Dear Prof. Murrell, Thanks a lot! This function is exactly what I need. Though I am not working with geographic data, this package seems to have other functions that I may need for my task.
Thanks! Regards, Shu Fai On Mon, Aug 11, 2025 at 6:42 AM Paul Murrell <p...@stat.auckland.ac.nz> wrote: > > Hi > > The 'sf' package can calculate the intersection ... > > library(grid) > x1 <- c(0.25, 0.25, 0.75, 0.75) > x2 <- c(0.05, 0.15, 0.25, 0.35) > y1 <- c(0.25, 0.75, 0.75, 0.25) > a <- xsplineGrob(x1, y1, shape = 1, gp=gpar(col="grey")) > b <- xsplineGrob(x2, y1, shape = .5, gp=gpar(col="grey")) > > ## Draw the lines to check that the answer is correct > grid.newpage() > grid.draw(a) > grid.draw(b) > > ## Use 'sf' to find point of intersection > library(sf) > aPts <- xsplinePoints(a) > bPts <- xsplinePoints(b) > aLine <- st_linestring(cbind(aPts$x, aPts$y)) > bLine <- st_linestring(cbind(bPts$x, bPts$y)) > int <- st_intersection(aLine, bLine) > > ## Draw the intersection > grid.circle(int[1], int[2], default.units="in", > r=unit(1, "mm"), gp=gpar(fill="black")) > > Hope that helps > > Paul > > On 9/08/25 06:36, Jeff Newmiller via R-help wrote: > > "Spline" is a class of curve generating techniques. The grid library > > implements two of these techniques to interpolate a curve through some > > points (b-spline and x-spline, differing primarily in the control > > variables). As a graphics library it has a focus on graphical output, > > though it does export the xsplinePoints() function with which you can > > approximate the curve with short line segments and proceed to do your math > > starting from there. There are some options discussed here [1]. > > > > [1] > > <https://stackoverflow.com/questions/68786700/calculate-2d-spline-path-in-r-with-customizable-number-of-interpolation-points> > > > > On August 8, 2025 7:16:03 AM PDT, Shu Fai Cheung <shufai.che...@gmail.com> > > wrote: > >> Hi All, > >> > >> I am new to the grid package. Please pardon me for asking a question > >> that may be a trivial one. > >> > >> If two curves created by grid::xsplineGrob() intersect, is there any > >> existing function that can find this/these point(s)? > >> > >> This is an example: > >> > >> library(grid) > >> > >> x1 <- c(0.25, 0.25, 0.75, 0.75) > >> x2 <- c(0.05, 0.15, 0.25, 0.35) > >> y1 <- c(0.25, 0.75, 0.75, 0.25) > >> > >> a <- xsplineGrob(x1, y1, shape = 1) > >> b <- xsplineGrob(x2, y1, shape = .5) > >> > >> grid.draw(a) > >> grid.draw(b) > >> > >> Specifically, I would like to find these points without drawing the > >> two curves using grid.draw(). > >> > >> I found methods to find this/these point(s) on the internet. But this > >> is a frequently asked question, so I guess maybe there are existing > >> functions/packages that can do this. However, I could not yet find > >> any. > >> > >> Regards, > >> Shu Fai Cheung > >> > >> ______________________________________________ > >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > >> https://stat.ethz.ch/mailman/listinfo/r-help > >> PLEASE do read the posting guide > >> https://www.R-project.org/posting-guide.html > >> and provide commented, minimal, self-contained, reproducible code. > > > > -- > Dr Paul Murrell (he/him) > Te Kura Tatauranga | Department of Statistics > Waipapa Taumata Rau | The University of Auckland > Private Bag 92019, Auckland 1142, New Zealand > 64 9 3737599 x85392 > p...@stat.auckland.ac.nz > www.stat.auckland.ac.nz/~paul/ > ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide https://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
