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  \item{id.lengths}{A numeric vector used to separate locations in \code{x} and
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  \item{id.lengths}{A numeric vector used to separate locations in \code{x} and
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    \code{y} into multiple xspline.  Specifies consecutive blocks of
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    \code{y} into multiple xspline.  Specifies consecutive blocks of
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    locations which make up separate xsplines.}
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    locations which make up separate xsplines.}
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  \item{default.units}{A string indicating the default units to use
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  \item{default.units}{A string indicating the default units to use
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    if \code{x} or \code{y}
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    if \code{x} or \code{y}
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    are only given as numeric vectors.}
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    are only given as numeric vectors.}
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  \item{shape}{A numeric vector of values between -1 and 1, which
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  \item{shape}{A numeric vector of values between -1 and 1, which
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    control the shape of the spline relative to the control points.}
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    control the shape of the spline relative to the control points.}
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  \item{open}{A logical value indicating whether the spline is
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  \item{open}{A logical value indicating whether the spline is
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    a line or a closed shape.}
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    a line or a closed shape.}
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  \item{arrow}{A list describing arrow heads to place at either end
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  \item{arrow}{A list describing arrow heads to place at either end
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    of the xspline, as produced by the \code{arrow} function.}
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    of the xspline, as produced by the \code{arrow} function.}
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  \item{repEnds}{A logical value indicating whether the first and
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  \item{repEnds}{A logical value indicating whether the first and
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    last control points should be replicated for drawing the curve
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    last control points should be replicated for drawing the curve
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  actually replicated before the curve is drawn.  A curve is drawn
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  actually replicated before the curve is drawn.  A curve is drawn
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  between (interpolating or approximating)
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  between (interpolating or approximating)
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  the second and third of each set of four control points, so
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  the second and third of each set of four control points, so
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  this default behaviour ensures that
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  this default behaviour ensures that
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  the resulting curve starts at the first control point you have
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  the resulting curve starts at the first control point you have
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  specified and ends at the last control point.
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  specified and ends at the last control point.
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  The default
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  The default
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  behaviour can be turned off via the \code{repEnds} argument,
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  behaviour can be turned off via the \code{repEnds} argument,
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  in which case the curve that is drawn starts (approximately) at
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  in which case the curve that is drawn starts (approximately) at
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  the second control point and ends (approximately) at the first and
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  the second control point and ends (approximately) at the first and
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  second-to-last control point.
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  second-to-last control point.
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  The \code{repEnds} argument is ignored for closed xsplines.
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  The \code{repEnds} argument is ignored for closed xsplines.
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  Missing values are not allowed for \code{x} and \code{y}
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  Missing values are not allowed for \code{x} and \code{y}
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  (i.e., it is not valid for a control point to be missing).
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  (i.e., it is not valid for a control point to be missing).
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  For closed xsplines, a curve is automatically drawn
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  For closed xsplines, a curve is automatically drawn
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  between the final control point and the initial control point.
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  between the final control point and the initial control point.
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}
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}
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\references{
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\references{
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  Blanc, C. and Schlick, C. (1995),
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  Blanc, C. and Schlick, C. (1995),
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  "X-splines : A Spline Model Designed for the End User",
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  "X-splines : A Spline Model Designed for the End User",
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  in \emph{Proceedings of SIGGRAPH 95}, pp. 377--386.
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  in \emph{Proceedings of SIGGRAPH 95}, pp. 377--386.
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  \url{http://dept-info.labri.fr/~schlick/DOC/sig1.html}
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  \url{http://dept-info.labri.fr/~schlick/DOC/sig1.html}
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}
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}
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\seealso{
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\seealso{
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  \link{Grid},
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  \link{Grid},
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  \code{\link{viewport}},
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  \code{\link{viewport}},