R/addAlpha.R

In brry/berryFunctions: Function Collection Related to Plotting and Hydrology

#' Color transparency
#' 
#' Make existing colors semi-transparent (add alpha)
#' 
#' @param col Vector of color names (\code{\link{colors}}), hexadecimal or integer that can be interpreted by \code{\link{col2rgb}}
#' @param alpha Level of semi-transparency. between 0 (transparent) and 1 (intransparent). Can also be a vector. DEFAULT: 0.3

#' @return character vector with hexadecimal color codes.
#' @author Berry Boessenkool, \email{berry-b@@gmx.de}, June 2014 Based on suggestion by Mathias Seibert, Dec. 2013
#' @seealso \code{\link{addFade}}, \code{\link{rgb}}, \code{\link{colors}}, \code{\link{col2rgb}}
#' @keywords dplot color
#' @export
#' @importFrom grDevices col2rgb rgb
#' @examples
#' 
#' addAlpha("red", c(0.1, 0.3, 0.6, 1))
#' addAlpha(1:3)
#' addAlpha(1:3, 1:3/3)
#' NewColors <- addAlpha(c("red","blue","yellow","green", "purple"), 0:200/200)
#' plot(runif(1000), col=NewColors, pch=16, cex=2)
#' 
#' # use addFade for line segments, because of overlapping dots
#' set.seed(1); x <- cumsum(rnorm(30))  ; y <- x-2
#' plot(x, type="n")
#' segments(x0=1:29,y0=head(x,-1), x1=2:30,y1=x[-1], col=addAlpha(4, 29:0/30), lwd=10)
#' segments(x0=1:29,y0=head(y,-1), x1=2:30,y1=y[-1], col=addFade (4, 29:0/30), lwd=10)
#' 
addAlpha <- function(
  col,
  alpha=0.3
  )
{
if(any(alpha<0 | alpha>1)) stop("alpha must be between 0 and 1, not ",
                                toString(alpha[alpha<0|alpha>1]))
rgb2 <-  function(x) rgb(x[1], x[2], x[3], alpha=alpha)
output <- apply(X=sapply(col,col2rgb)/255, MARGIN=2, FUN=rgb2)
if( length(alpha)==1 | length(col)==1)  return(as.vector(output)) else
if( length(alpha) == length(col) ) return(diag(output)) else
warning("col and alpha had different lengths"); return(output)
}