grid.xspline              package:grid              R Documentation

Draw an Xspline

Description:

     These functions create and draw an xspline, a curve drawn relative
     to control points.

Usage:

     grid.xspline(...)
     xsplineGrob(x = c(0, 0.5, 1, 0.5), y = c(0.5, 1, 0.5, 0),
                 id = NULL, id.lengths = NULL,
                 default.units = "npc",
                 shape = 0, open = TRUE, arrow = NULL, repEnds = TRUE,
                 name = NULL, gp = gpar(), vp = NULL)

Arguments:

       x: A numeric vector or unit object specifying x-locations of
          spline control points.

       y: A numeric vector or unit object specifying y-locations of
          spline control points.

      id: A numeric vector used to separate locations in 'x' and 'y'
          into multiple xsplines.  All locations with the same 'id'
          belong to the same xspline.

id.lengths: A numeric vector used to separate locations in 'x' and 'y'
          into multiple xspline.  Specifies consecutive blocks of
          locations which make up separate xsplines.

default.units: A string indicating the default units to use if 'x' or
          'y' are only given as numeric vectors.

   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  a line or a
          closed shape.

   arrow: A list describing arrow heads to place at either end of the
          xspline, as produced by the 'arrow' function.

 repEnds: A logical value indicating whether the first and last control
          points should be replicated for drawing the curve (see
          Details below).

    name: A character identifier. 

      gp: An object of class 'gpar', typically the output from a call
          to the function 'gpar'.  This is basically a list of
          graphical parameter settings.

      vp: A Grid viewport object (or NULL).

     ...: Arguments to be passed to 'xsplineGrob'.

Details:

     Both functions create an xspline grob (a graphical object
     describing an xspline), but only 'grid.xspline' draws the xspline.

     An xspline 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 xsplines, 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 xsplines, 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, in which
     case the curve that is drawn starts (approximately) at the second
     control point and ends (approximately) at the first and
     second-to-last control point.

     The 'repEnds' argument is ignored for closed xsplines.

     Missing values are not allowed for 'x' and 'y' (i.e., it is not
     valid for a control point to be missing).

     For closed xsplines, a curve is automatically drawn  between the
     final control point and the initial control point.

Value:

     A grob object.

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:

     Grid, 'viewport', 'arrow'.

     'xspline'.

Examples:

     x <- c(0.25, 0.25, 0.75, 0.75)
     y <- c(0.25, 0.75, 0.75, 0.25)

     xsplineTest <- function(s, i, j, open) {
       pushViewport(viewport(layout.pos.col=j, layout.pos.row=i))
       grid.points(x, y, default.units="npc", pch=16, size=unit(2, "mm"))
       grid.xspline(x, y, shape=s, open=open, gp=gpar(fill="grey"))
       grid.text(s, gp=gpar(col="grey"),
                 x=unit(x, "npc") + unit(c(-1, -1, 1, 1), "mm"),
                 y=unit(y, "npc") + unit(c(-1, 1, 1, -1), "mm"),
                 hjust=c(1, 1, 0, 0),
                 vjust=c(1, 0, 0, 1))
       popViewport()
     }

     pushViewport(viewport(width=.5, x=0, just="left",
                           layout=grid.layout(3, 3, respect=TRUE)))
     pushViewport(viewport(layout.pos.row=1))
     grid.text("Open Splines", y=1, just="bottom")
     popViewport()
     xsplineTest(c(0, -1, -1, 0), 1, 1, TRUE)
     xsplineTest(c(0, -1,  0, 0), 1, 2, TRUE)
     xsplineTest(c(0, -1,  1, 0), 1, 3, TRUE)
     xsplineTest(c(0,  0, -1, 0), 2, 1, TRUE)
     xsplineTest(c(0,  0,  0, 0), 2, 2, TRUE)
     xsplineTest(c(0,  0,  1, 0), 2, 3, TRUE)
     xsplineTest(c(0,  1, -1, 0), 3, 1, TRUE)
     xsplineTest(c(0,  1,  0, 0), 3, 2, TRUE)
     xsplineTest(c(0,  1,  1, 0), 3, 3, TRUE)
     popViewport()
     pushViewport(viewport(width=.5, x=1, just="right",
                           layout=grid.layout(3, 3, respect=TRUE)))
     pushViewport(viewport(layout.pos.row=1))
     grid.text("Closed Splines", y=1, just="bottom")
     popViewport()
     xsplineTest(c(-1, -1, -1, -1), 1, 1, FALSE)
     xsplineTest(c(-1, -1,  0, -1), 1, 2, FALSE)
     xsplineTest(c(-1, -1,  1, -1), 1, 3, FALSE)
     xsplineTest(c( 0,  0, -1,  0), 2, 1, FALSE)
     xsplineTest(c( 0,  0,  0,  0), 2, 2, FALSE)
     xsplineTest(c( 0,  0,  1,  0), 2, 3, FALSE)
     xsplineTest(c( 1,  1, -1,  1), 3, 1, FALSE)
     xsplineTest(c( 1,  1,  0,  1), 3, 2, FALSE)
     xsplineTest(c( 1,  1,  1,  1), 3, 3, FALSE)
     popViewport()

