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.   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. <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)

