R/ellipsoid-contrast.R

In antiphon/ellipsoid: Fit ellipsoids to point data

#' Testing contrast for ellipsoids
#' 
#' Give this to confint.ellipsoid to compute the contrast for
#'  testing semi_i - 0.5 * (semi_j + semi_k) = 0
#' 
#'  @param els list of ellipsoid estimates
#'  @param ijk on of c(1,2,3), c(2,1,3) or c(3,1,2). Default: c(3,1,2)
#'  @param out if >0 Use outlier detection to stabilise (useful with large variances)
#' 
#' @details 
#' 
#' The default contrast will compare third semi-axis to the average of first two. This seems to be the best, as the semi-axes are returned in an increasing order by the OLS-algorithm.
#' 
#' The 'out' is used like 'coef' in boxplot.stats, see there for details.
#' @export

ellipsoid_contrast_3d <- function(els, ijk = c(3,1,2), out=TRUE, z0=1.85){ # this is best
  #
  if(!all(1:3%in%ijk)) stop("ijk should be one of c(1,2,3), c(2,1,3) or c(3,1,2)")
  
  # determine what happends to x,y and z axis
  axes <- sapply(els, function(e){r<- diag(e$A);r/prod(r)} )
  #
  # the contrast
  e <- axes[ijk[1],]-0.5*(axes[ijk[2],]+axes[ijk[3],])
  
  if(out){
    m <- median(e, na.rm=TRUE)
    out <- abs(e-m) > z0 * abs(quantile(e-m, prob=3/4, na.rm=TRUE))
    e<-e[!out]
  }
  na.omit(e)
}