conics (version 0.3)

conicPlot: Plot a conic

Description

Plot a conic (ellipse, hyperbola, or parabola) specified by a quadratic polynomial or by a symmetric 3x3 matrix.

Usage

conicPlot(x, type='l', npoints=100,  sym.axes=FALSE, center=FALSE, asymptotes=FALSE,  add=FALSE, xlim=NULL, ylim=NULL,  ax.lty=1, ax.col=palette()[1],  as.lty=1, as.col=palette()[1], ...)

Arguments

x

a 6-length vector or a symmetric 3x3 matrix

type

(character) the type of plot to draw (same meaning as with the plot function)

npoints

(numeric) number of points to draw

sym.axes

(logical) if TRUE, display the axes of the conic

center

(logical) if TRUE, display the center of the conic (if any)

asymptotes

(logical) if TRUE, display the asymptotes (hyperbolas)

add

(logical) if TRUE, plot over the current graphical device

xlim

(vector) interval for the x-coordinate

ylim

(vector) interval for the y-coordinate

ax.lty

(character or numeric) line type of the axes

ax.col

(character or numeric) color of the axes

as.lty

(character or numeric) line type of the asymptotes

as.col

(character or numeric) color of the asymptotes

...

other parameters passed to the plot function

Value

The return value is invisible, i-e it is not printed on the console by default but can be stored in a variable. It is a list of relevant computed values corresponding to various elements of the conic. The following elements can be found in the return list, depending on the kind of the conic:

kind

the kind of the conic: "ellipse", "hyperbola", "parabola", or "lines".

axes

the symmetry axes. See also the function conicAxes.

center

the center of the conic. See also the function conicCenter.

asymptotes

the slopes of the asymptotes. See also the function conicAsymptotes.

vertices

the vertices of the conic.

foci

the focal points of the conic.

eccentricity

the eccentricity of the conic.

intercepts

the intercepts in the case of parallel lines.

points

the coordinates of the points used to plot the conic.

The points component is returned only if the type option is equal to n and if the conic is non-degenerate. In that case, nothing is drawn.

Details

The conicPlot function identifies the type of the conic and plots it in the current graphical device.

The conic is specified either by a 6-length vector representing the coefficients of the quadratic polynomial, or by the symmetric matrix representing the associated quadratic form. See the function conicMatrix to build this matrix given the coefficients of the polynomial.

It is usually a good idea to set explicitely the aspect ratio to 1 (as an additional argument asp=1 in the conicPlot function) in order to avoid distorsions between the units of the x-axis and the y-axis. See examples below.

See Also

conicAsymptotes, conicAxes, conicCenter, conicMatrix

Examples

# Ellipse
# -------
# Equation: 2*x_1^2 + 2*x_1*x_2 + 2*x_2^2 - 20*x_1 - 28*x_2 + 10 = 0
v <- c(2,2,2,-20,-28,10)
conicPlot(v)
v[6] <- 20
conicPlot(v, type='p', col="red", add=TRUE)

# Symmetric matrix
m <- rbind( c(5, -3, -21),
            c(-3, 5, -19),
            c(-21, -19, 93) )
conicPlot(m)

# Hyperbola
# ---------
# Equation: 2*x_1^2 + 2*x_1*x_2 - 2*x_2^2 - 20*x_1 + 20*x_2 + 10 = 0
v <- c(2,2,-2,-20,20,10)
conicPlot(v, center=TRUE, sym.axes=TRUE, asp=1)
conicPlot(v, asymptote=TRUE, as.col="grey30", as.lty=2,
          sym.axes=TRUE, ax.col="red", ax.lty=6, col="blue", asp=1)

# Parabola
# --------
# Equation: 4*x_1^2 + 4*x_1*x_2 + 1*x_2^2 + 20*x_1 + 20*x_2 + 20 = 0
v <- c(4,4,1,20,20,20)
conicPlot(v, sym.axes=TRUE, ax.lty=2, asp=1)

# Degenerate conics
# -----------------
# Intersecting lines
# Equation:  x_1^2 - 2*x_1*x_2 - 8*x_2^2 - 2*x_1 + 14*x_2 - 3 = 0
v <- c(1,-2,-8,-2,14,-3)
conicPlot(v)
# Parallel lines
# Equation: x_1^2 - 2*x_1*x_2 + x_2^2 + 4*x_1 - 4*x_2 + 3 = 0
v <- c(1,-2,1,4,-4,3)
conicPlot(v)
# Coincident lines
# Equation: 4*x_1^2 + 12*x_1*x_2 + 9*x_2^2 - 4*x_1 - 6*x_2 + 1 = 0
v <- c(4,12,9,-4,-6,1)
conicPlot(v)

# Return value
# ------------
v <- c(2,2,2,-20,-28,10)
cp <- conicPlot(v)
cp$kind
cp$vertices
cp$center
cp$axes
cp <- conicPlot(v,type='n')
cp$points