R/hull2line.R

In babichmorrowc/hull2spatial: Convert Alpha Hulls to Spatial* Objects

#' Convert an arc into line segments given the center of the arc, the radius, the vector, and the angle (radians)
#'
#' @param center The coordinates of the center
#' @param r The radius of the arc
#' @param vector The vector of the arc
#' @param theta The angle of the arc in radians
#' @param npoints Number of points along the arc
#' @return The Line object that approximates the given arc
#' @export
#' @examples
#' arc2line(center = c(0.5, -0.1), r = 0.2, vector = c(0.1, 1), theta = 0.04)

arc2line <- function(center, r, vector, theta, npoints = 100) {
  # Get the angles at the extremes of the arcs
  angles <- alphahull::anglesArc(vector, theta)
  # Generate sequence of angles along the arc to determine the points
  seqang <- seq(angles[1], angles[2], length = npoints)
  # Generate x coordinates for points along the arc
  x <- center[1] + r * cos(seqang)
  # Generate y coordinates for points along the arc
  y <- center[2] + r * sin(seqang)
  coords.xy <- cbind(x,y)
  line <- sp::Line(coords = coords.xy)
  return(line)
}

#' Convert alpha hulls into SpatialLines
#'
#' @param hull Alpha hull
#' @return The SpatialLines object
#' @export
#' @examples
#' library(alphahull)
#' set.seed(123)
#' x <- matrix(runif(100), nc = 2)
#' ahull_02 <- ahull(x, alpha = 0.2)
#' ahull_line_02 <- ahull2lines(ahull_02)

# Function to convert alpha hulls into SpatialLines
ahull2lines <- function(hull){
  arclist <- hull$arcs
  lines <- list()
  for (i in 1:nrow(arclist)) {
    # Extract the attributes of arc i
    center_i <- arclist[i, 1:2]
    radius_i <- arclist[i, 3]
    vector_i <- arclist[i, 4:5]
    theta_i <- arclist[i, 6]
    # Convert arc i into a Line object
    line_i <- arc2line(center = center_i, r = radius_i, vector = vector_i, theta = theta_i)
    list_length <- length(lines)
    if(list_length > 0){
      # If a line has already been added to the list of lines
      # Define last_line_coords as the coordinates of the last line added to the list before the ith line
      last_line_coords <- lines[[list_length]]@coords
    }
    if(i == 1){
      # Add the first line to the list of lines
      lines[[i]] <- line_i
    } else if(isTRUE(all.equal(line_i@coords[1,], last_line_coords[nrow(last_line_coords),]))){
      # If the first coordinate in the ith line is equal to the last coordinate in the previous line
      # then those lines should be connected
      # Row bind the coordinates for the ith line to the coordinates of the previous line in the list
      lines[[list_length]]@coords <- rbind(last_line_coords, line_i@coords[2:nrow(line_i@coords),])
    } else {
      # If the first coordinate in the ith line does not match the last coordinate in the previous line
      # then the ith line represents a new line
      # Add the ith line to the list as a new element
      lines[[length(lines) + 1]] <- line_i
    }
  }
  # Convert the list of lines to a Line object
  lines <- sp::Lines(lines, ID = 'l')
  # Convert the Line object to a SpatialLines object
  sp_lines <- sp::SpatialLines(list(lines))
  return(sp_lines)
}