How to generate a list of segments from a list of

Question

I'm using X=rpoisline(4) to generate lines and plot them with plot(X). With X$ends I have their coordinates and their intersection points with selfcrossing.psp(X) (In R with spatstat : library(spatstat)).

I need to get a list of segments and their coordinates and be able to manipulate them (change their orientation, position, intersection...). Those segments have to be defined by the intersection of a line with an other line and with the window.

So, am I missing a simple way to convert a psp of few intersecting lines in a psp of non intersecting segments (I hope it's clear) ?

If you have a non-simple way, I'm interested to !

Thanks for your time !

edit :

Here are the lines I have :

And here are the kind of random stuff I think I can produce if I manage to handle each segments (one by one). So I need to get a list of segments from my list of random lines.

Answer 1

The spatstat function selfcut.psp is designed for exactly this purpose.

Y <- selfcut.psp(X)

For further information about manipulating line segment patterns, see section 4.4 in the spatstat book.

Answer 2

Ok, several coffeebreaks later, here's some buggy code that does what you want. The cleanup I'll leave to you.

ranpoly <- function(numsegs=10,plotit=TRUE) {

require(spatstat)
# temp fix: put the first seg into segset. Later make it a constrained random.
segset<-psp(c(0,1,1,0,.25),c(0,0,1,1,0),c(1,1,0,0,1),c(0,1,1,0,.75),owin(c(0,1),c(0,1)) ) #frame the frame
for (jj in 1: numsegs) {
    # randomly select a segment to start from, a point on the seg, the slope,and direction
    # later... watch for slopes that immediately exit the frame
    endx <-sample(c(-0.2,1.2),1)  #force 'x1' outside the frame
# watch that sample() gotcha
    if(segset$n<=5) sampset <- c(5,5) else sampset<-5:segset$n
    startseg<-sample(sampset,1) #don't select a frame segment
    # this is slope of segment to be constructed
    slope <- tan(runif(1)*2*pi-pi) # range +/- Inf 
    # get length of selected segment
    seglen<-lengths.psp(segset)[startseg]
    startcut <- runif(1) 
    # grab the coords of starting point (similar triangles)
    startx<- segset$ends$x0[startseg] + (segset$ends$x1[startseg]-segset$ends$x0[startseg])*startcut #seglen
    starty<- segset$ends$y0[startseg] + (segset$ends$y1[startseg]-segset$ends$y0[startseg])*startcut #seglen
    # make a psp object with that startpoint and slope; will adjust it after finding intersections
    endy <- starty + slope*(endx-startx)
    newpsp<-psp(startx,starty,endx,endy,segset$window,check=FALSE)
    # don't calc crossing for current element of segset
    hits <- crossing.psp(segset[-startseg],newpsp)
    segdist <- dist(cbind(c(startx,hits$x),c(starty,hits$y)))
    # dig back to get the crosspoint desired -- have to get matrixlike object out of class "dist" object
    # And, as.matrix puts a zero in location 1,1 kill that row.
    cutx <- hits$x[ which.min( as.matrix(segdist)[-1,1] )]
    cuty <- hits$y[which.min(as.matrix(segdist)[-1,1] )]
    segset <- superimpose(segset,psp(startx,starty,cutx,cuty,segset$window))

} #end jj loop
if(plotit) plot(segset,col=rainbow(numsegs))
return(invisible(segset))
}