R/coordturn.R

In JVAdams/jvamisc: A Collection of Utility Functions

#' Rotate Coordinates
#'
#' Rotate coordinates clockwise around a pivot point.
#' @param pts
#'   A numeric matrix with two columns of x and y coordinates to be rotated.
#' @param pvt
#'   A numeric vector of length 2, the x and y coordinates of the pivot point
#'   around which \code{pts} will be rotated.
#' @param rot
#'   A numeric scalar indicating the amount of clockwise rotation, in radians.
#' @return
#'   A numeric matrix with same dimension as \code{pts} with the rotated
#'   x and y coordinates.
#' @export
#' @seealso
#'   \code{\link[jvamisc]{coordplot}}, \code{\link[jvamisc]{coordmove}},
#'   \code{\link[jvamisc]{coordflip}}, \code{\link[jvamisc]{coordtri}}.
#' @references
#'   Modification of code posted by Sage on 3 March 2011 on the website benn.org
#'   \href{http://benn.org/2007/01/06/rotating-coordinates-around-a-centre/}{[link]}.
#' @examples
#' # starting coordinates
#' test <- matrix(c(0, 4, 1, 0, 2, 3), ncol=2,
#'  dimnames=list(LETTERS[1:3], NULL))
#' coordplot(test)
#' # rotate the coordinates clockwise 45 degrees
#' # around the first point (the origin)
#' rottest <- coordturn(test, test[1, ], rot=pi/4)
#' coordplot(rottest)
#' # rotate the coordinates counterclockwise 45 degrees
#' # around the first point (the origin)
#' rottest <- coordturn(test, test[1, ], rot=-pi/4)
#' coordplot(rottest)

coordturn <- function(pts, pvt, rot) {
  r1 <- -rot
  xRot <- pvt[1] + cos(r1) * (pts[, 1] - pvt[1]) - sin(r1) * (pts[, 2] - pvt[2])
   yRot <- pvt[2] + sin(r1) * (pts[, 1] - pvt[1]) + cos(r1) * (pts[, 2] - pvt[2])
  m <- cbind(xRot, yRot)
  dimnames(m) <- dimnames(pts)
  m
}