Coverer.CellUnion(Point) can have multiple cells?

[jsmorph]

In S2, in some cases, a Coverer.CellUnion() of a Point can contain two cells. Is that the correct behavior? If so, what's the story?

Example:

package main

import (
    "fmt"

    "github.com/golang/geo/s2"
)

func main() {
    var (
        level   = 7
        coverer = s2.RegionCoverer{
            MinLevel: level,
            MaxLevel: level,
        }
        ll    = s2.LatLng{Lat: -0.8248776721189366, Lng: -2.038611228784389}
        p     = s2.PointFromLatLng(ll)
        union = coverer.CellUnion(p)
    )
    fmt.Println(len(union))
}

At the playground.

[jsmorph]

5/0030111 and 5/0101000 both contain s2.LatLng{Lat: -0.8248776721189366, Lng: -2.038611228784389} per the playground. Surprised me!

[rsned]

The RegionCoverer operates on Regions, and even though you are only using a single Point, it is interpreted as a Region. The Coverer uses the CellUnionBound() for the given Point which is a CellID slice of (top left, top right, bottom left, bottom right) as the bounding box on the point at the lowest level (30).

Even though the bounding box is microscopic, it does cross over the boundary leading to it being in two cells at the same time. You can see in the attached images a view of the two level 7 cells with a tiny tiny tiny spot. Zooming in, the spot turns out to be the 4 cells.

fmt.Printf("%+v\n", p.CellUnionBound()):

[5/01010003233230110323000103222 5/01010003233230110323000100111

5/00301110322301001032333010333 5/00301110322301001032333011000]

You happened to get lucky (or unlucky) in that the LatLng you used happened to be close enough to the edge of the two cells so that it's bounding box spilled over.

-- Robert

On Sun, Jan 19, 2020 at 9:47 PM Jamie Stephens notifications@github.com wrote:

5/0030111 and 5/0101000 both contain s2.LatLng{Lat: -0.8248776721189366, Lng: -2.038611228784389} per the playground https://play.golang.org/p/DKMl-pYCoea. Surprised me!

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[rsned]

[jsmorph]

@rsned, thanks for the quick, detailed response.

For posterity:

Since Cell.ContainsPoint(Point) reports true for those two cells and that point, I decided to read the documentation, which says to convert a Cell to a Loop. That works.