I have a set of four 2d points and want to create a quadrilateral. Currently, I manually order the input points to ensure it is convex. Is there a function in GeometryBasiscs that crates a convex polygon from an unordered set of 2d points? If I don’t sort them properly then the lines intersect like an X

If you have a set of unordered 2d points, that are vertices of a convex polygon, then you must first reorder them

as follows:

- calculate the center of mass, c, of the given set of points;
- calculate the coordinates of vectors \vec{cp}=p-c, where p runs over

the`points`

, given as a vector of vectors; - calculate the angle of each vector \vec{cp}=(x,y) with the positive x-axis, as
`atan(y, x)`

. `sortperm`

the the computed angles, α, and a get the index vector`idx`

.`points[idx]`

represent the ordered points, and now you can plot the associated polygon with your favorite plotting library.

I’m using PlotlyJS:

```
using PlotlyJS, Statistics
function reorderpoints(points)
x= getindex.( points, 1)
y= getindex.( points, 2)
c = [mean(x), mean(y)]
v = [p-c for p in points]
α =[atan(u[2], u[1]) for u in v]
idx=sortperm(α)
points[idx]
end
points= [[-0.35, 1.6],
[1.0, 0.0],
[-0.4, -1.3],
[-1.5, -1.2],
[0.75, 1.5],
[-1.0, 0.8]]
pts =reorderpoints(points)
push!(pts, pts[1])
fig =Plot(scatter(x=getindex.(pts, 1),
y=getindex.(pts, 2),
fill="toself",
fillcolor="RoyalBlue"),
Layout(width=400, height=400))
```

3 Likes