adding 3D arrows to 3D plots

Greetings,

I would like to add 3D arrows (i.e. arrow-headed vectors linking X1Y1Z1 to X2,Y2,Z2) to a 3D plot; ideally the sort of plot that can be rotated interactively.  Is this possible using plot3d, or another 3d plotter in R?  
While it is easy to draw segments in plot3d (e.g. below), I haven't figured out how to add arrow heads, or to create 3d arrows from scratch.  

##two headless segments:
library(rgl)
matrix(1:2,2,3)->segment1
matrix(1:3,2,3)->segment2
plot3d(segment2,type="l",col="red",xlim=c(0,3),ylim=c(0,3),zlim=c(0,3))
plot3d(segment1,type="l",add=TRUE,col="blue")

  

Have a go with this:

arrow3d <- function(p0=c(0,1,0),p1=c(1,1,1),s=0.1,theta=pi/4,n=3,...){
 ##    p0: start point
 ##    p1: end point
 ##     s: length of barb as fraction of line length
 ## theta: opening angle of barbs
 ##     n: number of barbs
 ##   ...: args passed to lines3d for line styling

 require(geometry)
 require(rgl)

 ## rotational angles of barbs
 phi=seq(0,2*pi,len=n+1)[-1]

 ## length of line
 lp = sqrt(sum((p1-p0)^2))

 ## point down the line where the barb ends line up
 cpt=(1-(s*lp*cos(theta)))*(p1-p0)

 ## draw the main line
 line = lines3d(c(p0[1],p1[1]),c(p0[2],p1[2]),c(p0[3],p1[3]),...)

 ## need to find a right-angle to the line. So create a random point:
 rpt = jitter(c(
   runif(1,min(p0[1],p1[1]),max(p0[1],p1[1])),
   runif(1,min(p0[2],p1[2]),max(p0[2],p1[2])),
   runif(1,min(p0[3],p1[3]),max(p0[3],p1[3]))
   ))

 ## and if it's NOT on the line the cross-product gives us a vector
at right angles:
 r = extprod3d(p1-p0,rpt)
 ## normalise it:
 r = r / sqrt(sum(r^2))

 ## now compute the barb end points and draw:
 pts = list()
 for(i in 1:length(phi)){
   ptb=rotate3d(r,phi[i],(p1-p0)[1],(p1-p0)[2],(p1-p0)[3])
   lines3d(
           c(p1[1],cpt[1]+p0[1]+lp*s*sin(theta)*ptb[1]),
           c(p1[2],cpt[2]+p0[2]+lp*s*sin(theta)*ptb[2]),
           c(p1[3],cpt[3]+p0[3]+lp*s*sin(theta)*ptb[3]),
           ...
           )
 }
 return(line)
}

This creates a line with 'n' arrow barbs at one end, equally spaced
when you look at the line end-on. The barb length is 's' times the
length of the line, and the opening angle is theta. Just do arrow3d()
to get something.

Might be useful, plus I wanted to brush up on my vector geometry anyway...

Barry