While a polygon vector can hold more or less any combination of rings, only a certain subset of these are valid and will produce correct answer to the various functions. Algorithms that require a valid polygon will attempt to fix invalid polygons beforehand, but it is often better to do it once after construction to avoid repeated repairs every time an algorithm is called on the polygon.
Value
is_valid() returns a logical vector of the same length as x.
make_valid() returns a polyclid_polygon() vector of the same length as
x.
Details
The requirement for a valid polygon in polyclid are as follow:
The outer boundary must be relatively simple and all holes must be simple
The outer boundary must be oriented counter-clockwise and all holes must be clockwise
The holes and the outer boundary must be pairwise disjoint except at vertices. All holes must be contained inside the outer boundary and holes must not partly or fully overlap
make_valid() attempts to correct an invalid polygon in the following way:
Checks if the outer boundary is relatively simple. If not it returns
NA.Checks and corrects the orientation of the outer boundery.
Checks if holes are simple. If any are not it returns
NA.Checks and fixes the orientation of all holes.
One by one uses boolean difference to cut out holes in the boundary. This means that holes outside the boundary are ignored, overlapping holes are merged, and holes intersecting with the boundary will become part of the boundary.
If 5 has resulted in cutting up the polygon in multiple disjoint parts only one will be returned. If the polygon remains connected then that is of course returned.
When calling is_valid() the result is cached in the polygon to avoid
repeated checks. Any modification of the polygon will of course clear the
cache. If a polygon has a cached result that indicates that the polygon is
valid it is returned as is.
Examples
poly <- polygon(
c(0, 10, 10, 0),
c(10, 10, 0, 0)
)
# Outer boundary must be counterclockwise for the polygon to be valid
is_clockwise(poly)
#> [1] TRUE
is_valid(poly)
#> [1] FALSE
poly <- reverse_orientation(poly)
is_valid(poly)
#> [1] TRUE
h <- polygon(
c(2, 4, 2),
c(2, 2, 4)
)
# Holes must be clockwise
is_counterclockwise(h)
#> [1] TRUE
hole(poly) <- h
is_valid(poly)
#> [1] FALSE
hole(poly, 1) <- reverse_orientation(h)
is_valid(poly)
#> [1] TRUE
# Holes must be completely contained and non-overlapping with the boundary
h <- as_polygon(circle(point(8, 8), 15))
plot(c(poly, h))
hole(poly) <- h
is_valid(poly)
#> [1] FALSE
# Use make_valid() to resolve holes (and orientation)
poly <- make_valid(poly)
plot(poly)
is_valid(poly)
#> [1] TRUE
# not all polygons can be made valid, specifically rings that are not
# relatively simple cannot be fixed
poly <- polygon(
c(0, 10, 10, 0),
c(0, 10, 0, 10)
)
is_valid(poly)
#> [1] FALSE
plot(poly, force_valid = FALSE)
# Trying to make them valid will return NA
make_valid(poly)
#> <2D polyclid_polygons [1]>
#> [1] <NA>