% File src/library/grDevices/man/colorRamp.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2017 R Core Team
% Distributed under GPL 2 or later

\name{colorRamp}
\alias{colorRamp}
\alias{colorRampPalette}
\title{Color interpolation}
\description{
  These functions return functions that interpolate a set of given
  colors to create new color palettes (like \code{\link{topo.colors}}) and
  color ramps, functions that map the interval \eqn{[0, 1]} to colors
  (like \code{\link{grey}}).
}
\usage{
colorRamp(colors, bias = 1, space = c("rgb", "Lab"),
          interpolate = c("linear", "spline"), alpha = FALSE)
colorRampPalette(colors, \dots)
}
\arguments{
  \item{colors}{colors to interpolate; must be a valid argument to
    \code{\link{col2rgb}()}.}
  \item{bias}{a positive number.  Higher values give more widely spaced
    colors at the high end.}
  \item{space}{a character string; interpolation in RGB or CIE Lab
    color spaces.}
  \item{interpolate}{use spline or linear interpolation.}
  \item{alpha}{logical: should alpha channel (opacity) values be
    returned?   It is an error to give a true value if
    \code{space} is specified.}
  \item{\dots}{arguments to pass to \code{colorRamp}.}
}
\details{
  The CIE Lab color space is approximately perceptually uniform, and so
  gives smoother and more uniform color ramps. On the other hand,
  palettes that vary from one hue to another via white may have a more
  symmetrical appearance in RGB space.

  The conversion formulas in this function do not appear to be
  completely accurate and the color ramp may not reach the extreme
  values in Lab space.  Future changes in the \R color model may change
  the colors produced with \code{space = "Lab"}.
}
\value{
  \code{colorRamp} returns a \code{\link{function}} with argument a
  vector of values between 0 and 1 that are mapped to a numeric matrix
  of RGB color values with one row per color and 3 or 4 columns.
  
  \code{colorRampPalette} returns a function that takes an integer
  argument (the required number of colors) and returns a character
  vector of colors (see \code{\link{rgb}}) interpolating the given sequence
  (similar to \code{\link{heat.colors}} or \code{\link{terrain.colors}}).
}

\seealso{
  Good starting points for interpolation are the \dQuote{sequential}
  and \dQuote{diverging} ColorBrewer palettes in the
  \CRANpkg{RColorBrewer} package.

  \code{\link{splinefun}} or \code{\link{approxfun}} are used for
  interpolation.
}

\examples{
## Both return a *function* :
colorRamp(c("red", "green"))( (0:4)/4 ) ## (x) , x in [0,1]
colorRampPalette(c("blue", "red"))( 4 ) ## (n)
## a ramp in opacity of blue values
colorRampPalette(c(rgb(0,0,1,1), rgb(0,0,1,0)), alpha = TRUE)(8)

require(graphics)

## Here space="rgb" gives palettes that vary only in saturation,
## as intended.
## With space="Lab" the steps are more uniform, but the hues
## are slightly purple.
filled.contour(volcano,
               color.palette =
                   colorRampPalette(c("red", "white", "blue")),
               asp = 1)
filled.contour(volcano,
               color.palette =
                   colorRampPalette(c("red", "white", "blue"),
                                    space = "Lab"),
               asp = 1)

## Interpolating a 'sequential' ColorBrewer palette
YlOrBr <- c("#FFFFD4", "#FED98E", "#FE9929", "#D95F0E", "#993404")
filled.contour(volcano,
               color.palette = colorRampPalette(YlOrBr, space = "Lab"),
               asp = 1)
filled.contour(volcano,
               color.palette = colorRampPalette(YlOrBr, space = "Lab",
                                                bias = 0.5),
               asp = 1)

## 'jet.colors' is "as in Matlab"
## (and hurting the eyes by over-saturation)
jet.colors <-
  colorRampPalette(c("#00007F", "blue", "#007FFF", "cyan",
                     "#7FFF7F", "yellow", "#FF7F00", "red", "#7F0000"))
filled.contour(volcano, color = jet.colors, asp = 1)

## space="Lab" helps when colors don't form a natural sequence
m <- outer(1:20,1:20,function(x,y) sin(sqrt(x*y)/3))
rgb.palette <- colorRampPalette(c("red", "orange", "blue"),
                                space = "rgb")
Lab.palette <- colorRampPalette(c("red", "orange", "blue"),
                                space = "Lab")
filled.contour(m, col = rgb.palette(20))
filled.contour(m, col = Lab.palette(20))
}
\keyword{color}