R/simutil-rotate_points.r

In jsta/glatos: A package for the Great Lakes Acoustic Telemetry Observation System

#' @title Rotate points in a 2-d plane
#' 
#' @description
#' Rotate points around a point in a 2-d plane
#'
#' @param x A numeric vector of x coordinates; minimum of 2.
#' @param y A numeric vector of y coordinates; minimum of 2.
#' @param theta A numeric scalar with the angle of rotation in degrees; 
#'   positive is clockwise.
#' @param focus A numeric vector of x (first element) and y (second element) 
#' coordinates for the point around which \code{x} and \code{y} will rotate.
#'
#' @details
#' Points are shifted to be centered at the focus, then rotated using a
#' rotation matrix, then shifted back to original focus.
#' 
#' @return A two-column data frame containing:
#' \item{x}{x coordinates}
#' \item{y}{y coordinates}
#'
#' @note
#' This function is called from \code{\link{crw_in_polygon}}
#' 
#' @author C. Holbrook (cholbrook@usgs.gov) 
#'
#' @examples
#' x <- runif(10,0,10)
#' y <- runif(10,0,10)
#' plot(x,y,type="b",pch=20)
#' foo <- rotate_points(x, y, 20, c(5, 5))
#' points(foo$x,foo$y,type="b",pch=20,col="red")
#'
#' @export
rotate_points <- function(x, y, theta, focus){
	pos <- cbind(x,y) #original points
	theta.rad <- theta*(pi/(180)) #convert to radians
	focus <- as.numeric(focus) #coerce to vector if not
	#make rotation matrix - for clockwise rotation
	rot <- matrix(c(cos(theta.rad), -sin(theta.rad), sin(theta.rad), cos(theta.rad)),ncol=2)
	pos[,1] <- pos[,1] - focus[1] #shift points to center at focus
	pos[,2] <- pos[,2] - focus[2] #shift points to center at focus
	pos <- t(rot%*%t(pos)) #rotate
	pos[,1] <- pos[,1] + focus[1] #shift back
	pos[,2] <- pos[,2] + focus[2] #shift back
	colnames(pos) <- c("x","y")
	return(as.data.frame(pos))
}