contourLines Calculate Contour Lines

Description

Calculate contour lines for a given set of data.

Usage

contourLines(x = seq(0, 1, length.out = nrow(z)),
             y = seq(0, 1, length.out = ncol(z)),
             z, nlevels = 10,
             levels = pretty(range(z, na.rm = TRUE), nlevels))

Arguments

x, y	locations of grid lines at which the values in z are measured. These must be in ascending order. By default, equally spaced values from 0 to 1 are used. If x is a list, its components x$x and x$y are used for x and y, respectively. If the list has component z this is used for z.
z	a matrix containing the values to be plotted (NAs are allowed). Note that x can be used instead of z for convenience.
nlevels	number of contour levels desired iff levels is not supplied.
levels	numeric vector of levels at which to draw contour lines.

Details

contourLines draws nothing, but returns a set of contour lines.

There is currently no documentation about the algorithm. The source code is in ‘R_HOME/src/main/plot3d.c’.

Value

A list of contours, each itself a list with elements:

level	The contour level.
x	The x-coordinates of the contour.
y	The y-coordinates of the contour.

See Also

options("max.contour.segments") for the maximal complexity of a single contour line.

contour: Its ‘Examples’ demonstrate how contourLines() can be drawn and are the same (as those from contour()).

Examples

x <- 10*1:nrow(volcano)
y <- 10*1:ncol(volcano)
cl <- contourLines(x, y, volcano)
## summarize the sizes of each the contour lines :
cbind(lev = vapply(cl, `[[`, .5, "level"),
       n  = vapply(cl, function(l) length(l$x), 1))

z <- outer(-9:25, -9:25)
pretty(range(z), 10) # -300 -200 ... 600 700
utils::str(c2 <- contourLines(z))
   # no segments for {-300, 700};
   #  2 segments for {-200, -100, 0}
   #  1 segment  for  100:600