Get SpatialPolygons after applying buffer width

Get SpatialPolygons after applying buffer width

get_sp_buffer(
  sp,
  sp_buffer = -0.5,
  steps = 50,
  relative_size = TRUE,
  verbose = FALSE,
  ...
)

Arguments

sp

sp::SpatialPolygons or equivalent object as input.

sp_buffer

numeric value, usually negative, which will create a buffer inside the sp polygon.

steps

integer number of steps to iterate between sp_buffer and 0.

verbose

logical indicating whether to print verbose output.

...

additional arguments are ignored.

relative

logical indicating whether sp_buffer is a fraction of the buffer width required to reduce the sp polygon to zero area.

Details

This function is intended to help apply negative buffer width, where the intended buffer width may result in a zero-size polygon, in which case a progressively smaller buffer width is applied until a polygon can be returned with non-zero area.

An added bonus, when relative=TRUE it will determine the actual buffer value that would barely result in non-zero area, and define that as 1x buffer, and the sp_buffer will be scaled to that value. So sp_buffer=-0.5 will return an interior polygon at half the width it would take to reduce the polygon to zero size. See examples.

Ultimately if no buffer can be applied, then the input sp object is returned without change.

This function is useful to draw a line segment from outside into a polygon, without drawing it to the center point of the polygon, where a text label may already be positioned.

See also

Other venndir spatial: degrees_to_adj(), diff_degrees(), display_angles(), get_largest_polygon(), intersect_polygons(), mean_degree_arc(), mean_degrees(), nudge_sp(), rescale_coordinates(), rescale_sp(), sp_circles(), sp_ellipses(), spread_degrees(), union_polygons()

Examples

sp <- get_venn_shapes(counts=c(A=1))
plot(sp, col="red")
sp_inside <- get_sp_buffer(sp, -100, relative=FALSE)
plot(sp_inside, col="cornsilk", lty="dotted", add=TRUE);


# by default relative=TRUE
sp1 <- sp_ellipses(c(3.5, 2), c(2, 3), xradius=c(2, 3), yradius=c(3, 1))
sp <- rgeos::gDifference(sp1[1], sp1[2]);
opar <- par("mfrow"=c(2,2));
on.exit(par(opar));
for (i in -c(0.9, 0.75, 0.5, 0.25)) {
   plot(sp, col="red",
      main=paste0("sp_buffer=", i));
   plot(get_sp_buffer(sp, i, relative=TRUE),
      col="cornsilk", lty="dotted", add=TRUE);
}




par(opar);