# xspline

##### Draw an X-spline

Draw an X-spline, a curve drawn relative to control points.

- Keywords
- aplot

##### Usage

```
xspline(x, y = NULL, shape = 0, open = TRUE, repEnds = TRUE,
draw = TRUE, border = par("fg"), col = NA, …)
```

##### Arguments

- x,y
vectors containing the coordinates of the vertices of the polygon. See

`xy.coords`

for alternatives.- shape
A numeric vector of values between -1 and 1, which control the shape of the spline relative to the control points.

- open
A logical value indicating whether the spline is an open or a closed shape.

- repEnds
For open X-splines, a logical value indicating whether the first and last control points should be replicated for drawing the curve. Ignored for closed X-splines.

- draw
logical: should the X-spline be drawn? If false, a set of line segments to draw the curve is returned, and nothing is drawn.

- border
the color to draw the curve. Use

`border = NA`

to omit borders.- col
the color for filling the shape. The default,

`NA`

, is to leave unfilled.- …
graphical parameters such as

`lty`

,`xpd`

,`lend`

,`ljoin`

and`lmitre`

can be given as arguments.

##### Details

An X-spline is a line drawn relative to control points. For each control point, the line may pass through (interpolate) the control point or it may only approach (approximate) the control point; the behaviour is determined by a shape parameter for each control point.

If the shape parameter is greater than zero, the spline approximates the control points (and is very similar to a cubic B-spline when the shape is 1). If the shape parameter is less than zero, the spline interpolates the control points (and is very similar to a Catmull-Rom spline when the shape is -1). If the shape parameter is 0, the spline forms a sharp corner at that control point.

For open X-splines, the start and end control points must have a shape of 0 (and non-zero values are silently converted to zero).

For open X-splines, by default the start and end control points are
replicated before the curve is drawn. A curve is drawn between
(interpolating or approximating) the second and third of each set of
four control points, so this default behaviour ensures that the
resulting curve starts at the first control point you have specified
and ends at the last control point. The default behaviour can be
turned off via the `repEnds`

argument.

##### Value

If `draw = TRUE`

, `NULL`

otherwise a list with elements
`x`

and `y`

which could be passed to `lines`

,
`polygon`

and so on.

Invisible in both cases.

##### Note

Two-dimensional splines need to be created in an isotropic coordinate system. Device coordinates are used (with an anisotropy correction if needed.)

##### References

Blanc, C. and Schlick, C. (1995),
*X-splines : A Spline Model Designed for the End User*,
in *Proceedings of SIGGRAPH 95*, pp.377--386.
http://dept-info.labri.fr/~schlick/DOC/sig1.html

##### See Also

`par`

for how to specify colors.

##### Examples

`library(graphics)`

```
# NOT RUN {
## based on examples in ?grid.xspline
xsplineTest <- function(s, open = TRUE,
x = c(1,1,3,3)/4,
y = c(1,3,3,1)/4, ...) {
plot(c(0,1), c(0,1), type = "n", axes = FALSE, xlab = "", ylab = "")
points(x, y, pch = 19)
xspline(x, y, s, open, ...)
text(x+0.05*c(-1,-1,1,1), y+0.05*c(-1,1,1,-1), s)
}
op <- par(mfrow = c(3,3), mar = rep(0,4), oma = c(0,0,2,0))
xsplineTest(c(0, -1, -1, 0))
xsplineTest(c(0, -1, 0, 0))
xsplineTest(c(0, -1, 1, 0))
xsplineTest(c(0, 0, -1, 0))
xsplineTest(c(0, 0, 0, 0))
xsplineTest(c(0, 0, 1, 0))
xsplineTest(c(0, 1, -1, 0))
xsplineTest(c(0, 1, 0, 0))
xsplineTest(c(0, 1, 1, 0))
title("Open X-splines", outer = TRUE)
par(mfrow = c(3,3), mar = rep(0,4), oma = c(0,0,2,0))
xsplineTest(c(0, -1, -1, 0), FALSE, col = "grey80")
xsplineTest(c(0, -1, 0, 0), FALSE, col = "grey80")
xsplineTest(c(0, -1, 1, 0), FALSE, col = "grey80")
xsplineTest(c(0, 0, -1, 0), FALSE, col = "grey80")
xsplineTest(c(0, 0, 0, 0), FALSE, col = "grey80")
xsplineTest(c(0, 0, 1, 0), FALSE, col = "grey80")
xsplineTest(c(0, 1, -1, 0), FALSE, col = "grey80")
xsplineTest(c(0, 1, 0, 0), FALSE, col = "grey80")
xsplineTest(c(0, 1, 1, 0), FALSE, col = "grey80")
title("Closed X-splines", outer = TRUE)
par(op)
x <- sort(stats::rnorm(5))
y <- sort(stats::rnorm(5))
plot(x, y, pch = 19)
res <- xspline(x, y, 1, draw = FALSE)
lines(res)
## the end points may be very close together,
## so use last few for direction
nr <- length(res$x)
arrows(res$x[1], res$y[1], res$x[4], res$y[4], code = 1, length = 0.1)
arrows(res$x[nr-3], res$y[nr-3], res$x[nr], res$y[nr], code = 2, length = 0.1)
# }
```

*Documentation reproduced from package graphics, version 3.6.1, License: Part of R 3.6.1*