[R] adding 3D arrows to 3D plots

[Duncan Murdoch]

Eben J. Gering wrote:
> 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")
>
>   

There isn't any current code to do that in rgl, but if you have a 
particular shape of arrow in mind, you could probably draw it with some 
combination of functions.  The main problem is that I've never seen any 
very appealing rotatable 3d arrows.  There's a Matlab function to draw 
some here:  
http://www.mathworks.com/matlabcentral/fileexchange/8396-draw-3d-arrows; 
it could probably be translated into rgl if you wanted that.

Duncan Murdoch