ST_DFullyWithin

[Paul Ramsey]

https://github.com/postgis/postgis/pull/757

On Wed, Jan 24, 2024 at 12:53 PM Paul Ramsey <pramsey at cleverelephant.ca> wrote:
>
> In fixing https://trac.osgeo.org/postgis/ticket/5639, it has become clear to me that our definition of ST_DFullyWithin is probably “wrong”, that is, it’s not what people think they are getting when they call it.
>
> Right now, ST_DFullyWithin(A, B, radius) can be expressed as:
>   ST_MaxDistance(A, B) < radius
>
> However, the distance https://postgis.net/docs/ST_MaxDistance.html returns is the maximum distance between the vertices of A and the vertices of B. In the limit, for example, the ST_MaxDistance(A, A) is not zero, it’s the longest distance between any two vertices of A. This is fine, there’s a place for this number, but applied to ST_DFullyWithin, it doesn’t return what I think people expect.
>
> Another way of expressing ST_DWithin(A, B, radius) is:
>   ST_Intersects(ST_Buffer(A, radius), B)
>
> Another way of expressing what I *think* people expect from ST_DFullyWithin(A, B, radius) is:
>   ST_Contains(ST_Buffer(A), B)
>
> The canonical use case of something like ST_DFullyWithin(A, B, radius) would be checking to see if two line segments were “similar”, so a road conflation problem.  The current implementation will very much not help with that.
>
> I propose changing ST_DFullyWithin to
> (a) apply the buffer recipe
> (b) once a new version of GEOS with an oriented hausdorf is available, use that implementation instead to calculate the distance metric the buffer recipe implies
>
> The function will remain index-enabled, fortunately that logic still makes sense.
>
> P