Compute direction of points in a polygon.
This is (private, not exported) R code replacing the call to maptools:::.ringDirxy, which links to C code.
.ringDirxy(xy)
xy |
a two-column matrix with x and y coordinates of at least 3 points which form a polygon |
Returns the integer -1 if the direction is counter-clockwise and 1 if the direction is clockwise.
The function uses vectorization and should not be very much slower than the original.
Daniel Sabanes Bove
## create test polygon ordered clockwise
nPoints <- 200
sequence <- seq(from=1, to=10, length=nPoints)
firstHalf <- sequence[1:(nPoints/2)]
test <- cbind(x=c(sequence, rev(sequence)),
y=c(firstHalf, rev(firstHalf), -firstHalf, -rev(firstHalf)))
plot(test,
type="n")
polygon(test)
## compute direction with this and the original function
stopifnot(identical(BayesX:::.ringDirxy(test), as.integer(+1)))
## Not run: maptools:::.ringDirxy(test)
## now transform to counter-clockwise and try that
test[, 2] <- - test[, 2]
stopifnot(identical(BayesX:::.ringDirxy(test), as.integer(-1)))
## Not run: maptools:::.ringDirxy(test)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.