Function S2CellId.expandedByDistanceUV

Expands a rectangle in (u,v)-space so that it contains all points within the given distance of the boundary, and return the smallest such rectangle. If the distance is negative, then instead shrink this rectangle so that it excludes all points within the given absolute distance of the boundary.

static s2.r2rect.R2Rect expandedByDistanceUV (
  in const(s2.r2rect.R2Rect) uv,
  in const(S1Angle) distance
);

Distances are measured *on the sphere*, not in (u,v)-space. For example, you can use this method to expand the (u,v)-bound of an S2CellId so that it contains all points within 5km of the original cell. You can then test whether a point lies within the expanded bounds like this:

R2Point uv; if (S2::FaceXYZtoUV(face, point, &uv) && bound.Contains(uv)) { ... }

Limitations

- Because the rectangle is drawn on one of the six cube-face planes (i.e., {x,y,z} = +/-1), it can cover at most one hemisphere. This limits the maximum amount that a rectangle can be expanded. For example, S2CellId bounds can be expanded safely by at most 45 degrees (about 5000 km on the Earth's surface).

- The implementation is not exact for negative distances. The resulting rectangle will exclude all points within the given distance of the boundary but may be slightly smaller than necessary.