add 2d point buffer

[aryehlev]

adding a buffer for 2d points, -> this method creates a polygon by distributing the number of point equaly around a point #

[twpayne]

Closing due to age.

[SaTae66]

Hey, I would be interested in having this feature in the lib and do the implementation. Although, from your short discussion above I could not infer on how much rework of the above code would actually be required. Would appreciate it if you could give me a TLDR @twpayne. Of course only IF this is a feature you want to have in this lib.

[twpayne]

This code needs a lot of changes before it can be added to the library, and I would instead recommend implementing your own version in your own project specific to your needs instead.

As well as the numerical accumulation errors in the code here, there are several other problems:

• The code here approximates the buffer (which is a circle) with an n-sided regular polygon. However, this consistently underestimates the actual buffer, i.e. the polygon lies entirely within the actual buffer. Depending on how you want to use the buffer, you may wish to increase the radius to minimize the overall error.
• The code here would be more efficient if it used go-geom's flat coords format.
• The code should be in the xy module.

Here's roughly what I think the code should look like:

// RegularPolygon returns an n-sided regular polygon centered at center with radius r.
func RegularPolygon(n int, center *geom.Point, r float64) *geom.Polygon {
    pointFlatCoords := center.FlatCoords()
    stride := center.Stride()
    flatCoords := make([]float64, (n+1)*stride)
    for i := 0; i < n; i++ {
        theta := 2 * math.Pi * float64(i) / float64(n)
        flatCoords[i*stride] = pointFlatCoords[0] + r*math.Cos(theta)
        flatCoords[i*stride+1] = pointFlatCoords[1] + r*math.Sin(theta)
        copy(flatCoords[i*stride+2:(i+1)*stride], pointFlatCoords[2:stride])
    }
    copy(flatCoords[n*stride:(n+1)*stride], flatCoords[:stride])
    return geom.NewPolygonFlat(center.Layout(), flatCoords, []int{(n + 1) * stride})
}

[SaTae66]

Thanks a lot for your input! I will look into the implementation for our internal project only then.