Apply a winding direction to polygon rings

Rewinds each polygon so its exterior and interior rings follow a consistent direction. ga_orient() keeps the geometry type of the input.

Usage

ga_orient(geometry, direction = "default")

Arguments

geometry: a GeoArrow polygon or multipolygon array
direction: one of "default", "reversed", "ccw", or "cw"

Value

a GeoArrow array of the same geometry type as geometry

Details

With "default", ga_orient() winds the exterior ring counter-clockwise and the interior rings clockwise, which is the winding the OGC simple features and GeoJSON specifications call for. With "reversed" it does the opposite. It accepts "ccw" and "cw" as aliases, which refer to the exterior ring.

ga_orient() does not change which points a polygon covers, only the order of its coordinates. It keeps a null geometry null.

References

Orient

Examples

# a clockwise square
ring <- rbind(c(0, 0), c(0, 1), c(1, 1), c(1, 0), c(0, 0))
p <- sf::st_polygon(list(ring))
g <- geoarrow::as_geoarrow_array(sf::st_sfc(p))

ga_winding_order(g)
#> <nanoarrow_array string[1]>
#>  $ length    : int 1
#>  $ null_count: int 0
#>  $ offset    : int 0
#>  $ buffers   :List of 3
#>   ..$ :<nanoarrow_buffer validity<bool>[null] ``
#>   ..$ :<nanoarrow_buffer data_offset<int32>[2][8 b]> `0 9`
#>   ..$ :<nanoarrow_buffer data<string>[9 b]> `clockwise`
#>  $ dictionary: NULL
#>  $ children  : list()
ga_winding_order(ga_orient(g, "default"))
#> <nanoarrow_array string[1]>
#>  $ length    : int 1
#>  $ null_count: int 0
#>  $ offset    : int 0
#>  $ buffers   :List of 3
#>   ..$ :<nanoarrow_buffer validity<bool>[null] ``
#>   ..$ :<nanoarrow_buffer data_offset<int32>[2][8 b]> `0 16`
#>   ..$ :<nanoarrow_buffer data<string>[16 b]> `counterclockwise`
#>  $ dictionary: NULL
#>  $ children  : list()