# Ellipse Overlap

#### Andrew L Jackson

#### 2026-02-09

## Calculating the Area of Overlap Between Two Ellipses

    # remove previously loaded items from the current environment and remove previous graphics.
    rm(list=ls())
    graphics.off()

    # Here, I set the seed each time so that the results are comparable.
    # This is useful as it means that anyone that runs your code, *should*
    # get the same results as you, although random number generators change
    # from time to time.
    set.seed(1)

    # load SIBER
    library(SIBER)
    library(viridis)

    # set a new three-colour palette from the viridis package
    palette(viridis::viridis(3))

    # load in the included demonstration dataset
    data("demo.siber.data")
    #
    # create the siber object
    siber.example <- createSiberObject(demo.siber.data)

    # Or if working with your own data read in from a *.csv file, you would use
    # This *.csv file is included with this package. To find its location
    # type
    # fname <- system.file("extdata", "demo.siber.data.csv", package = "SIBER")
    # in your command window. You could load it directly by using the
    # returned path, or perhaps better, you could navigate to this folder
    # and copy this file to a folder of your own choice, and create a
    # script from this vingette to analyse it. This *.csv file provides
    # a template for how your own files should be formatted.

    # mydata <- read.csv(fname, header=T)
    # siber.example <- createSiberObject(mydata)

    # Create lists of plotting arguments to be passed onwards to the
    # plotting functions. With p.interval = NULL, these are SEA. NB not SEAc though
    # which is what we will base our overlap calculations on. This implementation
    # needs to be added in a future update. For now, the best way to plot SEAc is to
    # add the ellipses manually following the vignette on this topic.
    group.ellipses.args  <- list(n = 100, p.interval = NULL, lty = 1, lwd = 2)

    par(mfrow=c(1,1))
    plotSiberObject(siber.example,
                      ax.pad = 2,
                      hulls = F, community.hulls.args,
                      ellipses = T, group.ellipses.args,
                      group.hulls = F, group.hull.args,
                      bty = "L",
                      iso.order = c(1,2),
                      xlab = expression({delta}^13*C~'permille'),
                      ylab = expression({delta}^15*N~'permille')
                      )

In order now to calculate the overlap between two (or more) ellipses, we need to know the coordinates of each ellipse. This is done by calling `addEllipse(..., do.plot = FALSE)`. See the associated help file and the vignette Customising-Plots-Manually for more information on optional inputs to addEllipse for different types of ellipse. Also, bear in mind that the option `n` controls how many points are used to draw the ellipse, and hence low `n` means clunky, edgier ellipses, compared with rounder, smoother ellipses for higher `n`. A higher `n` is more suitable when ellipses are more eccentric as their curvature is greater at the tips. The new functions `maxLikOverlap` and `bayesianOverlap` are wrapper functions that take care of the calls to `addEllipse` and the actual polygon overlap function in the package `spatstat.utils`. The functions `maxLikOverlap` and `bayesianOverlap` return three values each: the computationally calculated area of the first ellipse, second ellipse, and the overlap between them. It is not entirely obvious to me that there is a single choice if you wish to express your overlap as a proportion, since there are several options for the choice of denominator. One can imagine that expressing the overlap as a proportion of the sum of the non-overlapping areas of the ellipses seems suitable in a general sense, since this will range from 0 when the ellipses are completely distinct, to 1 when the ellipses are completely coincidental.

    # In this example, I will calculate the overlap between ellipses for groups 2
    # and 3 in community 1 (i.e. the green and yellow open circles of data).

    # The first ellipse is referenced using a character string representation where
    # in "x.y", "x" is the community, and "y" is the group within that community.
    # So in this example: community 1, group 2
    ellipse1 <- "1.2"

    # Ellipse two is similarly defined: community 1, group3
    ellipse2 <- "1.3"

    # The overlap of the maximum likelihood fitted standard ellipses are
    # estimated using
    sea.overlap <- maxLikOverlap(ellipse1, ellipse2, siber.example,
                                 p.interval = NULL, n = 100)

    # the overlap betweeen the corresponding 95% prediction ellipses is given by:
    ellipse95.overlap <- maxLikOverlap(ellipse1, ellipse2, siber.example,
                                 p.interval = 0.95, n = 100)

    # so in this case, the overlap as a proportion of the non-overlapping area of
    # the two ellipses, would be
    prop.95.over <- ellipse95.overlap[3] / (ellipse95.overlap[2] +
                                              ellipse95.overlap[1] -
                                              ellipse95.overlap[3])

The function `bayesianOverlap` returns multiple rows of these three numbers, each representing the values for a particular draw from the posterior estimates so that you can build up a picture of the distribution of the estimated overlap. Calculating this overlap is computationally time consuming, and there are going to be thousands of posterior samples collected in a typical analysis. For this example, I will calculate the posterior overlap on the first 100 samples, but in reality you would probably want to do this on at least a few hundred, if not all your posterior samples in a longer (perhaps over-lunch or over-night) run.

    # options for running jags
    parms <- list()
    parms$n.iter <- 2 * 10^4   # number of iterations to run the model for
    parms$n.burnin <- 1 * 10^3 # discard the first set of values
    parms$n.thin <- 10     # thin the posterior by this many
    parms$n.chains <- 2        # run this many chains

    # define the priors
    priors <- list()
    priors$R <- 1 * diag(2)
    priors$k <- 2
    priors$tau.mu <- 1.0E-3

    # fit the ellipses which uses an Inverse Wishart prior
    # on the covariance matrix Sigma, and a vague normal prior on the
    # means. Fitting is via the JAGS method.
    ellipses.posterior <- siberMVN(siber.example, parms, priors)
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model
    #>
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model
    #>
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model
    #>
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model
    #>
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model
    #>
    #> Compiling model graph
    #>    Resolving undeclared variables
    #>    Allocating nodes
    #> Graph information:
    #>    Observed stochastic nodes: 30
    #>    Unobserved stochastic nodes: 3
    #>    Total graph size: 45
    #>
    #> Initializing model

    # and teh corresponding Bayesian estimates for the overlap between the
    # 95% ellipses is given by:
    bayes95.overlap <- bayesianOverlap(ellipse1, ellipse2, ellipses.posterior,
                                       draws = 100, p.interval = 0.95, n = 100)

    # a histogram of the overlap
    hist(bayes95.overlap[,3], 10)

    # and as above, you can express this a proportion of the non-overlapping area of
    # the two ellipses, would be
    bayes.prop.95.over <- (bayes95.overlap[,3] / (bayes95.overlap[,2] +
                                                    bayes95.overlap[,1] -
                                                    bayes95.overlap[,3])
                           )

    hist(bayes.prop.95.over, 10)