How to generate a list of segments from a list of random self-intercept lines (psp in R)?

I am using X=rpoisline(4)

to generate strings and plot them with plot(X)

. Since X$ends

I have their coordinates and their intersection points with selfcrossing.psp(X)

(In R with putstat :) library(spatstat)

.

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

So I missed an easy way to convert psp from multiple intersecting lines to psp non-intersecting segments (hopefully this is clear)?

If you have a tricky way, I'm wondering!

Thank you for your time!

change:

Here are the lines I have:

here is what I have

And here is some random stuff that I think I can create if I manage to process each segment (one by one). So I need to get a list of segments from my list of random strings.

and here is what I need

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2 answers


The function is spatstat

selfcut.psp

designed for this very purpose.

Y <- selfcut.psp(X)

      



For more information on manipulating line segment patterns, see Section 4.4 in the Flattening Book .

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Okay, a few coffee breaks later, here is some buggy code that does what you want. The cleaning 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))
}

      

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