2D slice from extruded polygon after substraction

[Ion]

Hello

First of all, would like to thank the developers who made this framework possible.

I am starting off with a linearly extruded polygon and then subtract a cube out of it.

Then I want to get a 2d slice in vertical plane. To achieve this I am going through all polygons and all vertices from a very thin slice ignoring all polygons with same z-axis (walls) and extract the x,y and ignore the z altogether.

this.profile3D.polygons.forEach(function (polygon, index) {
      let local_this2 = local_this

      polygon.vertices.forEach(function (v, i) {
        let av_arr = polygon.vertices.map(function (x) {
          return parseInt(x[1] as any)
        }) as Array<number>

        let is_vertical = new Set(av_arr).size < 2
        if (!is_vertical) {
          local_this2.profile2D.push([v[0], v[2]])
        }
      })
    })
  }

Finally I sort of get the 2d slice but with some points "tangled" (shape front end)

How can I make sense of the mesh's structure to be able to walk the mesh so points are in order and correctly joined?
I've excluded the "walls". The mesh seems to be a 4 pct polygon and sometimes a 3 point polygon. More complex shapes turn out more tangled points.
Is there a common layout of the mesh that I can lookup somewhere else? I have tried checkout out the code but could not figure it out.

I have also tried extracting the x,y of any 2 corners of the 4 pct polygon which are not "vertical walls/sides"

this.profile3D.polygons.forEach(function (polygon, index) {
      let local_this2 = local_this

      polygon.vertices.forEach(function (v, i) {
        let av_arr = polygon.vertices.map(function (x) {
          return parseInt(x[1] as any)
        }) as Array<number>

        let is_vertical = new Set(av_arr).size < 2
        if (!is_vertical) {
          local_this2.profile2D.push([polygon.vertices[2][0], polygon.vertices[0][2]])
        }
      })
    })
  }

Artefacts like this started to come out but no luck with any combination.

Any suggestions are appreciated. Thanks!
P.S Wish my skills would be good enough to contribute.

[z3dev]

@Ion Welcome!

You are already doing things that 'normal' designers don't attempt.

There's a piece of code that does a very simular thing, converting 3D polgons (from boolean operations) to 2D sides. This might help you to understand a little more about the 2D geometry (geom2).

https://github.com/jscad/OpenJSCAD.org/blob/V2/packages/modeling/src/operations/booleans/fromFakePolygons.js

[gilboonet]

Hello, did you try to unify the polygons ? I have some code that does the same kind of thin slicing and the 2d slice is an union of the polygons.

[Ion 0]

@gilboonet The whole 2d contour is one single polygon of points. where should i get multiple polygons from? from the sides?

[Ion 0]

@z3dev thanks!

[gilboonet]

@Ion-0 If I approximate your design, and get a slice of it, I have a set of 3 polygons

[Ion 0]

@z3dev
I have got it working using the routine pointed at by you. it was a bit confusing as it throws error for polygons with more than "2 dimension" and the axis was different.

[Ion 0]

@gilboonet Thanks for the response. I believe you mean the "sides" are rendered using polygons. indeed they are lots of polygons. I might give that a try using the 2d constructive geometry. i've gotten the result right using the fromfakepolygons routine.

[gilboonet]

@Ion-0 That's great !