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Alpha channel in colorRamp() and colorRampPalette()

2 messages · Alberto Krone-Martins, Brian Ripley

Alberto Krone-Martins
# 
Hi all, 

 I had the need to create a colorbar considering the alpha channel of the colors, but colorRamp() and colorRampPalette() ignored the alpha argument in rgb(). So I performed some minor modifs. in their codes, as to support the interpolation using the alpha channel.
 
 I guess that those simple modifications might be useful for other people, so perhaps it would be worth to add them to colorRamp and colorRampPalette codes in grDevices? the modified functions follows.
 
 Cheers,
 
 Alberto.
 
colorRampPalette <- function (colors, ...) {
    ramp <- colorRamp(colors, ...)
    function(n) {
        x <- ramp(seq.int(0, 1, length.out = n))
        rgb(x[, 1], x[, 2], x[, 3], x[, 4], maxColorValue = 255)
    }
}

colorRamp <- function (colors, bias = 1, space = c("rgb", "Lab"), interpolate = c("linear", "spline")) {
    if (bias <= 0) 
        stop("'bias' must be positive")
    colors <- t(col2rgb(colors, alpha=T)/255)
    space <- match.arg(space)
    interpolate <- match.arg(interpolate)
    if (space == "Lab") {
        colors <- convertColor(colors, from = "sRGB", to = "Lab")
    }
    interpolate <- switch(interpolate, linear = stats::approxfun, spline = stats::splinefun)
    if ((nc <- nrow(colors)) == 1L) {
        colors <- colors[c(1L, 1L), ]
        nc <- 2L
    }
    x <- seq.int(0, 1, length.out = nc)^bias
    palette <- c(interpolate(x, colors[, 1]), interpolate(x, colors[, 2]), interpolate(x, colors[, 3]), interpolate(x, colors[, 4]))
    roundcolor <- function(rgb) pmax(pmin(rgb, 1), 0)
    if (space == "Lab") {
        function(x) {
            roundcolor(convertColor(cbind(palette[[1L]](x), palette[[2L]](x), 
                palette[[3L]](x), palette[[4L]](x)), from = "Lab", to = "sRGB")) * 
                255
        }
    }
    else {
        function(x) {
            roundcolor(cbind(palette[[1L]](x), palette[[2L]](x), 
                palette[[3L]](x), palette[[4L]](x))) * 255
        }
    }
}

____________________________________________________

 Universidade de Lisboa - Laborat?rio SIM
 Alberto Krone-Martins
 http://www.astro.iag.usp.br/~algol
Brian Ripley
# 
Thank you for the suggestion. But editing deparsed code is of almost no 
use to us: you need to edit the sources.  And even then mailers may 
distort it, so it really is much easier to file these things as 
attachments on bugs.r-project.org, as a wishlist item.

I have incorporated what I think you intended in R-devel, so please test it.
On 24/07/2013 16:00, Alberto Krone-Martins wrote: