directlabels - utility.function - Positioning Method - project.onto.segments

Given a point and a set of line segments representing a convex or alpha hull, calculate the closest point on the segments.

project.onto.segments <- function
### Given a point and a set of line segments representing a convex or
### alpha hull, calculate the closest point on the segments.
(m,
### m is 1 row, a center of a point cloud, we need to find the
### distance to the closest point on each segment of the convex
### hull.
 h,
### Data frame describing the line segments of the convex or alpha
### hull.
 debug=FALSE,
 ...
### ignored
 ){
  h$s <- (h$y2-h$y1)/(h$x2-h$x1)
  ## the closest point on the line formed by expanding this line
  ## segment (this expression is calculated by finding the minimum
  ## of the distance function).
  h$xstar <- (m$x + m$y*h$s + h$x1*h$s^2 - h$s*h$y1)/(h$s^2+1)
  h$minval <- apply(cbind(h$x1,h$x2),1,min)
  h$maxval <- apply(cbind(h$x1,h$x2),1,max)
  ## xopt is the closest point on the line segment
  h$xopt <- ifelse(h$xstar<h$minval,h$minval,
                   ifelse(h$xstar>h$maxval,h$maxval,h$xstar))
  h$yopt <- h$s*(h$xopt-h$x1)+h$y1
  ## distance to each point on line segment from the center
  h$d <- (m$x-h$xopt)^2+(m$y-h$yopt)^2
  i <- which.min(h$d)
  result <- with(h[i,],data.frame(x=xopt,y=yopt))
  if(debug){
    grid.segments(m$x,m$y,result$x,result$y,default.units="cm")
  }
  result
}