R/find_elbow.R

In fabotao/GapClust: Detecting rare cells from expression profiles by single cell RNA-seq

#' Detecting elbow point from density plot.
#'
#'
find_elbow <- function(x, y){
  n <- length(x)
  firstPoint <- c(x[1], y[1])
  lineVec = c(x[n]-x[1], y[n]-y[1])
  lineVecNorm = lineVec/(sqrt(sum(lineVec^2)))
  vecFromFirst = cbind(x-x[1], y-y[1])
  scalaProd =rowSums(vecFromFirst * cbind(rep(lineVecNorm[1], n), rep(lineVecNorm[2], n)))
  vecFromFirstParallel = outer(scalaProd, lineVecNorm)
  vecToLine = vecFromFirst - vecFromFirstParallel
  distToLine = sqrt(rowSums(vecToLine^2))
  idx = which.max(distToLine)
  return(x[idx])
}