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
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--
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
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p...@stat.auckland.ac.nz
www.stat.auckland.ac.nz/~paul/
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