xspline               package:graphics               R Documentation

Draw an X-spline

Description:

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

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.  The default, 'NULL', means to
          use 'par("fg")'.  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
     without warning).

     For open X-splines, by default the start and end control points
     are actually 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.

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. <URL: http://dept-info.labri.fr/~schlick/DOC/sig1.html>

See Also:

     'polygon'.

     'par' for how to specify colors.

Examples:

     ## 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)

