#' Create ternary maps based on the HSV or HCL color space
#'
#' Creates a map based on HSV or HCL color space and returns an image stack
#' with three bands containing the HSV/HCL colors converted back to RGB
#' to easier plotting
#'
#' @param H Hue raster usually representing angles in the HSV color wheel and within
#' the [0,1] interval
#' @param S Saturation raster for HSV and within [0,1] interval
#' @param V Value raster for HSV and within [0,1] interval
#'
#' @param hue Hue raster usually representing angles in the HCL color wheel and within
#' the [0,360] interval
#' @param chroma Chroma raster for HCL and within [0,100] interval
#' @param luminance Luminance raster for HCL and within [0,100] interval
#'
#' @param colorspace Color space to use. Either 'HSV' (default) or 'HCL'
#' @param rescale Re-scale values of S and V or chroma and luminance to 0 - 1 range?
#' Rescaling is performed as: (x-min(x)) / (max(x)-min(x))? (default: FALSE)
#' @param hueUnits Units of angle in the H (HSV) or hue (HCL) component?
#' Either 'deg' (degrees) or 'rad' (radians)
#' @param verbose Print progress messages? (default: TRUE)
#' @param filename A filename to which write the output RGB raster stack (default: NULL).
#' @param ... Extra parameters to pass in writeRaster
#'
#' @return A RasterStack object with three bands (for R,G, and B values in the 0-255 range)
#'

ternaryMaps <- function(H=NULL, S=NULL, V=NULL, hue=NULL, chroma=NULL, luminance=NULL, colorspace="HSV",
                        rescale=FALSE, hueUnits="deg", verbose=TRUE, filename=NULL, ...){

  require(raster)

  if(colorspace=="HSV"){
    if(any(c(is.null(H), is.null(S), is.null(V)))){
      stop("H, S and V must be non-null! Please check input arguments.")
    }
    c1 <- H
    c2 <- S
    c3 <- V
  }
  if(colorspace=="HCL"){
    if(any(c(is.null(hue), is.null(chroma), is.null(luminance)))){
      stop("hue, chroma and luminance must be non-null! Please check input arguments.")
    }
    c1 <- hue
    c2 <- chroma
    c3 <- luminance
  }

  if(!inherits(c1,"RasterLayer") || !inherits(c2,"RasterLayer") || !inherits(c3,"RasterLayer")){
    stop("H/S/V or hue/chroma/luminance must be RasterLayer objects!")
  }

  if(!compareRaster(c1,c2,c3,stopiffalse = FALSE)){
    stop("Rasters in H/S/V or hue/chroma/luminance are different! Please check input arguments.")
  }

  if(rescale){

    if(verbose) message("Standardizing raster data....")

    rescale01 <- function(x, ...){(x - cellStats(x, 'min', ...)) / (cellStats(x, 'max', ...) - cellStats(x, 'min', ...))}
    #c1 <- rescale01(c1)
    if(hueUnits=="deg"){
      c1 <- c1 / 360
    }else if(hueUnits=="rad"){
      c1 <- c1 / (2*pi)
    }else{
      stop("Invalid units in hueUnits! Either 'deg' (degrees) or 'rad' (radians) options are valid.")
    }
    c2 <- rescale01(c2)
    c3 <- rescale01(c3)

    if(verbose) message("Finished!")
  }


  if(verbose) message("Stacking and loading raster data....")
  rstDF <- values(stack(c1,c2,c3))
  if(verbose) message("Finished!")


  if(verbose) message("Calculating colors....")
  ind <- complete.cases(rstDF)

  if(colorspace=="HSV"){
    # Generate HSV color codes
    colVec <- hsv(h = rstDF[ind,1], s = rstDF[ind,2], v = rstDF[ind,3])
    # Convert color code to RGB and transpose the color matrix from wide to long
    rgbCol <- t(col2rgb(colVec))
  }

  if(colorspace=="HCL"){
    # Generate HCL color codes
    colVec <- hcl(h = rstDF[ind,1]*360, c = rstDF[ind,2]*100, l = rstDF[ind,3]*100)
    # Convert color code to RGB and transpose the color matrix from wide to long
    rgbCol <- t(col2rgb(colVec))
  }
  if(verbose) message("Finished!")


  if(verbose) message("Attributing colors to the output RGB raster....")

  r <- c1
  values(r) <- NA
  values(r)[ind] <- rgbCol[,1] # red component

  g <- c1
  values(g) <- NA
  values(g)[ind] <- rgbCol[,2] # green component

  b <- c1
  values(b) <- NA
  values(b)[ind] <- rgbCol[,3] # blue component

  rstOut <- stack(r,g,b)

  if(verbose) message("Finished!")

  if(!is.null(filename)){
    if(verbose) message("Writing raster data to file....")

    writeRaster(rstOut, filename = filename, ...)

    if(verbose) message("Finished!")
  }
  return(rstOut)
}