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$ends
I have their coordinates and their intersection points with selfcrossing.psp(X)
(In R with putstat :) library(spatstat)
.
I need to get a list of segments and their coordinates and be able to manipulate them (change their orientation, position, intersection ...). These segments should be defined by the intersection of a line with another line and with a window.
So I missed an easy way to convert psp from multiple intersecting lines to psp non-intersecting segments (hopefully this is clear)?
If you have a tricky way, I'm wondering!
Thank you for your time!
change:
Here are the lines I have:
And here is some random stuff that I think I can create if I manage to process each segment (one by one). So I need to get a list of segments from my list of random strings.
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The function is spatstat
selfcut.psp
designed for this very purpose.
Y <- selfcut.psp(X)
For more information on manipulating line segment patterns, see Section 4.4 in the Flattening Book .
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Okay, a few coffee breaks later, here is some buggy code that does what you want. The cleaning I'll leave to you.
ranpoly <- function(numsegs=10,plotit=TRUE) {
require(spatstat)
# temp fix: put the first seg into segset. Later make it a constrained random.
segset<-psp(c(0,1,1,0,.25),c(0,0,1,1,0),c(1,1,0,0,1),c(0,1,1,0,.75),owin(c(0,1),c(0,1)) ) #frame the frame
for (jj in 1: numsegs) {
# randomly select a segment to start from, a point on the seg, the slope,and direction
# later... watch for slopes that immediately exit the frame
endx <-sample(c(-0.2,1.2),1) #force 'x1' outside the frame
# watch that sample() gotcha
if(segset$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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