Polygon orientation

Polygon rings can have two different orientations: clockwise and counterclockwise. In a clockwise oriented polygon the polygon lies to the right as you traverse the boundary in order, while the reverse is true for a counterclockwise oriented polygon. Both orientations are basically valide representations of a polygon but many algorithms assumes a certain orientation of the boundary and holes in order to work. The outer boundary must be counterclockwise and all holes must be clockwise. This will ensure that the polygon always lies to the left as you traverse all the borders of the polygon. reverse_orientation() reverses the orientation of both the outer boundary and the holes. is_clockwise() and is_counterclockwise() only reports the orientation of the outer boundary. To get the orientation of a hole, first extract it with hole().

Usage

reverse_orientation(x)

is_clockwise(x)

is_counterclockwise(x)

Arguments

x

A polyclid_polygon vector

Value

A new polyclid_polygon vector for reverse_orientation() and a logical vector for is_clockwise() and is_conterclockwise()

Examples

poly <- polygon(
  c(0, 10, 10, 0),
  c(10, 10, 0, 0)
)

is_clockwise(poly)
#> [1] TRUE
poly <- reverse_orientation(poly)
is_counterclockwise(poly)
#> [1] TRUE