# 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
}
```
 Please contact Toby Dylan Hocking if you are using directlabels or have ideas to contribute, thanks! Documentation website generated from source code version 2021.2.24 (git revision bb6db07 Mon, 14 Jun 2021 22:38:45 +0530) using inlinedocs. validate