Identifies which cells are pentagons. A hexagonal grid on the icosahedron has exactly 12 pentagons, one at each icosahedron vertex, with 5 neighbors instead of 6. A grid on the octahedron has none: its six vertex cells are squares.
Details
H3 backend: Uses the vendored H3 isPentagon function.
ISEA backend: A vertex cell sits at a vertex of the solid, which is the (i, j) = (0, 0) cell of its quad, and has one side per face meeting there. Each input cell ID is decoded to its own (quad, i, j) and tested, rather than by re-deriving each vertex's cell ID (the forward direction has aperture-specific quirks – e.g. aperture 7's substrate/surrogate coordinate distinction – that make a single fixed formula for "the (0,0) cell ID of quad Q" unreliable across apertures).
See also
get_neighbors() for neighbor finding (pentagons have 5 neighbors)
Examples
# \donttest{
# H3 pentagon detection
g <- hex_grid(resolution = 1, type = "h3")
cells <- grid_global(g)
pent <- is_pentagon(cells$cell_id, g)
sum(pent) # Should be 12
# }