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Concave and convex hulls for LAS objects. st_convex_hull extends sf::st_convex_hull for LAS
objects. Both functions return a sfc_POLYGON. concaveman is very a fast 2D concave hull algorithm
for a set of points.
st_concave_hull(x, method = "concaveman", ...)# S3 method for LAS
st_convex_hull(x)
concaveman(x, y = NULL, concavity = 2, length_threshold = 0)
A sfc_POLYGON from sf or a data.frame in the case of concaveman
An object of class LAS or XY coordinates of points in case of concaveman. This can be
specified as two vectors x and y, a 2-column matrix x, a list with two components, etc.
string. currently supports "concaveman".
Propagate to the method.
numeric a relative measure of concavity. 1 results in a relatively detailed shape, Infinity results in a convex hull. You can use values lower than 1, but they can produce pretty crazy shapes.
numeric. When a segment length is below this threshold, it stops being considered for further detailed processing. Higher values result in simpler shapes.
The concaveman algorithm is based on ideas from Park and Oh (2012). A first implementation in JavaScript was proposed by Vladimir Agafonkin in mapbox. This implementation dramatically improved performance over the one stated in the paper using a spatial index. The algorithm was then ported to R by Joël Gombin in the R package concaveman that runs the JavaScript implementation proposed by Vladimir Agafonkin. Later, a C++ version of Vladimir Agafonkin's JavaScript implementation was proposed by Stanislaw Adaszewski in concaveman-cpp. This concaveman function uses Stanislaw Adaszewski's C++ code making the concaveman algorithm an order of magnitude (up to 50 times) faster than the Javascript version.
Park, J.-S & Oh, S.-J. (2013). A New Concave Hull Algorithm and Concaveness Measure for n-dimensional Datasets. Journal of Information Science and Engineering. 29. 379-392.
x <- runif(35)
y <- runif(35)
hull <- concaveman(x,y)
plot(x,y, asp = 1)
lines(hull, lwd = 3, col = "red")
LASfile <- system.file("extdata", "Megaplot.laz", package="lidR")
las = readLAS(LASfile, filter = "-drop_z_below 1")
hull = st_concave_hull(las, length_threshold = 10)
plot(hull)
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