon Nov 13 '17 at 6:20
  • 4
    $\begingroup$ For workaround, you can convert coords to numerical values. lines = N[lines]. $\endgroup$ – halmir Nov 13 '17 at 14:04

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