ellipse3d.axes: Draw axes of a 3D ellipsoid

Description

A function to draw the major axes of a 3D ellipsoid from a correlation, covariance or sums of squares and cross products matrix.

Usage

ellipse3d.axes(
  x,
  centre = c(0, 0, 0),
  center = centre,
  scale = c(1, 1, 1),
  level = 0.95,
  t = sqrt(qchisq(level, 3)),
  which = 1:3,
  labels = TRUE,
  label.ends = c(2, 4, 6),
  ...
)

Value

Returns a 6 x 3 matrix containing the end points of the three axis lines in pairs by rows.

Arguments

x: A square positive definite matrix at least 3x3 in size. It will be treated as the correlation or covariance of a multivariate normal distribution.
centre, center: The center of the ellipse
scale: If x is a correlation matrix, then the standard deviations of each parameter can be given in the scale parameter. This defaults to c(1, 1, 1), so no rescaling will be done.
level: The coverage level of a simultaneous region. The default is 0.95, for a 95% region. This is used to control the size of the ellipsoid.
t: The size of the ellipsoid may also be controlled by specifying the value of a t-statistic on its boundary, which defaults to the square root of a chi-square statistic for a given level on 3 degrees of freedom.
which: An integer vector to select which variables from the object will be plotted. The default is the first 3.
labels: Either a logical value, a character string, or a character vector of length 3. If TRUE, the default, the axes are labeled PC1, PC2, PC3. If a single character string, the digits 1, 2, 3 are pasted on the end.
label.ends: A vector of length 3 indicating which ends of the axes should be labeled, corresponding to a selection of rows of the 6 x 3 matrix of axes end points. Default: c(2,4,6).
...: Other arguments passed to segments3d and text3d.

Author

Michael Friendly

Examples

Run this code


data(iris)
iris3 <- iris[,1:3]
cov <- cov(iris3)
mu <- colMeans(iris3)
col <-c("blue", "green", "red")[iris$Species]

library(rgl)
plot3d(iris3, type="s", size=0.4, col=col, cex=2, box=FALSE, aspect="iso")
plot3d( ellipse3d(cov, centre=mu, level=0.68), col="gray", alpha=0.2,  add = TRUE)

axes <- ellipse3d.axes(cov, centre=mu, level=0.68, color="gray", lwd=2)